How to design a heating

system

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CIBSE Knowledge Series — How to design a heating system

How to design a
heating system

CIBSE Knowledge Series: KS8

Principal author
Gay Lawrence Race

Editor
Helen Carwardine

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
1.1 Use of this guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

The heating design process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
2.1 The design process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

2.2 Heating system design process . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

2.3 Key heating design calculation sequence . . . . . . . . . . . . . . . . . . . .8

2.4 Thermal comfort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Key design steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
3.1 Step 1: pre-design and design brief . . . . . . . . . . . . . . . . . . . . . . .10

3.2 Step 2: gather design information . . . . . . . . . . . . . . . . . . . . . . . . .11

3.3 Step 3: design data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

3.4 Step 4: building thermal performance analysis . . . . . . . . . . . . . . .13

5 Step 5: heating system option analysis and selection . . . . . . . . . .15

3.6 Step 6: space heat losses and heat load . . . . . . . . . . . . . . . . . . . .20

3.7 Step 7: equipment sizing and selection . . . . . . . . . . . . . . . . . . . . .23

3.8 Step 8: heating load analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

3.9 Step 9: plant sizing and selection . . . . . . . . . . . . . . . . . . . . . . . . .27

3.10 Step 10: system analysis and control performance . . . . . . . . . .27

3.11 Step 11: Final value engineering and energy targets assessment 29

3.12 Step 12: design review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

Developing the design — key issues . . . . . . . . . . . . . . . . . . . . . . .31
4.1 Design data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

4.2 Design margins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

4.3 Energy efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

4.4 Quality control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

CIBSE Knowledge Series — How to design a heating system

Introduction

In cooler climates the provision of heating is an essential part of creating

comfortable internal environments, and therefore heating system design is a

fundamental part of building services design.

Heating is a major sector within mechanical building services. There are some

21 million domestic properties in the UK with gas-fired central heating, and a

further 200,000 commercial properties with heating. The UK market for

heating systems is substantial, with around 1.65 million new domestic boilers

installed per year and around 23,500 commercial boilers. There are around 9

million radiators installed per year with a further 22 million metres of

underfloor heating pipe (2005 figures)

(1)

Heating is also a major consumer of energy within the UK, with space heating

accounting for over 40% of all non-transport energy use and over 60% of

domestic energy use

(2)

, rising to over 80% if hot water is included (see Figure

1). As major energy users, heating and hot water also generate a substantial

proportion of CO

emissions, delivering almost half the CO

emissions from

non-domestic buildings.

Given the current requirements to limit energy consumption and CO

production, good design of heating systems is essential to ensure that systems

operate efficiently and safely and make effective use of energy. Historically

there have been problems with oversizing of heating systems which can lead

to inefficient operation, particularly at part load operation, to control

problems and to a reduction in plant operating life

(3)

. The energy

consumption for oversized plant can be 50% more than necessary.

Although heating is often considered to be a simple, basic system, there are

many options and permutations to be considered. The majority of UK

buildings will require heating but different building types and locations will

have very different requirements and constraints — consider for example the

choices possible for a small ground floor flat in a city centre development

against those for a holiday cottage in one of the National Parks, or the

choices for an urban industrial unit against those for a rural agricultural unit

and farm shop.

The fundamental components of any heating system are:

—

a means of generating heat, i.e. the heat source

—

a means of distributing the heat around the building or buildings, i.e.

the distribution medium and network

—

a means of delivering the heat into the space to be heated, i.e. the
heat emitter.

Heating

In 2005:

1.65 million new domestic boilers

23,500 commercial boilers

9 million radiators

22 million metres of underfloor
heating pipe

were installed in the UK alone.

Sources: BSRIA domestic boiler
marketing report March 2006, BSRIA
commercial boiler marketing report
March 2006.

Figure 1:

UK non-transport energy
use (2002 figures) million
tonnes of oil equivalent

Space heating
Water
Cooking/catering
Lighting appliances
Process use
Motors/drivers
Drying/separation
Other non-transport

41·4

11·8

4·4

2·4

15·0

9·7

3·3

12·9

Source: DTI Energy consumption tables:
overall energy consumption. URN No:
05/2008 Table 1.2 Non-transport energy
consumption by end use, 1990, 2000, 2001
and 2002

There are many possible options to be considered, some of which are listed

in Table 1 below. These can give many permutations, from the simple use of

electric panel heating, using electricity both as the heat source and

distribution medium, to a conventional gas boiler system distributing low

temperature water to a convector system. A more complex system would be

one serving various buildings by using oil as the heat source to generate high

temperature water for the main distribution, which is then reduced in

temperature and pressure to low temperature water, via heat exchangers, to

serve a radiator system.

Whilst heating systems may seem relatively simple, in practice there are many

factors to be considered during the design process, in order to achieve a

well-designed system that delivers both the required comfort conditions and

level of control whilst still minimising energy consumption. This publication,

together with other CIBSE guidance, aims to assist the designer in achieving

that aim.

1.1

Use of this guidance

This publication provides a clear, step-by-step overview of the whole heating

design sequence:

—

section 2 maps the heating design process, with flowcharts illustrating

the design steps sequence, and sets this in the context of the overall

building process

—

section 3 outlines the key design procedures for each design step, and

CIBSE Knowledge Series — How to design a heating system

Good design

Good design of heating systems is
essential to ensure that systems operate
efficiently and safely and make effective
use of energy.

Factors to consider

Building type:

domestic

school

apartment building

retail

hospital

factory

office

Location:

city centre

urban

suburban

rural

Heat source

gas

CHP

LPG

solar

oil

biomass

coal

off-peak electricity

electricity

wind

air or water via heat pump

ground via ground source heat pump

Distribution medium

water: low, medium or high temperature

air

steam

electricity

Emitter

radiators

ceiling panels

fan convectors

natural convectors

panel heaters

underfloor heating coils

unit heaters

storage heaters

high temperature radiant panels

Table 1:

Heating systems

CIBSE Knowledge Series — How to design a heating system

provides guidance on data requirements and sources, design outputs,

key design issues and potential problem points

—

section 4 addresses additional design issues that affect the design

process.

The publication links to the CIBSE Guides and also cross-references other

key industry sources of design procedure guidance. Other relevant titles in

the Knowledge Series are:

—

KS04: Understanding controls

—

KS06: Comfort

—

KS: Energy efficient heating (forthcoming title).

This guidance is intended to enable and assist building services engineers

involved in design, installation and commissioning to appreciate the key

decisions and design steps involved in heating system design. It is likely to be

of particular benefit to junior engineers and those whose main experience lies

within other sectors of building services design. It can also be used by building

services engineers to facilitate discussion on design requirements and design

decisions with their clients.

The publication answers the following questions, which can be used to help

you find the most relevant sections to you:

—

What are the key stages in the heating design process? (Section 2.2)

—

What are the design criteria for thermal comfort? (Sections 2.4 and

3.3)

—

What should I consider when selecting a heating system? (Section 3.5)

—

How do I determine preheat requirements? (Section 3.6)

—

What should I consider to determine the required heating load?

(Section 3.8)

—

When should I consider load diversity? (Section 3.8)

—

What else should I consider during design? (Section 4).

Finally, a selected bibliography is provided for those who want further reading

on the subject, subdivided to cover the main design steps and key topics such

as design data, design calculations, design checks, heating plant and controls.

Detailed technical information on heating system design and design data can

be found in CIBSE Guide A (2006) and CIBSE Guide B (2001-2), chapter 1.

The heating design process

2.1

The design process

Design involves translating ideas, proposals and statements of needs and

requirements into precise descriptions of a specific product

(4)

, which can then

be delivered. (See Figure 2.) Two major features characterise the design

process in general. Firstly, design tends to evolve through a series of stages

during which the solution is increasingly designed at greater levels of detail,

moving from broad outline through to fine detail. Secondly, design tends to

contain iterative cycles of activities during which designs, or design

components, are continually trialled, tested, evaluated and refined. Feedback

is therefore an essential component of the design process, as shown in

Figure 2.

Within construction, design is a part of the larger construction process, as

shown in Figure 3. Both the RIBA Plan of Work Stages

(5)

and the ACE

Conditions of Engagement Agreements A(2) and B(2)

(6)

, which are commonly

used for mechanical and electrical building services design, divide design into

the separate stages of outline design, scheme design and further/detail design.

In practice, therefore, the construction design process is invariably iterative,

with many design steps being revisited and revised as the design evolves and

develops, and this necessitates constant communication and clarification

between team members.

CIBSE Knowledge Series — How to design a heating system

Figure 2:

The design process

1. Client

need

The design process

3. Design

Design

performance

Feedback/

review

Feedback/

review

Inform

Develop

Select

Implement

2. Design

requirements

4. Design

delivery

CIBSE Knowledge Series — How to design a heating system

2.2

Heating design process

The problem with the standard design process is that it is both complex and

lacking in design task details. Although design is a clear part of the process,

detail of the design tasks involved is not given beyond global statements such

as ‘develop the design and prepare sufficient drawings…’.

Therefore, a simple straightforward design sequence for heating design has

been developed (see Figure 4 over the page) to both clarify the process and

allow detail of specific design tasks to be added. This gives a simplified linear

design sequence, from the pre-design stage through the various analysis,

decision and calculation steps through to the final solution, enabling design

tasks to be clearly linked to both preceding and succeeding actions. Although

some feedback loops are shown, in practice there are often feedback loops

between all tasks and even within specific tasks, reflecting the more iterative

nature of real-life design. Further detail on all of these steps is available in

section 3.

It is important to still set this in the context of the full design process. In practice

there are several design repetitions within the various stages, and overlaps from

one stage to another. For example, information on overall space requirements

and plant structural loadings is often required by other team members at the

outline design stage. This degree of detail is unknown at this early stage

therefore often assumptions and approximations have to be made in order to

provide information. It is vital that these are checked as the design progresses.

RIBA plan of work (1999)

ACE Agreements A(2) &

B(2) (2002)

A Inception/Identification of

client requirements

B Strategic brief

C1 Appraisal stage

C2 Strategic briefing

re-design

C Outline proposals

D Detailed proposals

E Final proposals

C3 Outline proposals stage

C4 Detailed proposals stage

C5 Final proposals stage

Design

F Production information

C6 Production information

stage

G Tender documentation

H Tender action

C7 Tender documentation and

tender action stage

Construction

J Mobilisation/Project

planning

K Construction to practical

completion

L After practical completion

C8 Mobilisation, construction

and completion stage

Figure 3:

Construction process
stages

CIBSE Knowledge Series — How to design a heating system

Step no. Key

design steps

Design tasks

Pre-design

Obtain design brief.

Identify client and building user needs and requirements.

Refer to feedback and lessons learned from previous projects

Gather design information

Gather information about site, including utilities provision and fuel options.

Obtain information on use of building, occupancy hours and on possible building

form, fabric, etc

Establish and confirm key design requirements including Regulations and Codes of

Practice. Establish planning conditions for use of on-site renewables

Design data

Establish the key design data and parameters that relate to the design of the

heating system, including building air tightness data, and potential use of renewables.

Develop

room design data sheets

Check that design parameters comply with legislation, energy targets, etc

Building thermal performance

Analyse building – establish fabric thermal performance and infiltration

analysis

Determine whether intermittent operation is likely and consider potential

pre-heat requirements

Estimate approximate building total heat loss to inform system selection process

Heating system option

Consider zoning requirements. Consider alternative heat source (fuel) and heating

analysis and selection

system options. Establish contribution from renewable sources

Consider operating and control strategies, and building usage and layout data.

Assess

options against client requirements, performance, risk, energy use,

etc

Select proposed system

Design calculations

Calculate space heat losses. Assess ventilation requirements and provision. Assess

Space heat losses and heat load

HWS provision

Check system selection choice still appropriate.

Determine pre-heat requirements

Equipment selection and sizing

Consider suitable emitter positions and connections.

Check

distribution layout considering balancing and regulating requirements.

Consider circuit layouts and connections and pumping choices – variable or constant

volume.

Develop

control requirements

Size and select emitters and distribution network and determine any distribution

losses

Design calculations

Determine other loads such as HWS and process.

Heating load analysis Calculate

main heating loads. Analyse load diversity and pre-heat requirement and

determine the total heating load

Plant sizing and selection

Consider any standby requirement. Determine number of boilers /modules

required and size and select main plant. Finalise control requirements

Check layouts and services co-ordination for clashes and ease of commissioning

and maintenance

Design calculations Review

system design and check predicted system performance.

System analysis Check

part load performance

Control performance

Check that the selected controls are capable of achieving the required level of

control, response and energy efficiency, particularly at part load

Final value engineering and

Check that final system and components meet client requirements for

energy targets assessment

performance, quality, reliability, etc at acceptable cost; and also meet required

energy targets and comply with Regulations, such as meeting the seasonal efficiency

requirements

Review

Design review

Figure 4:

Heating system design
process

CIBSE Knowledge Series — How to design a heating system

As the design develops, these design steps are revisited and further detail

added with more accurate analysis as additional information becomes available.

The steps and amount of repetition involved will differ from design to design

but an example is illustrated in Figure 5. This uses the same design step

numbers as Figure 4 to show how the different steps are repeated and

revisited as the design develops. The detailed design tasks at each step have

been omitted to keep the diagram to a manageable size.

Step no.

7

9

Design stage

Pre-design

Outline design

Scheme / Detail design

Design development/Final
proposals/Production
information

Key outputs

Design brief
Outline drawings
and schematics.
Provisional cost plan

Design drawings and
schematics.
Cost plan

Design drawings and
specification for tender
purposes.
Possibly co-ordination
drawings.
Final cost appraisal

Key design steps

Pre design: obtain client brief.
Refer to feedback and lessons learned from previous projects

Gather design information and establish key design requirements.
Establish planning requirements

Establish key design data

Initial building thermal performance analysis.
Approximate heat loss

Heating system – consider options and fuel choices

Consider system requirements, potential layout, etc

Approximate total loads and plant size to arrive at cost plans, provide
space requirements and structural load information, etc.

Gather further necessary design information and establish key design
requirements

Establish key design data

Detailed building thermal performance analysis

Heating system choice and selection

Design calculations: space heat losses

Equipment selection and sizing – emitters and distribution network.
Control requirements

Design calculations: heating load analysis, possibly including thermal
modelling

Initial plant and control selection

Value engineering workshops

Interim design review

Further building thermal performance analysis, to assist in modelling
dynamic building and system performance (if required)

Final equipment selection and sizing

Final heating load calculation and analysis

Plant selection. Control requirements. Preparation of detailed design
drawings and specifications for plant and equipment

Design calculations.
System performance analysis, including part load performance and
predicted energy use. Possible final dynamic modelling of building and
system performance.
Control performance

Final value engineering exercise

Final design review

Post-occupancy review

Figure 5:

Heating design process
mapped against the main
design work stages

2.3

Key heating design calculation sequence

Within the overall heating design sequence there are some specific

calculations that will need to be carried out, and the sequence of these can

also be illustrated as shown in Figure 6. These mainly take place during steps

4, 6 and 8 — building performance analysis, heat losses and load analysis;

continuing into system and equipment sizing in steps 7 and 9, and system

analysis in step 10.

CIBSE Knowledge Series — How to design a heating system

Infiltration

heat loss

Fabric heat

loss

Site weather

data

Internal and external

design conditions

Space heat

loss

Space heating

load

Pre-heat

margin

Emitter

sizing

Infiltration load

diversity

Distribution system

sizing

Maximum simultaneous

space heating load

Standby capacity

(if required)

Fuel supply

system sizing

Distribution system

losses

Part load

performance

Final system and

control performance

analysis

Load diversity

analysis

Total heating

load

Boiler/heating

plant sizing

Flue

sizing

Natural ventilation

load (if any)

Internal gains (only

if both heating and

gains are continuous)

Building thermal

response analysis

Intermittent

operation

assessment

Building air-

tightness details

Fabric

details

Condensation

risk analysis

U-values

Central fresh air

ventilation heating load

HWS

load

Process

load

Figure 6:

Key steps for heating
design calculation
sequence

CIBSE Knowledge Series — How to design a heating system

2.4

Thermal comfort

For heating design, thermal comfort could be regarded as the main output of

the design process, as shown in Figure 7. Certainly most clients do not ask

for a heating system as part of their design brief — their focus is on what

systems deliver and not how they do it. What clients really require is the

building services design to deliver comfortable working or living conditions to

enable their business to function efficiently. An understanding of thermal

comfort is therefore central to good heating system design.

Although there are many factors to take into account, thermal comfort is

fundamentally about how people interact with their thermal environment.

Generally, a reasonable level of comfort is achieved where there is broad

satisfaction with the thermal environment, i.e. most people are neither too

hot nor too cold.

The four main environmental factors that affect thermal comfort are:

—

air temperature (t

)

—

relative humidity

—

mean radiant temperature (t

)

—

air velocity (v).

All of these are affected by the choice of heating system and the way it

delivers heat to the space.

Building designers should aim to provide comfortable conditions for the

greatest possible number of occupants and to minimise discomfort. This is

achieved by considering comfort requirements and setting appropriate design

criteria.

For the thermal environment, these would usually be the operative

temperature and humidity, together with a fresh air supply rate. A typical

initial winter design condition might therefore be written as 21 °C and 50%

RH for operative temperature and relative humidity respectively, with 10 l/s

per person of fresh air required. More often some variation is allowed, i.e.

21 °C ±1 °C and 50% RH ±10%. Example design criteria for a range of

building types are given in section 3.3.

For a further discussion of comfort, see CIBSE Knowledge Series KS06:

Comfort, and CIBSE Guide A, chapter 1.

Design

process

Client

need

Thermal

comfort

Input

Output

Figure 7:

Design output

Thermal comfort

‘That condition of mind which expresses
satisfaction with the thermal environment
and is assessed by subjective evaluation.’

ASHRAE Standard 55-2004

Key factors in thermal comfort

temperature

humidity

air movement

air quality.

Key design steps

This section covers the key steps in the heating design process given in

sections 2.2 and 2.3 in more detail to give some further guidance. Key design

outputs from each stage are summarised and additional reference sources

provided.

3.1

Step 1: pre-design/design brief

Depending on the type of project, the design brief may evolve during the

course of the initial project stages. However, design briefs do not usually ask

for specific heating systems, they tend to concentrate on the outcomes that

must be achieved, i.e. the internal conditions that must be delivered. The

brief may simply ask for a heated building, with specific comfortable working

conditions. Design of any system must therefore relate to the functional brief,

and be seen in the context of the full design requirements.

During the initial design process the building services engineer can potentially

provide input on ways to optimise building performance and reduce energy

loads, including advice on:

—

building form and orientation to optimise the impact of solar gain

—

building air tightness, to reduce infiltration

—

fabric insulation

—

optimisation of glazing, balancing daylighting needs against thermal

performance

—

building thermal mass.

Much design data and information can be gained from the client brief and

occasionally additional input will be needed from the client to clarify points or

to provide missing data in order to develop the design brief. Some client

briefs will include the necessary initial design data such as internal design

conditions, in some cases this will need to be advised. In both cases it is

sensible to check any data provided against good design practice.

Input to the design brief can include advice on:

—

future need design requirements

—

comfort requirements

—

ventilation strategy

—

spatial requirements

—

standards and regulations

—

energy strategy, including the use of renewable energy sources

—

operating strategy including facilities maintenance requirements

—

plant life expectancy and replacement strategies

CIBSE Knowledge Series — How to design a heating system

—

control strategy.

Information required from the design brief can include:

—

required functional performance

—

occupancy

—

usage details and potential internal loads

—

internal design conditions

—

cost plan.

(Further detail of this is given in step 2.)

3.2

Step 2: gather design information

A large amount of information is necessary to inform the various design

stages, and as such this task is ongoing throughout the design stages. Much of

the information is available from the original client brief or statement of

requirements, and additional information can be sought by additional

questions. Other data must be gathered from other sources such as site

visits, etc. Some key initial information is given in Figure 8.

Key design outputs for step 1:

pre-design

functional design brief.

Key design outputs for step 2:

information gathering

key design requirements

necessary information to establish
internal and external design data

site assessment and utility
provision

statutory and regulatory design
requirements and targets.

Site
information

Client
brief

Standards and
regulations

Location: Geographical location and
height above sea level
Local microclimate, wind
Information on local conditions – pollution,
noise

Specific information required

Outputs

Operating strategy: Client approach to
building design and operation including
sustainability, energy strategy, control,
maintenance, etc

Orientation: Details of surrounding
buildings, shading, etc

Services: Utilities provision and positions

Functional performance: Specific
deliverables

Costs: Cost plans and budgets

Future needs: Future proofing and
flexibility requirements

Building use: Tasks, office equipment, etc

Occupancy: Information on occupancy
activity and density
Hours of occupation, etc

Access: Access to site

External design
conditions

Available
services

Cost budgets
and constraints

Internal design
conditions
Assessment of
intermittent
system operation
Internal loads –
small power,
lighting, etc

Possible comfort
or energy
requirements

Additional system
requirements

Possible system
constraints or
requirements

Statutory and regulatory
requirements

Design requirements
Energy targets,
including % energy
to be provided from
renewable sources

Figure 8:

Information gathering

The building services engineer will also need to provide information to other

design team members throughout the project. As outlined in section 3.1, at

the initial design stages this can include advice on optimising building

performance, and can also include information on potential spatial

requirements, which can be refined as the design develops.

The new Building Regulations Part L (2006) requires that both fabric and

services heat losses are limited and that energy efficient services with

effective controls are provided. Details are provided in the second tier

documents such as the Non-domestic heating, cooling and ventilation
compliance guide and the Domestic heating compliance guide.

3.3

Step 3: design data

The fundamental initial design data needed for design of a heating system to

deliver comfortable conditions are the:

—

internal design conditions

—

external design conditions.

The design conditions selected can have a substantial impact on both system

loads and subsequent system performance and therefore care must be taken

to select appropriate values. See section 4.1 for further discussion.

Internal design criteria may be specified in the brief, or a required functional

performance may be asked for and the designer will have to specify the

required conditions. In either case these will need to be checked against good

practice design standards.

Table 2 gives example winter internal design conditions for thermal comfort

for a range of common building types. More detailed guidance for a wider

range of building and room types is given in CIBSE Guide A, Table 1.5, which

also relates the design guidance to the expected clothing and metabolic rates

of occupants to achieve a predicted percentage persons dissatisfied (PPD) of

around 5%. For design purposes reference should be made to the full table

together with the associated footnotes.

CIBSE Knowledge Series — How to design a heating system

Building Regulations Part L 2006

Heating systems should be designed to
minimise carbon emissions and make it
easier for the whole building to achieve a
building CO

emission rate (BER) lower

than the set target (TER) and thus
comply with Part L requirements, which
implement the EPBD directive.

Key design outputs for step 3:

design data

internal thermal comfort design
conditions

schedule of internal design
criteria for each space (e.g. on
room data sheets)

external design conditions.

CIBSE Knowledge Series — How to design a heating system

Selection of appropriate external design criteria requires information on the

site location, development details and local microclimate, as outlined in

section 3.2, as well as meteorological data. The type of building and the

thermal inertia will also help to determine what may be an acceptable risk of

exceedence of conditions, and this will need to be discussed and agreed with

the client. Further guidance is provided in CIBSE Guide A, chapter 2 and in

CIBSE Guide J (2002).

3.4

Step 4: building thermal performance analysis

The thermal performance of the building will need to be established to

enable the calculation of heat losses, assess preheat requirements and

calculate the heating loads. Some key information is given in Figure 9.

Building/room type

Winter operative temp

range °C

Suggested air supply rate

l/s per person

(unless stated otherwise)

Dwellings

bathrooms

20–22

15 l/s

bedrooms

17–19

0.4–1 ACH

halls, stairs

19–24

–

kitchen

17–19

60 l/s

living rooms

22–23

0.4-1 ACH

Offices

conference/board rooms

22–23

computer rooms

19–21

corridors

19–21

drawing office

19–21

entrance halls/lobbies

19–21

general office space

21–23

open plan

21–23

toilets

19–21

>5 ACH

Retail

department stores

19–21

small shops

19–21

supermarkets

19–21

shopping malls

12–19

Schools

teaching spaces

19–21

Notes:

1. ACH stands for air changes per hour.

2. For design purposes, please refer to the full Table 1.5 in CIBSE Guide A.

Table 2:

Recommended winter thermal
comfort criteria for some
selected building types

External design conditions

Appropriate design criteria should be
agreed with the client, taking into
consideration the acceptable risk of
exceedence of design conditions.

(Source: CIBSE Guide A, Table 1.5)

CIBSE Knowledge Series — How to design a heating system

Calculation procedures and data required to establish the fabric thermal

properties, including the transmittance details, i.e. the fabric and glazing U-
values, are given in CIBSE Guide A, chapter 3, together with U-values for
standard constructions. This information, together with the design conditions

from step 3 (section 3.3), and site data from step 2 (section 3.2), will also

enable the analysis of condensation risk, if this is part of the agreed design

duties. Key steps in the calculation sequence related to this and the building

thermal response are shown in Figure 10 in dark blue.

Building air-

tightness details

Site weather

data

Infiltration

heat loss

Internal and external

design conditions

Condensation

risk analysis

Fabric heat

loss

Space heat

loss

Fabric

details

U-values

Building thermal

response analysis

Figure 10:

Key steps to analyse building
thermal performance

Building
information

Fabric
information

Building plan and form: Details of
building plan and form
Building orientation and shading
Building layout
Glazing locations, etc

Specific information required

Outputs

Internal layout: Layout drawings
Potential space use and fit out
Partitioning

Fabric: Detail of building materials
and construction
Fabric thermal performance

Plant and distribution space:
Potential location and space required/
available (should be discussed and agreed
with rest of design team as early as
possible in the design)

Glazing: Glazing information – type,
dimensions, including glazing height,
and thermal performance

Air tightness: Construction quality
Building air tightness prediction

Room dimensions
Constraints on
emitter
positioning
Constraints on
distribution space

Possible system
constraints or
requirements

Thermal mass
assessment
(heavy or
light weight)
Fabric and glazing
U-values
Fabric admittance
Y-values
Window leakage
rates
Infiltration data

Layout
information to
inform services
location, zoning
strategy, etc

Figure 9:

Building form and fabric

Glazing height

Glazing height influences comfort within
the occupied space both due to
downdraughts and to cold radiation
which affects the mean radiant
temperature.

CIBSE Knowledge Series — How to design a heating system

As the thermal insulation performance of the building fabric has improved,

the infiltration component of heat loss can now comprise a substantial

percentage and therefore needs to be estimated as accurately as possible.

Although building air leakage testing will be required for most buildings, and

will form part of the design requirements, this sets an expected standard,

generally specified for a specific applied pressure difference such as 50 Pa,

and therefore does not provide data for infiltration calculations. Methods for

estimating infiltration rates are given in CIBSE Guide A, chapter 4, with

additional guidance in CIBSE AM10.

An initial assessment of building use and hours of occupancy will determine if

intermittent, rather than continuous, operation is likely. Details of the overall

building thermal response will be needed to determine the likely preheat

requirements and the impact on heating system performance (see also

section 3.6). More detailed modelling of the building and system dynamic

performance can then be carried out at a later design stage if required.

An initial estimate of total building heat loss can be useful at this stage to help

inform system choices, just to give an approximate global figure. The system

choices that are reasonable for a 50 kW loss can be very different from those

for a heat loss of 1500 kW, for example.

3.5

Step 5: heating system option analysis and selection

Heating system choice depends on many factors. These can be loosely

grouped into two areas relating to practical system installation and to

performance and use factors.

Installation factors include:

—

space required/available: both for plant and for distribution

—

potential plant room locations related to the spaces to be served

—

cost plan: capital cost of installation

—

zoning requirements

—

flexibility: any requirements for future change of use or changes in

fitout

—

ease of installation: access, materials, etc

—

ease of commissioning.

Performance and use factors include:

—

cost

—

comfort

—

control

—

convenience.

Key design outputs for step 4:
building thermal performance

fabric thermal transmittance
details, i.e. the fabric and glazing
U-values

building thermal response (and
dynamic thermal performance
characteristics including
admittance values, if required)

infiltration assessment for
individual spaces and for the
whole building

assessment of intermittent
operation to inform preheat
requirements

estimation of approximate total
building heat loss.

Infiltration estimation

A useful cross check for infiltration
estimation is to convert the estimated
infiltration total to a room or whole
building air change rate, as appropriate.

Zoning

Zoning strategy needs to be agreed with
the client. Some variation in internal
conditions may be acceptable, which can
help to minimise the number of zones
and improve operating efficiency.

To determine the most appropriate system to meet the client’s requirements,

an assessment of options against some of these factors can be helpful. System

choices can be compared using, for example, a ranking and weighting matrix

to assess suitability using some of the key usage factors related to system

choice. Information on the client’s operating and control strategy will also

inform the decision process.

Key design decisions will include the choice of:

—

heat media and distribution system

—

system: centralised or de-centralised

—

heat emitter

—

heat source.

Tables 5, 6 and 7 provide further information on some system options, giving

some characteristics and relative advantages and disadvantages together with

CIBSE Knowledge Series — How to design a heating system

Cost

operating and maintenance costs

energy efficiency

carbon emissions and energy usage.

Comfort

balance of radiant and convective heat output to provide
comfort conditions

time taken to achieve comfort conditions from start up

evenness of heat distribution throughout space

noise level.

Control

ability to provide accurate control of space temperature

ability to provide localised control

speed of response to changing conditions.

Convenience

ease of use

location

potential lettable/usable space taken up by emitters/outlets
and distribution

ease of maintenance.

Table 3:

System performance and use
factors

Table 4:

Heating system design choices

Heat media

the balance between radiant and convective output
required from the system

space required for distribution

speed of response to changing conditions, and on start up.

System

centralised or de-centralised – potential plant locations.

Heat emitter

characteristics including the balance between radiant and
convective output

location to provide uniform temperatures

noise level

space required.

Heat source

conventional boilers or other heat sources such as heat
pumps, CHP, etc

boiler and fuel type, any storage requirements

central plant location.

Key design outputs for step 5:

heating system selection

zoning strategy for building — to
give details of building zones and
required operating conditions —
hours of use and internal design
conditions

selection of heating system(s) in
principle — fuel/heat source,
system, distribution medium and
emitter types.

Low and zero carbon technologies

Part L (2006) of the Building Regulations
encourages the use of low and zero
carbon (LZC) technologies, such as
renewables, CHP and heat pumps, as a
way of meeting the required carbon
emission reductions, and implementing
the requirements of the EPBD directive.
Many local planning authorities also
encourage the use of these technologies,
in some cases making it a specific
planning requirement.

some selection flow charts for heating systems and fuels. Although not

included on the selection charts in Figures 11 and 12, note that, in addition to

CHP, other low and zero carbon technologies such as renewables should also

be considered as heat source options. Further information on heat emitters

and heating systems is given in CIBSE Guide B, chapter 1, with guidance on

renewable energy sources covered in CIBSE TM38: Renewable energy sources
for buildings (2006).

CIBSE Knowledge Series — How to design a heating system

Constraints on combustion appliances in workplace?

Considering CHP, waste fuel or local community
heating system available as source of heat?

Most areas have similar heating requirements
in terms of times and temperatures?

Significant spot heating
(>50% of heated space)?

Above average ventilation rates?

N Y

Non-sedentary workforce?

Radiant heat acceptable
to process?

Note: This selection chart is intended to give initial guidance only;

it is not intended to replace more rigorous option appraisal

Low temperature

radiant system

Medium or high temperature

radiant system

Convective

system

Convective

system

Centralised system

Start here

Decentralised system

Waste fuel or local community heating
available as source of heat?

Strategic need for back-up
fuel supply?

Natural gas required?

Radiant heat required?

Natural gas +

oil back-up

Community

or waste heat

Community

or waste with

oil or LPG

back-up

Community

or waste
with gas

back-up

Oil + LPG

electricity

back-up

Electricity for

high temperature

systems, LPG

for medium

temperature systems

Natural

gas

N Y

Oil or

LPG

N Y

Centralised system

Decentralised system

Figure 11:

Selection chart: heating systems

Source: CIBSE Guide B, chapter 1, Figure 1.2,
itself based on the Carbon Trust Good Practice
Guide 303

(7)

Figure 12:

Selection chart: fuel

Source: CIBSE Guide B, chapter 1, Figure 1.3,
itself based on the Carbon Trust Good Practice
Guide 303

(7)

CIBSE Knowledge Series — How to design a heating system

Medium

Principal characteristics

Advantages

Disadvantages

Air

Low specific heat capacity, low
density and small temperature
difference permissible between
supply and return, compared to
water, therefore larger volume
needed to transfer given heat
quantity

No heat emitters needed

No intermediate medium or
heat exchanger is needed

Large volume of air required —
large ducts require more
distribution space

Fans can require high energy
consumption

Water

High specific heat capacity, high
density and large temperature
difference permissible between
supply and return, compared to
air, therefore smaller volume
needed to transfer given heat
quantity. Usually classified
according to water temperature/
pressure:

Small volume of water required
— pipes require little
distribution space

Requires heat emitters to
transfer heat to occupied space

— LTHW (LPHW)

Low temperature/pressure hot
water systems operate at
temperatures of less than 90 °C
(approx.), and at low pressures
that can be generated by an
open or sealed expansion vessel

Generally recognised as simple
to install and safe in operation.

Use with condensing boilers to
maximise energy efficiency

Output is limited by system
temperatures

— MTHW (MPHW)

Medium temperature/pressure
hot water systems operate at
between 90–120 °C (approx.),
with a greater drop in water
temperature around the system.
This category includes
pressurisation up to 5 bar
absolute

Higher temperatures and
temperature drops give smaller
pipework, which may be an
advantage on larger systems

Pressurisation necessitates
additional plant and controls, and
additional safety requirements

— HTHW (HPHW)

High temperature/pressure hot
water systems operate at over
120 °C, often with higher
temperatures — perhaps up to
200 °C, with even greater
temperature drops in the
system. These temperatures will
require pressurisation up to
around 10 bar absolute

Higher temperatures and
temperature drops give even
smaller pipework

Safety requires that all pipework
must be welded, and to the
standards applicable to steam
pipework. This is unlikely to be a
cost-effective choice except for
the transportation of heat over
long distances

Steam

Exploits the latent heat of
condensation to provide very
high transfer capacity. Operates
at high pressures. Principally
used in hospitals and buildings
with large kitchens or processes
requiring steam

High maintenance and water
treatment requirements

Table 5:

Heat distribution media

CIBSE Knowledge Series — How to design a heating system

Table 6:

Centralised versus non-
centralised systems

Centralised

Non-centralised

Capital cost

Capital cost per unit output falls with increased
capacity of central plant.

Capital cost of distribution systems is high

Low overall capital cost, savings made on minimising
the use of air and water distribution systems

Space requirements

Space requirements of central plant and distribution
systems are significant, particularly ductwork

Large, high flues needed

Smaller or balanced flues can often be used

Flueing arrangements can be more difficult in some
locations

System efficiency

Central plant tends to be better engineered, operating
at higher system efficiencies (where load factors are
high) and more durable

As the load factor falls, the total efficiency falls as
distribution losses become more significant

Energy performance in buildings with diverse patterns
of use is usually better

System operation

Convenient for some institutions to have centralised
plant

Distribution losses can be significant

May require more control systems

Zoning of the systems can be matched more easily to
occupancy patterns

System maintenance
and operational life

Central plant tends to be better engineered, more
durable

Less resilience if no standby plant provided

Can be readily altered and extended

Equipment tends to be less robust with shorter
operational life

Plant failure only affects the area served

Maintenance less specialised

Fuel choice

Flexibility in the choice of fuel, boilers can be dual fuel

Better utilisation of CHP, etc

Some systems will naturally require central plant, e.g.
heavy oil and coal burning plant

Fuel needs to be supplied throughout the site

Boilers are single fuel

Based on data from CIBSE Guide F (2004), chapter 10.

3.6

Step 6: design calculations: space heat losses and heat load

The next step in the design sequence is to take the information on the

building fabric and infiltration performance from step 4 and use this to

establish both infiltration and fabric heat losses for each space to give an

individual heat loss for each building space that will require heating.

Information on the type of heating system and emitter selected is also

required, as both manual calculations and the majority of software packages

will require information on the relative radiant and convective outputs as part

of the input data.

CIBSE Guide A, chapter 5 provides details of the required calculation

procedures for heat losses, covering both a steady state heat loss approach

and a dynamic approach which can provide more detailed analysis if required,

including modelling of building and system thermal response. Section 5.6.2 of

CIBSE Guide A provides a worked example for the steady state heat loss

calculation.

Key steps in the calculation sequence for space heat loss are shown in Figure

13 in dark blue.

CIBSE Knowledge Series — How to design a heating system

Table 7:

Common emitter/system types

Design points

Advantages

Disadvantages

Radiators

Output up to 70% convective

Check for limit on surface
temperature in some applications,
e.g. hospitals

Good temperature control

Balance of radiant and convective
output gives good thermal comfort

Low maintenance

Cheap to install

Fairly slow response to control

Slow thermal response

Natural convectors

Quicker response to control

Skirting or floor trench convectors
can be unobtrusive

Can occupy more floor space

Can get higher temperature
stratification in space

Underfloor heating

Check required output can be
achieved with acceptable floor
surface temperatures

Unobtrusive

Good space temperature
distribution with little stratification

Heat output limited

Slow response to control

Fan convectors

Can also be used to deliver
ventilation air

Quick thermal response

Can be noisy

Higher maintenance

Occupies more floor space

Warm air heaters

Can be direct fired units

Quick thermal response

Can be noisy

Can get considerable temperature
stratification in space

Low temperature radiant
panels

Ceiling panels need relatively low
temperatures to avoid discomfort

Unobtrusive

Low maintenance

Slow response to control

High temperature radiant
heaters

Can be direct gas or oil fired units

Check that irradiance levels are
acceptable for comfort

Quicker thermal response

Can be used in spaces with high
air change rates and high ceilings

Need to be mounted at high level
to avoid local high intensity
radiation and discomfort

Heat losses

A useful cross check for heat losses is to
convert the calculated values to W/m

W/m

figures to check against reasonable

benchmarks.

CIBSE Knowledge Series — How to design a heating system

With better fabric insulation the infiltration heat loss can now account for up

to 50% of the total heat loss in some smaller buildings and therefore

infiltration rates need to be estimated as accurately as possible — see section

3.4.

To move from the heat loss to the heat load for a space, additional factors

need to be considered, including any additional loads within the space and

any preheat requirements, as shown in Figure 14.

An assessment of ventilation provision is required at this stage, as although

this is likely to be met by a separate system in most buildings, it will in some

cases be met by natural ventilation, in which case it will add an additional heat

load directly to the space. Further information on naturally ventilated

buildings is given in CIBSE AM10 and on mixed mode buildings in CIBSE

AM13.

A preliminary assessment of other loads that may also need to be met by the

main heating source, such as any HWS load, can also be made at this stage to

provide information for the next calculation step (see also section 3.8).

Heat losses — temperatures

Care needs to be taken when considering
the temperatures to use for heat loss
calculations. Design criteria are usually
given as operative temperatures (t

Fabric heat losses should use the internal
environmental temperature (t

) and

infiltration loss the internal air
temperature (t

). These can differ

substantially for some buildings and some
heating types. CIBSE provides a method
for steady state heat losses that applies
correction factors F

and F

to enable the

design internal operative temperature to
be used — see CIBSE Guide A, section
5.6.2. (Note: for very well insulated
buildings, without large areas of glass, and
with low air change rates, there is often
little difference between operative,
environmental and air temperatures.)

Building air-

tightness details

Site weather

data

Infiltration

heat loss

Internal and external

design conditions

U-values

Condensation

risk analysis

Fabric heat

loss

Space heat

loss

Fabric

details

Figure 13:

Key steps to establish individual
space heat losses

Infiltration

heat loss

Fabric

heat loss

Natural ventilation

load (if any)

Internal gains (only

if both heating and

gains are continuous)

Space

heat loss

Space

heating load

Pre-heat

margin

Intermittent

operation

assessment

Building thermal

response

analysis

Figure 14:

Key steps to establish space
heating loads

Radiant systems

For high temperature radiant systems the
standard heat loss calculation methods
are not appropriate for equipment
selection. Instead the distribution of
radiant energy in the space should be
determined, utilising a radiant polar
diagram for the emitter. Further guidance
is given in CIBSE Guide A, section
5.10.3.7 and CIBSE Guide B, section
1.4.6. Medium and low temperature
radiant systems can be sized using the
usual heat loss calculation methods.

Internal gains

Normally no allowance would be made for internal gains in establishing space

heating loads as a worst-case scenario is always considered, i.e. to bring the

unoccupied building up to temperature. However, exceptionally, if the heating

will be operating continuously and there are constant heat sources such as

electric lights and occupants in a continuously occupied building, then the

steady state heat requirement can be reduced by the amount of the constant

gains. However the risks of this should always be made overt to the client as

if any gains are removed or reduced or the building is operated intermittently

then the system may not be able to achieve the design temperatures.

Preheat requirements

The building thermal capacity will affect the way the building responds to

heat input, meaning the rate at which it warms up and cools down. For any

building that is heated intermittently this will need to be considered as the

building will cool down during the unoccupied periods and then need to be

brought back to temperature. For heavyweight buildings with a high thermal

capacity, and/or those intermittently occupied, some additional heating

capacity will be required to ensure that the building can warm up and achieve

the design temperature before the start of the occupied period: the preheat

time (see Figure 15). This additional capacity is required by the space heating

system, i.e. the emitters, as well as by the main heating plant.

In order to assess the preheat requirements, information on both

intermittent operation and on the building thermal response is needed. For

normal intermittent operation the plant and equipment will need to be larger

than the steady state requirements, with the required capacity calculated by

applying an ‘intermittency factor’, F

, based on the thermal response factor

for the building and the total hours of plant operation:

CIBSE Knowledge Series — How to design a heating system

HWS

HWS requirements and options should
be assessed, e.g. consider whether
storage or instantaneous water heating is
more appropriate. For hot water storage
consider the options of a dedicated boiler
or a standalone hot water generator
(direct-fired storage system). For
instantaneous hot water consider the
choice and availability of fuel and whether
point-of-use provision or multi-outlet is
more suitable.

Inside tem

erature

Plant off

Preheat

time

Optimised

start time

Start of

occupancy

Design inside
temperature

Time

Intermittent operation

Intermittent heating occurs when the
heating plant is switched off at or near
the end of a period of occupancy and
then turned back on at full capacity prior
to the next period of occupancy in order
to bring the building back to the design
temperature. There are two main types
of intermittent operation:

normal intermittent operation is
where the output is reduced
when the building is unoccupied
— for example to a level of
10 °C to protect the building
fabric and contents

highly intermittent is where the
building is occupied for short
periods only and therefore needs
to be brought back to
temperature quickly prior to use.

Figure 15:

Preheat

CIBSE Knowledge Series — How to design a heating system

Peak heating load = F

x space heat load

Details are given in CIBSE Guide A, section 5.10.3.3 and Appendix 5.A8, and

in CIBSE Guide B, section 1.4.7.3.

If the calculated value of F

is less than 1.2, CIBSE suggests that the value be

taken as 1.2 to ensure that a reasonable margin of 20% for preheat is

applied, although other values may be used, for example by using a dynamic

simulation model to more accurately assess the required excess capacity. Full

analysis of building thermal response can require dynamic rather than steady

state modelling and this is discussed further in CIBSE Guide A, chapter 5.

CIBSE suggests in Guide B, chapter 1 that acceptable values for F

lie in the range

1.2–2.0, with research

(8)

indicating that values over 2.0 cannot be economically

justified for most buildings and could result in considerably oversized plant. The

same research found that a value of 1.5 was a more typical economical value for

the cases investigated. For small buildings and small plants the optimum values

will be even lower. The use of optimum start control, as illustrated in Figure 15,

can help to ensure adequate preheat time in cold weather.

For highly intermittent systems, a steady state heat loss is inappropriate to

size the system and a dynamic simulation model that considers the way heat

is absorbed by the building fabric is required. Details are given in CIBSE

Guide A, section 5.10.3.3.

3.7

Step 7: equipment sizing and selection

Once the individual room losses and space heating loads have been

determined and decisions have been made on the system, emitters, etc, then

the system can be sized and emitters selected. Key steps for this are shown

in Figure 16 below. It is possible that alternative solutions are still being

investigated at this stage, in which case further comparison in terms of cost,

performance and energy efficiency may be required to reach a final decision.

Plant size ratio

The intermittency factor F

can also be

expressed as a plant size ratio (PSR)
defined as:

PSR =

installed heat emission

design peak steady state

heat load

Key design outputs for step 6: space

heat losses and heat load

schedule of individual space and
zone heat losses, subdivided into
fabric and infiltration losses,
together with details of the
internal design conditions

assessment of preheat
requirements for the building

schedule of space heating loads.

Emitter

sizing

Space heating

load

Infiltration

load diversity

Preheat

margin

Distribution

system sizing

Maximum

simultaneous

space heating load

Distribution

system losses

Figure 16:

Key steps for emitter and
distribution system sizing

Heat transfer correction factors

The type of heat emitter can have a
significant effect on the calculated design
steady state heating load, so it is essential
that appropriate values for the heat
transfer correction factors F

and F

were

used at step 6.

The heat output from the emitter, and therefore the size required, will be

affected by its position within the space and local effects such as furniture

positions, etc. For example if emitters are situated behind furnishings then

most of the immediate radiant heat output will be lost, and in some cases

even the convective heat output can be obstructed and reduced. Although

much of the heat will eventually enter the space it may not be available

during preheat and therefore an allowance may be need to be made and the

required heat output increased to compensate. Details are given in CIBSE

Guide A, section 5.10.3.2.

Some heating systems, such as warm air, can lead to considerable

temperature stratification in the space — see Figure 17. This means that the

inside temperature at high level is much higher than that used in heat loss

calculations and therefore the heat loss through the ceiling/roof will be

greater than anticipated. A correction to the heat loss, to allow for the height

of space and system used, will need to be applied — for example a 5–15%

increase in the fabric component of heat loss for a low level forced warm air

system used in a space 5–10 m high. Further guidance is given in CIBSE

Guide A, section 5.10.3.2 and in Table 5.15.

These corrections can now mean that, for certain heating systems, the

required emitter load is larger than the original space heating load. Once the

emitters have been sized then the distribution layout can be determined and

the system sized. Guidance on pipe and duct sizing is given in CIBSE Guide C

(2001), chapter 4. When determining the most appropriate layout for the

distribution system, balancing and regulating requirements should be

considered, e.g. the use of reverse return pipework layouts to aid system

balancing during commissioning.

The system distribution losses will need to be assessed. Those from within

the space can contribute to the required space heating load. However any

non-useful distribution losses will need to be allowed for within the overall

CIBSE Knowledge Series — How to design a heating system

15 20 25

15 20

Radiator

Underfloor heating

Warm air heater at high level

3·0

2·0

1·0

Room height / m

Air temperatures / °C

Heat emitters

Check that the manufacturer’s published
data is applicable to the conditions at
which the emitter will be operating and
apply any relevant corrections for space
temperature, water temperatures, etc.
Note that manufacturers’ outputs are
based on particular space and water
temperatures which may differ from the
design operating conditions.

Key design outputs for step 7:

emitter and distribution system

sizing

schedule of emitters with
required output, and with surface
and water temperature for
hydronic systems

initial control requirements

layout drawings with emitter
positions

schematic of pipework layouts
with required flowrates for
hydronic systems.

Figure 17:

Vertical air temperature
gradients for different heating
types

Source: CIBSE Guide A, Figure 5.6.

CIBSE Knowledge Series — How to design a heating system

heating load for the building. Whilst for energy efficiency distribution losses

should be minimised, for example by insulating pipes that run through non-

occupied areas, an allowance will still need to be made. Guidance is given in

CIBSE Guide C, chapter 3.

3.8

Step 8: design calculations — heating load analysis

Once individual space heating loads have been determined, and the emitters

and distribution system sized, an overall heating load can be determined. This

will require establishing all the various heat loads that may need to be met,

such as:

—

space heating loads

—

any system distribution losses

—

HWS load

—

central fresh air ventilation heating load (if ventilation air is provided

centrally by mechanical ventilation systems)

—

any potential process load.

The first step is to establish the maximum simultaneous space heating load —

see Figure 18. Having already considered the preheat requirements for the

space(s), and sized the emitters, an allowance needs to be made for any non-

useful distribution losses, as discussed in step 7.

Infiltration load diversity

For individual spaces the maximum heat loss is always required to size any

emitters for that space. However when considering the total space heating

load for sizing central plant, some diversity can be applied to infiltration, to

allow for the fact that infiltration of outdoor air will only take place on the

windward side of the building at any one time, with the flow on the leeward

side being outwards. This suggests that the total net infiltration load is usually

about half of the summation total for the individual spaces, although the

infiltration patterns for individual building configurations should always be

considered carefully. This exercise is important as, given current high levels of

Emitter

sizing

Space heating

load

Infiltration

load diversity

Preheat

margin

Distribution

system sizing

Maximum

simultaneous

space heating load

Distribution

system losses

Figure 18:

Key steps to establish the
maximum simultaneous space
heating load

fabric insulation, the infiltration component of heat loss is now substantial,

often accounting for up to 50% of the total in small buildings. CIBSE Guide A,

chapter 4 provides further guidance on infiltration.

The next step is to consider the other loads that may need to be met by the

heating plant and carry out an assessment and analysis of load diversity — see

Figure 19.

Load diversity analysis

An analysis of load diversity is needed as the maximum demands for each

separate part of the overall load are unlikely to coincide. In addition to

the infiltration diversity within the total space heating load, there can be

zone diversities, perhaps due to differing hours of occupancy. Process

loads could be intermittent and the HWS load could perhaps peak at the

middle or towards the end of the occupied period, rather than the

beginning.

The individual and zone space heating loads should be reviewed to check

when the peak demand occurs. While it is most likely that the worst case

scenario will be for all spaces to require heating at the same time it is

possible in certain buildings that there could be spaces or zones which only

have very occasional use and do not coincide with the main demand times

from other areas.

For intermittent heating, the period of maximum demand for the heating

systems will be during the preheat period. In practice the preheat periods

for all spaces and zones will generally be co-incident and therefore the

maximum space heating load will be the sum of these, after considering

infiltration diversity as discussed above.

For continuous heating some diversity can be expected between the various

zone heating loads. This is discussed in CIBSE Guide A, section 5.10.3.5, with

CIBSE Knowledge Series — How to design a heating system

Load diversity

analysis

Central fresh air

ventilation

heating load

Process

load

HWS

load

Preheat

margin

Part load

performance

Maximum

simultaneous

space heating load

Total heating

load

Figure 19:

Key steps to establish the total
heating load

Key design outputs for step 8:

heating load analysis

assessment and analysis of load
diversity

total heating load to enable boiler
or other heating plant to be
sized.

CIBSE Knowledge Series — How to design a heating system

Table 5.18 suggesting that central plant diversity factors ranging from 0.7–1.0

may be appropriate depending on building type and system control.

3.9

Step 9: plant sizing and selection

Once the overall heating load has been determined, then the heating plant

can be sized and selected, see Figure 20, together with other plant items

such as the flue and fuel supply system if required.

Standby capacity

Occasionally standby capacity may be required so that, in the event of

partial system failure or plant maintenance, the main loads can still be met

and the building continues to function. The decision on this can require risk

assessment. However this can add still more additional capacity to the

system increasing the overall risk of oversizing and poor performance,

therefore this should be considered together with the load diversity analysis

as there may already be sufficient capacity within the system. Where

further capacity is required careful consideration is needed of the load

breakdown to ensure that the various expected load combinations can be

met efficiently, for example considering the optimum module size for

modular boiler installations. If the heating plant consists of modular boilers

then adding one extra module may be sufficient to both meet the

requirement and still ensure system operating efficiency.

Control requirements should be finalised, considering the required system

operation. With the main system design layouts completed, the final layouts

and services co-ordination should be checked again for any clashes and for

ease of commissioning and maintenance.

3.10

Step 10: system analysis and control performance

With the system selected and plant and equipment sized and plant selected, it

is now possible to more accurately predict system performance and check

energy performance targets are still met.

Figure 20:

Key steps for boiler/heating plant
sizing and selection

Part load

performance

Total heating

load

Boiler/heating

plant sizing

Standby capacity

(if required)

Fuel supply

system sizing

Final system and

control performance

analysis

Flue

sizing

Key design outputs for step 9:

plant sizing and selection

schedule of plant, giving required
output, flowrates, etc

control requirements

schematic of plant layout,
connections, etc.

Control system

Both the heating system and its control
system should be appropriate for the
requirements of the building and the
operation it supports. Ideally the
approach should always be to use the
simplest control system that meets
building owner, operator and user needs
and capabilities, and efficiently delivers
the required quality of system operation.

Predicted system performance, including part load performance, should be

investigated to check that the selected systems can operate efficiently under

all predicted load conditions, see Figure 21. This is particularly important if

additional capacity has been added, for example for preheat or standby, as

this effectively adds a margin. It is important to check that this does not

unduly oversize the system, leading to poor performance at normal operating

conditions. It is also essential to check whether other margins have been

added at any stage in the design process, including those that will occur by

selecting standard plant sizes.

System control performance

In order to achieve an energy efficient building that delivers the required level

of functionality and occupant comfort it is essential to form a clear and

integrated control strategy at a very early design stage. In all cases the control

strategy should be set out first so that the control options can be evaluated

against the required level of functionality. As such, the controls should be

considered at an early stage as an integral part of the system design.

At this design stage the task is to carry out a final evaluation of the controls,

now that the final system design is complete and part-load performance

evaluated, to ensure that they can deliver the required level of control,

response and energy efficiency.

Controls are discussed further in CIBSE KS04: Understanding controls, which
also explains terms such as weather compensation, optimum start controls,

etc; with further information on heating system controls given in CIBSE

Guide B, chapter 1, CIBSE Guide F, CIBSE Guide H (2000), and in other texts

such as Heating systems — plant and control (2003).

CIBSE Knowledge Series — How to design a heating system

Normal system operation

The initial system design is often based
on design conditions that occur for less
than 1% of the occupied time. For the
majority of the heating season occupied
period the system will be operating on a
fraction of the installed load and
therefore it is essential to ensure that the
system can operate efficiently at these
low load conditions.

Part load

performance

Total heating

load

Boiler/heating

plant sizing

Flue

sizing

Maximum

simultaneous

space heating load

Final system and

control performance

analysis

Load diversity

analysis

Figure 21:

System analysis

Key design outputs for step 10:

system analysis

analysis of system part-load
performance

system control strategy statement
and flowcharts

schematics of plant and systems

required control system
functionality

control system specification.

CIBSE Knowledge Series — How to design a heating system

3.11

Step 11: final value engineering and energy targets assessment

Final value engineering assessment

Value engineering should be carried out at several stages within the project to

ensure that the design is on track to meet the client requirements for

performance, quality, reliability, etc at least cost. For example, value

engineering workshops can be held during both the scheme and detail design

stages to ensure that the design decisions made are the ones that achieve

best value.

Energy targets

The final system performance will need to be checked again to ensure it

complies with regulations and meets required energy targets, for example

meeting the seasonal efficiency requirements and achieving a building

emission rate (BER) less than the target emission rate (TER).

3.12

Step 12: design review

There are a number of different interim reviews that can be done throughout

the design stages of a project, from the feasibility and innovation review to

straightforward progress reviews, culminating in a post-project review after

project completion which can provide valuable feedback lessons to inform

future work.

During the design stages there should be review meetings of the design team

at regular intervals to review design progress, agree changes, check

compliance with the brief, etc. The intent of these is to monitor the progress

of the design against the programme and cost targets, anticipate potential

problems, and ensure that required information will be available when

needed. Review meetings can involve one or several design disciplines.

Some design practices hold a formal peer group in-house design review near

the end of the design stages, presenting to other design teams, perhaps from

other regional offices. This can be a useful part of the project quality checks,

and provide additional valuable cross-checks on the proposed design

solutions, as well as sharing experience and expertise within the organisation.

Post-project review is usually held by the in-house design team at the end of

the project, after completion and handover, to review the inputs and

outcomes and provide the opportunity to summarise key points learnt. This

can provide the opportunity to review both the technical content of the

design and the management of the design process to provide feedback to

inform future work, including the provision of design benchmark data for

Value engineering

A systematic approach to achieving the
required project functions at least cost
without detriment to quality,
performance and reliability.

Key design outputs for step 11:

value engineering and energy targets

value engineering review

energy target and emission value
calculations.

Safety in design

Reviews should include consideration of
safety in design to ensure that the
provision of the design can be
constructed, operated, maintained and
de-commissioned safely, to comply with
the Construction (Design and
Mangement) Regulations (CDM)
requirements. Helpful guidance on
designers’ responsibilities under CDM is
given on the HSE website:
www.hse.gov.uk/construction/designers/
index.htm.

future projects. A post-project review meeting can also be held with the

entire project team.

Sometimes there is the opportunity to obtain further feedback after handover

and occupation, e.g. via post-occupancy surveys. The client may also require

additional duties to include monitoring system operation. For example, the

energy performance of the system can be monitored using the CIBSE

logbook approach, and the actual operation of the system and comfort

performance monitored for compliance with the intended design operation.

This can provide valuable feedback to inform briefing and design guidance for

future projects. Further guidance on feedback can be found in BSRIA AG

21/98: Feedback for better building services design.

CIBSE Knowledge Series — How to design a heating system

Key design outputs for step 12:

design review

quality checks on the design
technical content

feedback lessons and design
benchmark data to inform future
work.

CIBSE Knowledge Series — How to design a heating system

Developing the design — key issues

This section covers some key areas relevant to the overall design of heating

systems.

4.1

Design conditions

The choice of both internal and external design conditions can have a

substantial impact on initial system loads and subsequent system

performance. These are a fundamental part of heating load calculations and

the choice should be very carefully considered. For example the difference

between using a temperature difference of 21 K (

–

1 °C to 20 °C) and one of

25 K (

–

4 °C to 21 °C) for a particular building is nearly a 20% increase in the

heat loss. By the time allowance has been made for reduction in emitter

output and preheat requirements the difference could be as much as 40%.

When considering energy efficiency the fundamentals need to be considered

first.

It is also important to consider what system performance criteria are

acceptable and agree this with the client. Establishing the required system

performance criteria at the briefing stage is one of the most critical tasks in

the design and it is vital that clients and their designers have a thorough

understanding of what conditions are required and what can practically be

achieved. For example the difference between specifying an internal

condition of 21 °C ± 1 °C or a condition of 21 °C ± 2 °C can have a

considerable impact on energy consumption, control choice and system

performance. The closer the control the more expensive the system. If

conditions can be relaxed a little and allowed to vary (within reasonable

limits) the system can be simpler and cheaper to install and to operate.

Further guidance can be found in CIBSE KS06: Comfort on practical issues on
temperature and design criteria, etc, with guidance on design conditions in

CIBSE Guide A, chapters 1 and 2, and on the margins that can occur at

different design stages in CIBSE RR04: Engineering design calculations and the
use of margins (1998).

4.2

Design margins

Margins should never be added during a calculation process without an

adequate reason for doing so and only with the approval of a senior engineer.

Excessive margins can result in system oversizing and poor operational

performance and control. If any margins are used they should be clearly

identified and a justification given for their use, which should be recorded in

the design file. It is also important to check for any inbuilt assumptions and

margins in software calculation packages. The use of margins should be

reviewed at several stages during the design process to check their

appropriateness and avoid any duplication or excess, e.g. at the end of a

calculation procedure, at design review stage, etc. Figure 22 illustrates the

consequences of oversizing for heating system performance.

(For more information on the use of margins in engineering design refer to

Design Checks for HVAC — a quality control framework for building services
engineers, topic sheet number 1 Design margins and CIBSE Research Report
RR04: Engineering design calculations and the use of margins.)

4.3

Energy efficiency

Energy efficiency should be considered throughout the design process. In

general, energy efficient heating should:

—

incorporate the most efficient primary plant to generate heat/hot

water

—

optimise the use of renewable energy sources

—

ensure that heat/hot water is distributed effectively and efficiently

—

include effective controls on primary plant and distribution systems to

ensure that heat is only provided when and where it is needed and at

the correct temperature

—

be responsive to changes in climate, solar gains, occupancy, activity and

internal gains.

CIBSE Knowledge Series — How to design a heating system

Variable temperature

If terminal units are oversized, space
temperatures drift higher than
required and energy is wasted. If
coils are oversized, too much water
is pumped through the system and
performance and control is
compromised if laminar flow results
when flow rate is reduced

Oversized pumps consume excess
energy as too much cold water is
pumped and/or they are inefficient
because they are not operating at
their most efficient operating point.
They can often cause balancing
problems during morning start-up
and constant temperature pumps
may turn off and on at maximum
demand

Return water temperatures are lower
than expected if oversized constant
temperature (constant flow)
radiators are installed. Boilers can
corrode if they are not protected

Variable flow constant

temperature

Constant flow

constant temperature

Boiler plant

Oversized valves reduce effective
control and fail prematurely. They
can often cause balancing problems
during morning start up

Boilers that are oversized will cycle
at maximum demand. Under
medium and low loads burner
fraction on-time is small (especially
if cycling rates are high) and
reduction in plant dynamic
efficiency occurs. Operating costs
increase because of the reduced
plant load operating efficiency.
Oversized plant permanently
operating at low loads can reduce
plant life. Accelerated wear can also
arise from unstable control caused
by plant oversizing. For example:
many oversized steam traps fail
prematurely because they operate
too close to their closed position

Variable temperature circuit and
variable flow circuit return water
temperatures are higher
than expected

Figure 22:

The impact of oversizing on
heating system performance

(Source: BSRIA AG 1/2000 Enhancing the
performance of oversized plant by Barry Crozier,
BSRIA 2000)

CIBSE Knowledge Series — How to design a heating system

Designers should:

—

select fuels and tariffs that promote efficiency and minimise running

costs

—

segregate hot water services generation wherever possible

—

consider de-centralised heating and hot water services generation plant

on large sites to reduce standing losses and improve load matching

—

locate plant to minimise distribution system and losses

—

ensure distribution systems are sized correctly to minimise pump and

fan energy consumption

—

insulate pipework, valves, etc effectively

—

ensure the base load is provided by the most efficient plant

—

utilise condensing boilers where feasible and appropriate

—

consider energy recovery where feasible, e.g. from air exhaust streams.

Further guidance is given in the CIBSE Knowledge Series on Energy efficient
heating and CIBSE Guide F, chapter 10.

4.4

Quality control

The design information, including the design calculations, is part of the design

process and therefore will form part of the project design file and records

and be subject to standard in-company quality assurance (QA) and quality

control (QC) procedures. As such all information and data should be properly

recorded and checked. Good practice includes:

—

clearly identify and record all data sources to enable input information

to be adequately verified

—

clearly state all assumptions, and identify, and flag, where more

accurate data will be required (e.g. from client, manufacturer, etc) as

the design progresses

—

review any assumptions as the design progresses to check they are still

valid, and replace with more accurate information as received

—

clearly identify, record and review the required design inputs and

design outputs

—

record calculations clearly, with sufficient detail to ensure the work can

be followed by others (be aware that if a problem arises on a project

this could mean revisiting calculations several years after they were

originally done)

—

identify and record calculation checks and cross-checks clearly

—

verify the design to ensure it can meet the design requirements

—

review the overall design.

Further guidance on design quality control is given in BSRIA AG 1/2002:

Design checks for HVAC.

References

BSRIA domestic boiler marketing report (Bracknell: BSRIA Ltd) (March 2006) and
BSRIA commercial boiler marketing report (Bracknell: BSRIA Ltd) (March 2006)

DTI UK Energy Consumption in the United Kingdom (www.dti.gov.uk/energy/statistics/
publications/energy-consumption/page17658.html)

Crozier B Enhancing the performance of oversized plant BSRIA AG 1/2000 (Bracknell:
BSRIA Ltd) (2000) and Brittain J Oversized heating plant BSRIA GN 12/97 (Bracknell: BSRIA
Ltd) (1997)

Cross N Design: principles and practice — product planning and the design brief (Open
University) (1995)

RIBA Plan of Work (London: Royal Institute of British Architects) (1999)

ACE Agreement A(2) 2002 and B(2) 2002 (revised 2004) Mechanical and Electrical
Engineering Services (London: Association for Consultancy and Engineering) (2002/2004)

The designer’s guide to energy-efficient buildings for industry GPG 303 (Carbon Trust) (2000)

Day A, Ratcliffe M and Shephed K Sizing central boiler plant using an economic
optimisation model (Proc CIBSE National Conference) (2001)

Selected bibliography

Overall heating system design process

Heating, ventilation, air conditioning and refrigeration CIBSE Guide B , chapter 1(London: Chartered

Institution of Building Services Engineers) (2001-2)

Lawrence Race, G, Design checks for HVAC BSRIA AG 1/02 (Bracknell: BSRIA Ltd) (2002)

Lawrence Race, G, Mitchell, S, A practical guide to HVAC building services calculations BSRIA/CIBSE

BG 30/03 (2003)

Building Regulations compliance guides

Domestic heating compliance guide (London: TSO) (2006)

Non-domestic heating, cooling and ventilation compliance guide (London: TSO) (2006)

Comfort

Comfort CIBSE KS06 (London: Chartered Institution of Building Services Engineers) (2006)

Environmental Design CIBSE Guide A (London: Chartered Institution of Building Services Engineers)

(2006), chapter 1

CDM guidance for designers

www.hse.gov.uk/construction/designers/index.htm

Design data

Environmental Design CIBSE Guide A , chapters 1 and 2 (London: Chartered Institution of Building

Services Engineers) (2006)

Reference data CIBSE Guide C (London: Chartered Institution of Building Services Engineers) (2001)

CIBSE Knowledge Series — How to design a heating system

Weather, solar and illuminance data CIBSE Guide J (London: Chartered Institution of Building Services

Engineers) (2002)

Design management

Parsloe, C, Wild, L, Project Management Handbook BSRIA AG 11/98 (Bracknell: BSRIA Ltd) (1998)

Parsloe, C, Allocation of design responsibilities for building engineering services BSRIA TN21/97

(Bracknell: BSRIA Ltd) (1997) (New edition due in 2006)

Design margins

Engineering design calculations and the use of margins CIBSE Research Report RR04 (London:

Chartered Institution of Building Services Engineers) (1998)

Design quality control

Lawrence Race, G, Design checks for HVAC BSRIA AG 1/02 (Bracknell: BSRIA Ltd) (2002)

Design review and feedback

Lawrence Race, G, Pearson, C, de Saulles, T, Feedback for better building services design BSRIA

AG 21/98 (Bracknell: BSRIA Ltd) (1998)

Domestic heating

CIBSE Domestic building services panel, Domestic heating design guide (London: Chartered Institution

of Building Services Engineers) (2003)

Energy efficiency

Energy efficiency in buildings CIBSE Guide F (London: Chartered Institution of Building Services

Engineers) (2004)

Energy efficient heating CIBSE KS (London: Chartered Institution of Building Services Engineers) (to

be published)

Fabric thermal performance

Environmental Design CIBSE Guide A , chapters 3 and 5 (London: Chartered Institution of Building

Services Engineers) (2006)

Heating design calculations with worked examples

Lawrence Race, G, Mitchell, S, A practical guide to HVAC building services calculations BSRIA/CIBSE,

BG 30/03 (2003)

Environmental Design CIBSE Guide A , chapters 3 and 5 (London: Chartered Institution of Building

Services Engineers) (2006)

Heating, ventilation, air conditioning and refrigeration CIBSE Guide B , chapter 1 (London: Chartered

Institution of Building Services Engineers) (2001-2)

Sands, J, Parsloe, C, Churcher, D, Model demonstration projectBSRIA BG 1/2006 (Bracknell: BSRIA

Ltd) (2006)

Heating plant and controls

Heating, ventilation, air conditioning and refrigeration CIBSE Guide B , chapter 1 (London: Chartered

CIBSE Knowledge Series — How to design a heating system

Institution of Building Services Engineers) (2001-2)

Building Control Systems CIBSE Guide H (London: Chartered Institution of Building Services

Engineers) (2000)

Understanding controls CIBSE KS04 (London: Chartered Institution of Building Services Engineers)

(2005)

Renewable energy issues for buildings CIBSE TM 38 (London: Chartered Institution of Building Services

Engineers) (2006)

Day, A, Ratcliffe, M, Shepherd , K, Heating systems — plant and control (Oxford: Butterworth-

Heinemann) (2003)

Heating systems

Heating, ventilation, air conditioning and refrigeration CIBSE Guide B (London: Chartered Institution of

Building Services Engineers) (2001-2), chapter 1

CIBSE Domestic building services panel, Underfloor heating design guide (London: Chartered

Institution of Building Services Engineers) (2004)

Sands, J, Underfloor heating — the designers guide BSRIA AG 12/01 (Bracknell: BSRIA Ltd) (2001)

Brown, R, Radiant Heating BSRIA AG3/96 (Bracknell: BSRIA Ltd) (1996)

Infiltration estimation

Environmental design CIBSE Guide A , chapter 4 (London: Chartered Institution of Building Services

Engineers) (2006)

Natural ventilation in non-domestic buildings CIBSE AM10 (London: Chartered Institution of Building

Services Engineers) (2005)

Natural ventilation

Natural ventilation in non-domestic buildings CIBSE AM10 (London: Chartered Institution of Building

Services Engineers) (2005)

Mixed mode ventilation CIBSE AM13 (London: Chartered Institution of Building Services Engineers)

(2000)

Renewable energy

Renewable energy sources for buildings CIBSE TM38 (London: Chartered Institution of Building

Services Engineers) (2006)

Value engineering

Hayden, G, Parsloe, C, Value engineering of building services BSRIA Application Guide 15/96

(Bracknell: BSRIA Ltd) (1996)

CIBSE Knowledge Series — Variable flow pipework systems

General textbooks covering heating

systems and design aspects

Oughton, D, Hodkinson, S, Faber and
Kell’s Heating and air-conditioning of
buildings, 9th ed (Oxford: Elsevier) (2002)

Moss K, Heating and water services design
in buildings (London: Taylor & Francis)
(2003)

Day, A, Ratcliffe, M, Shepherd K, Heating
systems — plant and control (Oxford:
Butterworth-Heinemann) (2003)

Kavanaugh, S, HVAC simplified (Atlanta
GA: American Society of Heating,
Refrigerating and Air-Conditioning
Engineers) (2006)

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