CIBSE KNOWLEDGE SERIES
How to design a heating
system
Direct and accessible guidance from key subject
overviews to implementing practical solutions
KS8 cover 28/9/06 10:24 Page 3
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© October 2006 The Chartered Institution of Building Services Engineers London
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ISBN-10: 1-903287-79-0
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How to design a
heating system
CIBSE Knowledge Series: KS8
Principal author
Gay Lawrence Race
Editor
Helen Carwardine
CIBSE Knowledge Series  How to design a heating system
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
1.1 Use of this guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
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
3 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
3.5 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
4 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
1 Introduction
In cooler climates the provision of heating is an essential part of creating
Heating
comfortable internal environments, and therefore heating system design is a
fundamental part of building services design.
In 2005:
1.65 million new domestic boilers
Heating is a major sector within mechanical building services. There are some
23,500 commercial boilers
21 million domestic properties in the UK with gas-fired central heating, and a
9 million radiators
further 200,000 commercial properties with heating. The UK market for
22 million metres of underfloor
heating systems is substantial, with around 1.65 million new domestic boilers
heating pipe
installed per year and around 23,500 commercial boilers. There are around 9
were installed in the UK alone.
million radiators installed per year with a further 22 million metres of
underfloor heating pipe (2005 figures)(1).
Sources: BSRIA domestic boiler
marketing report March 2006, BSRIA
commercial boiler marketing report
Heating is also a major consumer of energy within the UK, with space heating
March 2006.
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 CO2 emissions, delivering almost half the CO2 emissions from
non-domestic buildings.
Given the current requirements to limit energy consumption and CO2
production, good design of heating systems is essential to ensure that systems
Figure 1:
operate efficiently and safely and make effective use of energy. Historically
UK non-transport energy
there have been problems with oversizing of heating systems which can lead
use (2002 figures) million
to inefficient operation, particularly at part load operation, to control
tonnes of oil equivalent
problems and to a reduction in plant operating life(3). The energy
consumption for oversized plant can be 50% more than necessary.
11·8
Although heating is often considered to be a simple, basic system, there are
4·4
many options and permutations to be considered. The majority of UK
41·4
buildings will require heating but different building types and locations will
15·0
have very different requirements and constraints  consider for example the
choices possible for a small ground floor flat in a city centre development
9·7
12·9
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.
Space heating
Water
Cooking/catering
The fundamental components of any heating system are:
Lighting appliances
/
Process use
Motors/drivers
 a means of generating heat, i.e. the heat source
Drying/separation
Other non-transport
 a means of distributing the heat around the building or buildings, i.e.
Source: DTI Energy consumption tables:
the distribution medium and network
overall energy consumption. URN No:
 a means of delivering the heat into the space to be heated, i.e. the
05/2008 Table 1.2 Non-transport energy
consumption by end use, 1990, 2000, 2001
heat emitter.
and 2002
CIBSE Knowledge Series  How to design a heating system 1
2·4
3·3
There are many possible options to be considered, some of which are listed
Good design
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
Good design of heating systems is
essential to ensure that systems operate distribution medium, to a conventional gas boiler system distributing low
efficiently and safely and make effective
temperature water to a convector system. A more complex system would be
use of energy.
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.
Table 1:
Heat source gas CHP
Heating systems
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
Factors to consider
electricity
Emitter radiators ceiling panels
Building type:
fan convectors natural convectors
domestic
panel heaters underfloor heating coils
school
unit heaters storage heaters
apartment building
high temperature radiant panels
retail
hospital
factory Whilst heating systems may seem relatively simple, in practice there are many
office
factors to be considered during the design process, in order to achieve a
well-designed system that delivers both the required comfort conditions and
Location:
level of control whilst still minimising energy consumption. This publication,
city centre
together with other CIBSE guidance, aims to assist the designer in achieving
urban
that aim.
suburban
rural
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
2 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.
CIBSE Knowledge Series  How to design a heating system 3
2 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.
Figure 2:
1. Client
Feedback/
The design process
need Inform
review
Implement Design
performance
4. Design 2. Design
The design process
delivery requirements
Feedback/
review
Select Develop
3. Design
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.
4 CIBSE Knowledge Series  How to design a heating system
Figure 3:
ACE Agreements A(2) &
RIBA plan of work (1999)
B(2) (2002)
Construction process
stages
A Inception/Identification of C1 Appraisal stage
client requirements
C2 Strategic briefing
B Strategic brief
C Outline proposals C3 Outline proposals stage
D Detailed proposals C4 Detailed proposals stage
E Final proposals C5 Final proposals stage
F Production information C6 Production information
stage
G Tender documentation C7 Tender documentation and
tender action stage
H Tender action
J Mobilisation/Project C8 Mobilisation, construction
planning and completion stage
K Construction to practical
completion
L After practical completion
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.
CIBSE Knowledge Series  How to design a heating system 5
Design
Pre-design
Construction
Figure 4:
Heating system design
process
Step no. Key design steps Design tasks
1 Pre-design Obtain design brief.
Identify client and building user needs and requirements.
Refer to feedback and lessons learned from previous projects
2 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
3 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
4 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
5 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
6 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
7 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
8 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
9 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
10 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
11 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
12 Review Design review
6 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
Figure 5:
numbers as Figure 4 to show how the different steps are repeated and
Heating design process
revisited as the design develops. The detailed design tasks at each step have
mapped against the main
been omitted to keep the diagram to a manageable size.
design work stages
Step no.
Design stage Key design steps Key outputs
1
Pre-design Pre design: obtain client brief.
Refer to feedback and lessons learned from previous projects
2
Outline design Gather design information and establish key design requirements. Design brief
Establish planning requirements Outline drawings
and schematics.
3 Provisional cost plan
Establish key design data
4
Initial building thermal performance analysis.
Approximate heat loss
5
Heating system  consider options and fuel choices
7
Consider system requirements, potential layout, etc
9
Approximate total loads and plant size to arrive at cost plans, provide
space requirements and structural load information, etc.
Scheme / Detail design 2 Design drawings and
Gather further necessary design information and establish key design
schematics.
requirements
Cost plan
3
Establish key design data
4 Detailed building thermal performance analysis
5
Heating system choice and selection
6
Design calculations: space heat losses
7
Equipment selection and sizing  emitters and distribution network.
Control requirements
8
Design calculations: heating load analysis, possibly including thermal
modelling
9
Initial plant and control selection
11
Value engineering workshops
12
Interim design review
Design development/Final 4
Further building thermal performance analysis, to assist in modelling Design drawings and
proposals/Production specification for tender
dynamic building and system performance (if required)
information purposes.
Possibly co-ordination
7
Final equipment selection and sizing
drawings.
Final cost appraisal
8
Final heating load calculation and analysis
9
Plant selection. Control requirements. Preparation of detailed design
drawings and specifications for plant and equipment
10
Design calculations.
System performance analysis, including part load performance and
predicted energy use. Possible final dynamic modelling of building and
system performance.
Control performance
11
Final value engineering exercise
12
Final design review
Post-occupancy review
CIBSE Knowledge Series  How to design a heating system 7
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
Figure 6:
4, 6 and 8  building performance analysis, heat losses and load analysis;
Key steps for heating
continuing into system and equipment sizing in steps 7 and 9, and system
design calculation
analysis in step 10.
sequence
Building air- Internal and external Fabric
tightness details design conditions details
Condensation
U-values
risk analysis
Site weather
data
Infiltration Fabric heat
heat loss loss
Internal gains (only
Natural ventilation Space heat Building thermal
if both heating and
load (if any) loss response analysis
gains are continuous)
Emitter Space heating
Pre-heat
sizing load
margin
Distribution system Infiltration load
Intermittent
sizing diversity
operation
assessment
Distribution system Maximum simultaneous
HWS
losses space heating load
load
Load diversity
Process
analysis
load
Part load
performance
Total heating
Central fresh air
load
ventilation heating load
Boiler/heating
Standby capacity
plant sizing
(if required)
Final system and
control performance
Flue
analysis Fuel supply
sizing
system sizing
8 CIBSE Knowledge Series  How to design a heating system
Figure 7:
2.4 Thermal comfort
Design output
Input Output
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 Client Design Thermal
need process comfort
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
Thermal comfort
hot nor too cold.
 That condition of mind which expresses
The four main environmental factors that affect thermal comfort are:
satisfaction with the thermal environment
and is assessed by subjective evaluation.
 air temperature (ta) ASHRAE Standard 55-2004
 relative humidity
 mean radiant temperature (tr)
 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
Key factors in thermal comfort
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%
temperature
RH for operative temperature and relative humidity respectively, with 10 l/s
humidity
per person of fresh air required. More often some variation is allowed, i.e.
air movement
21 °C Ä…1 °C and 50% RH Ä…10%. Example design criteria for a range of
air quality.
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.
CIBSE Knowledge Series  How to design a heating system 9
3 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
10 CIBSE Knowledge Series  How to design a heating system
Key design outputs for step 1:
 control strategy.
pre-design
Information required from the design brief can include:
functional design brief.
 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.
Specific information required Outputs
Figure 8:
Location: Geographical location and
Information gathering
height above sea level
Local microclimate, wind
Information on local conditions  pollution,
External design
noise
conditions
Site Orientation: Details of surrounding
information buildings, shading, etc
Services: Utilities provision and positions Available
services
Access: Access to site
Possible comfort
Functional performance: Specific or energy
deliverables requirements
Operating strategy: Client approach to Possible system
building design and operation including constraints or
sustainability, energy strategy, control, requirements
maintenance, etc
Cost budgets
Costs: Cost plans and budgets and constraints
Client
Key design outputs for step 2:
brief
Internal design
information gathering
Occupancy: Information on occupancy conditions
activity and density Assessment of
Hours of occupation, etc intermittent
system operation
Internal loads  key design requirements
Building use: Tasks, office equipment, etc
small power,
necessary information to establish
lighting, etc
internal and external design data
Future needs: Future proofing and
Additional system
flexibility requirements
site assessment and utility
requirements
provision
statutory and regulatory design
Design requirements
Standards and Statutory and regulatory
requirements and targets.
Energy targets,
regulations requirements
including % energy
to be provided from
renewable sources
CIBSE Knowledge Series  How to design a heating system 11
The building services engineer will also need to provide information to other
Building Regulations Part L 2006
design team members throughout the project. As outlined in section 3.1, at
the initial design stages this can include advice on optimising building
Heating systems should be designed to
minimise carbon emissions and make it
performance, and can also include information on potential spatial
easier for the whole building to achieve a
requirements, which can be refined as the design develops.
building CO2 emission rate (BER) lower
than the set target (TER) and thus
comply with Part L requirements, which
The new Building Regulations Part L (2006) requires that both fabric and
implement the EPBD directive.
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
Key design outputs for step 3:
practice design standards.
design data
Table 2 gives example winter internal design conditions for thermal comfort
internal thermal comfort design
for a range of common building types. More detailed guidance for a wider
conditions
schedule of internal design range of building and room types is given in CIBSE Guide A, Table 1.5, which
criteria for each space (e.g. on
also relates the design guidance to the expected clothing and metabolic rates
room data sheets)
of occupants to achieve a predicted percentage persons dissatisfied (PPD) of
external design conditions.
around 5%. For design purposes reference should be made to the full table
together with the associated footnotes.
12 CIBSE Knowledge Series  How to design a heating system
Suggested air supply rate Table 2:
Winter operative temp
Building/room type l/s per person
Recommended winter thermal
range °C
(unless stated otherwise)
comfort criteria for some
Dwellings
selected building types
bathrooms 20 22 15 l/s
(Source: CIBSE Guide A, Table 1.5)
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 10
computer rooms 19 21 10
corridors 19 21 10
drawing office 19 21 10
entrance halls/lobbies 19 21 10
general office space 21 23 10
open plan 21 23 10
toilets 19 21 >5 ACH
Retail
department stores 19 21 10
small shops 19 21 10
supermarkets 19 21 10
shopping malls 12 19 10
Schools
teaching spaces 19 21 10
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.
External design conditions
Selection of appropriate external design criteria requires information on the
site location, development details and local microclimate, as outlined in Appropriate design criteria should be
agreed with the client, taking into
section 3.2, as well as meteorological data. The type of building and the
consideration the acceptable risk of
thermal inertia will also help to determine what may be an acceptable risk of exceedence of design conditions.
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.
CIBSE Knowledge Series  How to design a heating system 13
Figure 9: Specific information required Outputs
Building form and fabric
Layout
information to
Building plan and form: Details of inform services
building plan and form location, zoning
Building orientation and shading strategy, etc
Building layout
Glazing locations, etc
Room dimensions
Constraints on
Building Internal layout: Layout drawings emitter
information Potential space use and fit out positioning
Partitioning Constraints on
distribution space
Plant and distribution space:
Potential location and space required/
Possible system
available (should be discussed and agreed
constraints or
with rest of design team as early as
requirements
possible in the design)
Fabric: Detail of building materials Thermal mass
and construction assessment
Fabric thermal performance (heavy or
Glazing height
light weight)
Fabric and glazing
Glazing: Glazing information  type,
Fabric
U-values
dimensions, including glazing height,
information
Glazing height influences comfort within Fabric admittance
and thermal performance
Y-values
the occupied space both due to
Window leakage
downdraughts and to cold radiation
Air tightness: Construction quality
rates
which affects the mean radiant
Building air tightness prediction
Infiltration data
temperature.
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.
Figure 10:
Building air- Internal and external Fabric
Key steps to analyse building tightness details design conditions details
thermal performance
Site weather Condensation
U-values
data risk analysis
Infiltration Fabric heat
heat loss loss
Space heat Building thermal
loss response analysis
14 CIBSE Knowledge Series  How to design a heating system
Key design outputs for step 4:
As the thermal insulation performance of the building fabric has improved,
building thermal performance
the infiltration component of heat loss can now comprise a substantial
percentage and therefore needs to be estimated as accurately as possible.
fabric thermal transmittance
Although building air leakage testing will be required for most buildings, and
details, i.e. the fabric and glazing
U-values
will form part of the design requirements, this sets an expected standard,
building thermal response (and
generally specified for a specific applied pressure difference such as 50 Pa,
dynamic thermal performance
and therefore does not provide data for infiltration calculations. Methods for
characteristics including
admittance values, if required)
estimating infiltration rates are given in CIBSE Guide A, chapter 4, with
infiltration assessment for
additional guidance in CIBSE AM10.
individual spaces and for the
whole building
An initial assessment of building use and hours of occupancy will determine if
assessment of intermittent
operation to inform preheat
intermittent, rather than continuous, operation is likely. Details of the overall
requirements
building thermal response will be needed to determine the likely preheat
estimation of approximate total
requirements and the impact on heating system performance (see also
building heat loss.
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.
Infiltration estimation
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
A useful cross check for infiltration
choices that are reasonable for a 50 kW loss can be very different from those
estimation is to convert the estimated
infiltration total to a room or whole
for a heat loss of 1500 kW, for example.
building air change rate, as appropriate.
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:
Zoning
 space required/available: both for plant and for distribution
Zoning strategy needs to be agreed with
the client. Some variation in internal
 potential plant room locations related to the spaces to be served
conditions may be acceptable, which can
 cost plan: capital cost of installation
help to minimise the number of zones
and improve operating efficiency.
 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.
CIBSE Knowledge Series  How to design a heating system 15
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.
Table 3:
Cost operating and maintenance costs
System performance and use
energy efficiency
factors 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.
Key design outputs for step 5:
Control ability to provide accurate control of space temperature
heating system selection
ability to provide localised control
speed of response to changing conditions.
zoning strategy for building  to
give details of building zones and Convenience ease of use
required operating conditions 
location
hours of use and internal design
potential lettable/usable space taken up by emitters/outlets
conditions
and distribution
selection of heating system(s) in
ease of maintenance.
principle  fuel/heat source,
system, distribution medium and
emitter types.
Key design decisions will include the choice of:
 heat media and distribution system
 system: centralised or de-centralised
 heat emitter
 heat source.
Table 4:
Heat media the balance between radiant and convective output
required from the system
Heating system design choices
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
Low and zero carbon technologies
location to provide uniform temperatures
noise level
Part L (2006) of the Building Regulations
space required.
encourages the use of low and zero
carbon (LZC) technologies, such as
Heat source conventional boilers or other heat sources such as heat
renewables, CHP and heat pumps, as a
pumps, CHP, etc
way of meeting the required carbon
boiler and fuel type, any storage requirements
emission reductions, and implementing
the requirements of the EPBD directive.
central plant location.
Many local planning authorities also
encourage the use of these technologies,
in some cases making it a specific
Tables 5, 6 and 7 provide further information on some system options, giving
planning requirement.
some characteristics and relative advantages and disadvantages together with
16 CIBSE Knowledge Series  How to design a heating system
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
Figure 11:
for buildings (2006).
Selection chart: heating systems
Note: This selection chart is intended to give initial guidance only;
Start here
it is not intended to replace more rigorous option appraisal
N Y
Constraints on combustion appliances in workplace?
Considering CHP, waste fuel or local community
N Y
heating system available as source of heat?
Most areas have similar heating requirements N Y
in terms of times and temperatures?
Decentralised system Centralised system
N Y N Y
Significant spot heating
(>50% of heated space)?
N Y N Y
Above average ventilation rates?
N Y N Y
Non-sedentary workforce?
Radiant heat acceptable N Y
N Y N Y
to process?
Convective Medium or high temperature Convective Low temperature
system radiant system system radiant system
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
Decentralised system Centralised system
Waste fuel or local community heating N Y
available as source of heat?
N Y N Y
Strategic need for back-up
fuel supply?
N Y N Y N Y
Natural gas required?
Radiant heat required? N Y
Natural Natural gas +
gas oil back-up
Electricity for
Community Community
high temperature
or waste with or waste
Oil or
systems, LPG
Oil + LPG Community oil or LPG with gas
LPG
for medium
electricity or waste heat back-up back-up
temperature systems
back-up
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 17
Table 5:
Heat distribution media
Medium Principal characteristics Advantages Disadvantages
Air Low specific heat capacity, low No heat emitters needed Large volume of air required 
density and small temperature large ducts require more
No intermediate medium or
difference permissible between distribution space
heat exchanger is needed
supply and return, compared to
Fans can require high energy
water, therefore larger volume
consumption
needed to transfer given heat
quantity
Water High specific heat capacity, high Small volume of water required Requires heat emitters to
density and large temperature  pipes require little transfer heat to occupied space
difference permissible between distribution space
supply and return, compared to
air, therefore smaller volume
needed to transfer given heat
quantity. Usually classified
according to water temperature/
pressure:
 LTHW (LPHW) Low temperature/pressure hot Generally recognised as simple Output is limited by system
water systems operate at to install and safe in operation. temperatures
temperatures of less than 90 °C
Use with condensing boilers to
(approx.), and at low pressures
maximise energy efficiency
that can be generated by an
open or sealed expansion vessel
 MTHW (MPHW) Medium temperature/pressure Higher temperatures and Pressurisation necessitates
hot water systems operate at temperature drops give smaller additional plant and controls, and
between 90 120 °C (approx.), pipework, which may be an additional safety requirements
with a greater drop in water advantage on larger systems
temperature around the system.
This category includes
pressurisation up to 5 bar
absolute
 HTHW (HPHW) High temperature/pressure hot Higher temperatures and Safety requires that all pipework
water systems operate at over temperature drops give even must be welded, and to the
120 °C, often with higher smaller pipework standards applicable to steam
temperatures  perhaps up to pipework. This is unlikely to be a
200 °C, with even greater cost-effective choice except for
temperature drops in the the transportation of heat over
system. These temperatures will long distances
require pressurisation up to
around 10 bar absolute
Steam Exploits the latent heat of High maintenance and water
condensation to provide very treatment requirements
high transfer capacity. Operates
at high pressures. Principally
used in hospitals and buildings
with large kitchens or processes
requiring steam
18 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 Low overall capital cost, savings made on minimising
capacity of central plant. the use of air and water distribution systems
Capital cost of distribution systems is high
Space requirements Space requirements of central plant and distribution Smaller or balanced flues can often be used
systems are significant, particularly ductwork
Flueing arrangements can be more difficult in some
Large, high flues needed locations
System efficiency Central plant tends to be better engineered, operating Energy performance in buildings with diverse patterns
at higher system efficiencies (where load factors are of use is usually better
high) and more durable
As the load factor falls, the total efficiency falls as
distribution losses become more significant
System operation Convenient for some institutions to have centralised May require more control systems
plant
Zoning of the systems can be matched more easily to
Distribution losses can be significant occupancy patterns
System maintenance Central plant tends to be better engineered, more Can be readily altered and extended
and operational life durable
Equipment tends to be less robust with shorter
Less resilience if no standby plant provided 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 Fuel needs to be supplied throughout the site
Better utilisation of CHP, etc Boilers are single fuel
Some systems will naturally require central plant, e.g.
heavy oil and coal burning plant
Based on data from CIBSE Guide F (2004), chapter 10.
CIBSE Knowledge Series  How to design a heating system 19
Table 7:
Common emitter/system types
Design points Advantages Disadvantages
Radiators Output up to 70% convective Good temperature control Fairly slow response to control
Check for limit on surface Balance of radiant and convective Slow thermal response
temperature in some applications, output gives good thermal comfort
e.g. hospitals
Low maintenance
Cheap to install
Natural convectors Quicker response to control Can occupy more floor space
Skirting or floor trench convectors Can get higher temperature
can be unobtrusive stratification in space
Underfloor heating Check required output can be Unobtrusive Heat output limited
achieved with acceptable floor
Good space temperature Slow response to control
surface temperatures
distribution with little stratification
Fan convectors Can also be used to deliver Quick thermal response Can be noisy
ventilation air
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 Ceiling panels need relatively low Unobtrusive Slow response to control
panels temperatures to avoid discomfort
Low maintenance
High temperature radiant Can be direct gas or oil fired units Quicker thermal response Need to be mounted at high level
heaters to avoid local high intensity
Check that irradiance levels are Can be used in spaces with high
radiation and discomfort
acceptable for comfort air change rates and high ceilings
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
Heat losses
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
A useful cross check for heat losses is to
convert the calculated values to W/m2 or
CIBSE Guide A provides a worked example for the steady state heat loss
W/m3 figures to check against reasonable
calculation.
benchmarks.
Key steps in the calculation sequence for space heat loss are shown in Figure
13 in dark blue.
20 CIBSE Knowledge Series  How to design a heating system
Figure 13:
Building air- Internal and external Fabric
Key steps to establish individual
tightness details design conditions details
space heat losses
Site weather Condensation
U-values
data risk analysis
Infiltration Fabric heat
heat loss loss
Heat losses  temperatures
Space heat
Care needs to be taken when considering
loss
the temperatures to use for heat loss
calculations. Design criteria are usually
given as operative temperatures (to).
Fabric heat losses should use the internal
With better fabric insulation the infiltration heat loss can now account for up
environmental temperature (tei) and
infiltration loss the internal air
to 50% of the total heat loss in some smaller buildings and therefore
temperature (tai). These can differ
infiltration rates need to be estimated as accurately as possible  see section
substantially for some buildings and some
heating types. CIBSE provides a method
3.4.
for steady state heat losses that applies
correction factors F1 and F2 to enable the
design internal operative temperature to
To move from the heat loss to the heat load for a space, additional factors
be used  see CIBSE Guide A, section
need to be considered, including any additional loads within the space and
5.6.2. (Note: for very well insulated
buildings, without large areas of glass, and
any preheat requirements, as shown in Figure 14.
with low air change rates, there is often
little difference between operative,
environmental and air temperatures.)
Infiltration Fabric
heat loss heat loss
Figure 14:
Building thermal
Space response
Key steps to establish space
heat loss analysis
heating loads
Natural ventilation Internal gains (only Intermittent
load (if any) if both heating and operation
gains are continuous) assessment
Space Pre-heat
heating load margin
Radiant systems
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
For high temperature radiant systems the
cases be met by natural ventilation, in which case it will add an additional heat
standard heat loss calculation methods
are not appropriate for equipment
load directly to the space. Further information on naturally ventilated
selection. Instead the distribution of
buildings is given in CIBSE AM10 and on mixed mode buildings in CIBSE
radiant energy in the space should be
determined, utilising a radiant polar
AM13.
diagram for the emitter. Further guidance
is given in CIBSE Guide A, section
5.10.3.7 and CIBSE Guide B, section
A preliminary assessment of other loads that may also need to be met by the
1.4.6. Medium and low temperature
main heating source, such as any HWS load, can also be made at this stage to
radiant systems can be sized using the
usual heat loss calculation methods.
provide information for the next calculation step (see also section 3.8).
CIBSE Knowledge Series  How to design a heating system 21
HWS Internal gains
Normally no allowance would be made for internal gains in establishing space
HWS requirements and options should
be assessed, e.g. consider whether
heating loads as a worst-case scenario is always considered, i.e. to bring the
storage or instantaneous water heating is
unoccupied building up to temperature. However, exceptionally, if the heating
more appropriate. For hot water storage
consider the options of a dedicated boiler
will be operating continuously and there are constant heat sources such as
or a standalone hot water generator
electric lights and occupants in a continuously occupied building, then the
(direct-fired storage system). For
instantaneous hot water consider the
steady state heat requirement can be reduced by the amount of the constant
choice and availability of fuel and whether
gains. However the risks of this should always be made overt to the client as
point-of-use provision or multi-outlet is
more suitable.
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.
Figure 15:
Preheat
Design inside
temperature
Intermittent operation
Preheat
Intermittent heating occurs when the
time
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
Plant off Optimised Start of
temperature. There are two main types start time occupancy
of intermittent operation:
Time
normal intermittent operation is
where the output is reduced In order to assess the preheat requirements, information on both
when the building is unoccupied
intermittent operation and on the building thermal response is needed. For
 for example to a level of
10 °C to protect the building normal intermittent operation the plant and equipment will need to be larger
fabric and contents
than the steady state requirements, with the required capacity calculated by
highly intermittent is where the
applying an  intermittency factor , F3, based on the thermal response factor
building is occupied for short
periods only and therefore needs
for the building and the total hours of plant operation:
to be brought back to
temperature quickly prior to use.
22 CIBSE Knowledge Series  How to design a heating system
Inside temperature
Peak heating load = F3 x space heat load Plant size ratio
Details are given in CIBSE Guide A, section 5.10.3.3 and Appendix 5.A8, and
The intermittency factor F3 can also be
expressed as a plant size ratio (PSR)
in CIBSE Guide B, section 1.4.7.3.
defined as:
PSR = installed heat emission
If the calculated value of F3 is less than 1.2, CIBSE suggests that the value be
design peak steady state
taken as 1.2 to ensure that a reasonable margin of 20% for preheat is heat load
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 F3 lie in the range
Key design outputs for step 6: space
1.2 2.0, with research(8) indicating that values over 2.0 cannot be economically
heat losses and heat load
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 schedule of individual space and
zone heat losses, subdivided into
the cases investigated. For small buildings and small plants the optimum values
fabric and infiltration losses,
will be even lower. The use of optimum start control, as illustrated in Figure 15, together with details of the
internal design conditions
can help to ensure adequate preheat time in cold weather.
assessment of preheat
requirements for the building
For highly intermittent systems, a steady state heat loss is inappropriate to
schedule of space heating loads.
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.
Figure 16:
Emitter Space heating Preheat
Key steps for emitter and
sizing load margin
distribution system sizing
Distribution Infiltration
Heat transfer correction factors
system sizing load diversity
Distribution
The type of heat emitter can have a
system losses
significant effect on the calculated design
Maximum
steady state heating load, so it is essential
simultaneous
that appropriate values for the heat
space heating load
transfer correction factors F1 and F2 were
used at step 6.
CIBSE Knowledge Series  How to design a heating system 23
Heat emitters
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
Check that the manufacturer s published
positions, etc. For example if emitters are situated behind furnishings then
data is applicable to the conditions at
most of the immediate radiant heat output will be lost, and in some cases
which the emitter will be operating and
apply any relevant corrections for space
even the convective heat output can be obstructed and reduced. Although
temperature, water temperatures, etc.
much of the heat will eventually enter the space it may not be available
Note that manufacturers outputs are
based on particular space and water
during preheat and therefore an allowance may be need to be made and the
temperatures which may differ from the
required heat output increased to compensate. Details are given in CIBSE
design operating conditions.
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.
Figure 17: Radiator Underfloor heating Warm air heater at high level
3·0
Vertical air temperature
gradients for different heating
types
2·0
Source: CIBSE Guide A, Figure 5.6.
1·0
0
15 20 25 15 20 25 15 20 25
Air temperatures / °C
Key design outputs for step 7:
emitter and distribution system
These corrections can now mean that, for certain heating systems, the
sizing
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
schedule of emitters with
required output, and with surface the system sized. Guidance on pipe and duct sizing is given in CIBSE Guide C
and water temperature for
(2001), chapter 4. When determining the most appropriate layout for the
hydronic systems
distribution system, balancing and regulating requirements should be
initial control requirements
considered, e.g. the use of reverse return pipework layouts to aid system
layout drawings with emitter
positions
balancing during commissioning.
schematic of pipework layouts
with required flowrates for
The system distribution losses will need to be assessed. Those from within
hydronic systems.
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
24 CIBSE Knowledge Series  How to design a heating system
Room height / m
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.
Figure 18:
Emitter Space heating Preheat
Key steps to establish the
sizing load margin
maximum simultaneous space
heating load
Distribution Infiltration
system sizing load diversity
Distribution
system losses
Maximum
simultaneous
space heating load
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
CIBSE Knowledge Series  How to design a heating system 25
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.
Figure 19:
Preheat
margin
Key steps to establish the total
Maximum
heating load
simultaneous
HWS
space heating load
load
Process
Part load Load diversity
load
performance analysis
Central fresh air
Total heating ventilation
load heating load
Load diversity analysis
An analysis of load diversity is needed as the maximum demands for each
Key design outputs for step 8:
separate part of the overall load are unlikely to coincide. In addition to
heating load analysis
the infiltration diversity within the total space heating load, there can be
zone diversities, perhaps due to differing hours of occupancy. Process
assessment and analysis of load
diversity
loads could be intermittent and the HWS load could perhaps peak at the
total heating load to enable boiler
middle or towards the end of the occupied period, rather than the
or other heating plant to be
beginning.
sized.
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
26 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.
Figure 20:
Part load Total heating
performance load
Key steps for boiler/heating plant
Standby capacity
sizing and selection
(if required)
Final system and Boiler/heating
control performance plant sizing
analysis
Fuel supply
system sizing
Flue
sizing
Key design outputs for step 9:
plant sizing and selection
Standby capacity
schedule of plant, giving required
output, flowrates, etc
Occasionally standby capacity may be required so that, in the event of
control requirements
partial system failure or plant maintenance, the main loads can still be met
schematic of plant layout,
and the building continues to function. The decision on this can require risk connections, etc.
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
Control system
as there may already be sufficient capacity within the system. Where
further capacity is required careful consideration is needed of the load
Both the heating system and its control
breakdown to ensure that the various expected load combinations can be
system should be appropriate for the
requirements of the building and the
met efficiently, for example considering the optimum module size for
operation it supports. Ideally the
modular boiler installations. If the heating plant consists of modular boilers
approach should always be to use the
simplest control system that meets
then adding one extra module may be sufficient to both meet the
building owner, operator and user needs
requirement and still ensure system operating efficiency.
and capabilities, and efficiently delivers
the required quality of system operation.
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.
CIBSE Knowledge Series  How to design a heating system 27
Normal system operation
Predicted system performance, including part load performance, should be
investigated to check that the selected systems can operate efficiently under
The initial system design is often based
all predicted load conditions, see Figure 21. This is particularly important if
on design conditions that occur for less
additional capacity has been added, for example for preheat or standby, as
than 1% of the occupied time. For the
majority of the heating season occupied
this effectively adds a margin. It is important to check that this does not
period the system will be operating on a
unduly oversize the system, leading to poor performance at normal operating
fraction of the installed load and
therefore it is essential to ensure that the
conditions. It is also essential to check whether other margins have been
system can operate efficiently at these
added at any stage in the design process, including those that will occur by
low load conditions.
selecting standard plant sizes.
Maximum
simultaneous
space heating load
Figure 21:
System analysis
Load diversity
analysis
Part load Total heating
performance load
Final system and
Boiler/heating
control performance
plant sizing
analysis
Flue
sizing
System control performance
Key design outputs for step 10:
system analysis
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
analysis of system part-load
performance integrated control strategy at a very early design stage. In all cases the control
system control strategy statement
strategy should be set out first so that the control options can be evaluated
and flowcharts
against the required level of functionality. As such, the controls should be
schematics of plant and systems
considered at an early stage as an integral part of the system design.
required control system
functionality
At this design stage the task is to carry out a final evaluation of the controls,
control system specification.
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).
28 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
Value engineering should be carried out at several stages within the project to
A systematic approach to achieving the
required project functions at least cost
ensure that the design is on track to meet the client requirements for
without detriment to quality,
performance, quality, reliability, etc at least cost. For example, value
performance and reliability.
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.
Key design outputs for step 11:
Energy targets
value engineering and energy targets
The final system performance will need to be checked again to ensure it
value engineering review
complies with regulations and meets required energy targets, for example
energy target and emission value
meeting the seasonal efficiency requirements and achieving a building
calculations.
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
Safety in design
the design stages of a project, from the feasibility and innovation review to
straightforward progress reviews, culminating in a post-project review after Reviews should include consideration of
safety in design to ensure that the
project completion which can provide valuable feedback lessons to inform
provision of the design can be
future work. constructed, operated, maintained and
de-commissioned safely, to comply with
the Construction (Design and
During the design stages there should be review meetings of the design team Mangement) Regulations (CDM)
requirements. Helpful guidance on
at regular intervals to review design progress, agree changes, check
designers responsibilities under CDM is
compliance with the brief, etc. The intent of these is to monitor the progress given on the HSE website:
www.hse.gov.uk/construction/designers/
of the design against the programme and cost targets, anticipate potential
index.htm.
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
CIBSE Knowledge Series  How to design a heating system 29
future projects. A post-project review meeting can also be held with the
Key design outputs for step 12:
design review
entire project team.
quality checks on the design Sometimes there is the opportunity to obtain further feedback after handover
technical content
and occupation, e.g. via post-occupancy surveys. The client may also require
feedback lessons and design
additional duties to include monitoring system operation. For example, the
benchmark data to inform future
work.
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.
30 CIBSE Knowledge Series  How to design a heating system
4 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 Ä… 1 °C or a condition of 21 °C Ä… 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
CIBSE Knowledge Series  How to design a heating system 31
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
Figure 22:
calculation procedure, at design review stage, etc. Figure 22 illustrates the
The impact of oversizing on
consequences of oversizing for heating system performance.
heating system performance
Constant flow
If terminal units are oversized, space Return water temperatures are lower
constant temperature
temperatures drift higher than than expected if oversized constant
required and energy is wasted. If temperature (constant flow)
coils are oversized, too much water radiators are installed. Boilers can
is pumped through the system and corrode if they are not protected
Variable flow constant
performance and control is
temperature
compromised if laminar flow results
when flow rate is reduced
Variable temperature circuit and
variable flow circuit return water
temperatures are higher
than expected
Variable temperature
Oversized pumps consume excess
energy as too much cold water is
Boilers that are oversized will cycle
pumped and/or they are inefficient
at maximum demand. Under
because they are not operating at
medium and low loads burner
their most efficient operating point.
fraction on-time is small (especially
They can often cause balancing
if cycling rates are high) and
problems during morning start-up
reduction in plant dynamic
and constant temperature pumps
efficiency occurs. Operating costs
may turn off and on at maximum
increase because of the reduced
demand
plant load operating efficiency.
Oversized plant permanently
operating at low loads can reduce
plant life. Accelerated wear can also
arise from unstable control caused
Oversized valves reduce effective
by plant oversizing. For example:
control and fail prematurely. They
many oversized steam traps fail
can often cause balancing problems
prematurely because they operate
during morning start up
too close to their closed position
Boiler plant
(Source: BSRIA AG 1/2000 Enhancing the
performance of oversized plant by Barry Crozier,
(For more information on the use of margins in engineering design refer to
BSRIA 2000)
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.
32 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.
CIBSE Knowledge Series  How to design a heating system 33
References
1 BSRIA domestic boiler marketing report (Bracknell: BSRIA Ltd) (March 2006) and
BSRIA commercial boiler marketing report (Bracknell: BSRIA Ltd) (March 2006)
2 DTI UK Energy Consumption in the United Kingdom (www.dti.gov.uk/energy/statistics/
publications/energy-consumption/page17658.html)
3 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)
4 Cross N Design: principles and practice  product planning and the design brief (Open
University) (1995)
5 RIBA Plan of Work (London: Royal Institute of British Architects) (1999)
6 ACE Agreement A(2) 2002 and B(2) 2002 (revised 2004) Mechanical and Electrical
Engineering Services (London: Association for Consultancy and Engineering) (2002/2004)
7 The designer s guide to energy-efficient buildings for industry GPG 303 (Carbon Trust) (2000)
8 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)
34 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
35
CIBSE Knowledge Series  How to design a heating system
General textbooks covering heating
Institution of Building Services Engineers) (2001-2)
systems and design aspects
Building Control Systems CIBSE Guide H (London: Chartered Institution of Building Services
Engineers) (2000)
Oughton, D, Hodkinson, S, Faber and
Understanding controls CIBSE KS04 (London: Chartered Institution of Building Services Engineers)
Kell s Heating and air-conditioning of
buildings, 9th ed (Oxford: Elsevier) (2002)
(2005)
Moss K, Heating and water services design
Renewable energy issues for buildings CIBSE TM 38 (London: Chartered Institution of Building Services
in buildings (London: Taylor & Francis)
Engineers) (2006)
(2003)
Day, A, Ratcliffe, M, Shepherd K, Heating Day, A, Ratcliffe, M, Shepherd , K, Heating systems  plant and control (Oxford: Butterworth-
systems  plant and control (Oxford:
Heinemann) (2003)
Butterworth-Heinemann) (2003)
Kavanaugh, S, HVAC simplified (Atlanta
GA: American Society of Heating,
Heating systems
Refrigerating and Air-Conditioning
Heating, ventilation, air conditioning and refrigeration CIBSE Guide B (London: Chartered Institution of
Engineers) (2006)
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)
36 CIBSE Knowledge Series  Variable flow pipework systems