DOE/GO-102001-1113
FS143
June 2001
Air-Source
Heat Pumps
There are two common types of heat An air-source heat pump can provide effi-
pumps: air-source heat pumps and geot- cient heating and cooling for your home,
hermal heat pumps (GHPs). Either one especially if you live in a warm climate.
can keep your home warm in the winter When properly installed, an air-source
and cool in the summer. An air-source heat pump can deliver one-and-a-half to
heat pump pulls its heat indoors from the three times more heat energy to a home
outdoor air in the winter and from the compared to the electrical energy it con-
indoor air in the summer. A GHP extracts sumes. This is possible because a heat
heat from the indoor air when it's hot out- pump moves heat rather than converting
side, but when it's cold outside, it draws it from a fuel, like in combustion heating
heat into a home from the ground, which systems.
maintains a nearly constant temperature
of 50Ú to 60ÚF. This fact sheet focuses on
How They Work
air-source heat pumps, which comprise
You might be wondering how an air-
the majority of all residential heat pump
source heat pump uses the outdoor winter
applications.
air to heat a home. Believe it or not: heat
can be harvested from cold outdoor air
This home in Austin, Texas, features an air-source heat pump.
This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national laboratory. The
document was produced by the Information and Outreach Program at NREL for the DOE Office of Energy Efficiency and Renewable Energy. The Energy Efficiency
and Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc., for NREL / DOE. The statements contained herein are based on
information known to EREC and NREL at the time of printing. No recommendation or endorsement of any product or service is implied if mentioned by EREC.
Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 20% postconsumer waste
Sara Farrar, NREL/PIX05420
down to about 40°F. And this can
Types of Air-Source Heat Pumps
be accomplished through a process
You can use a central heat pump to heat
you're probably already familiar
and cool a whole house. Most central heat
with refrigeration.
pumps are split-systems that is, they
each have one coil indoors and one out-
Basically, a heat pump's refrigeration sys-
doors (see Fig. 1 below). Supply and
tem consists of a compressor, and two
return ducts connect to a central fan,
coils made of copper tubing, which are
which is located indoors. The fan, often
surrounded by aluminum fins to aid heat
called an air handler or blower, circulates
transfer. The coils look much like the radi-
air throughout the house. The fan also
ator in your car. Like in a refrigerator or
usually contains electric resistance coils
air-conditioner, refrigerant flows continu-
You can use a
(some units now have a gas-fired furnace
ously through pipes, back and forth from
option). The heated or cooled air circulates
the outdoor coils. In the heating mode, liq-
central heat pump to
from the fan to the supply ducts, and open-
uid refrigerant extracts heat from the out-
ings in the home called supply registers.
side coils and air, and moves it inside as it
heat and cool a whole
Return registers and ductwork return the
evaporates into a gas. The indoor coils
air to the fan to be heated.
transfer heat from the refrigerant as it con-
house.
denses back into a liquid (see Fig. 1
Some heat pumps are packaged systems.
below). A reversing valve, near the com-
These usually have both coils and the fan
pressor, can change the direction of the
outdoors. Heated or cooled air is delivered
refrigerant flow for cooling as well as for
to the interior from ductwork that
defrosting the outdoor coils in winter (see
protrudes through a wall or roof.
Fig. 2 on page 3).
Another packaged system is the ductless
When outdoor temperatures fall below
room heat pump. These pumps will effi-
40°F, a less-efficient panel of electric resis-
ciently heat or cool a room or small house
tance coils, similar to those in your toaster,
with an open floor plan. They are much
kicks in to provide indoor heating. This is
more common for apartments and motel
why air-source heat pumps aren't always
rooms than homes. They can be installed
very efficient for heating in areas with
in a window or through a hole in the
cold winters. Fuel-burning furnaces gener-
wall wall installations being preferable
ally can provide a more economical way
for appearances sake. Through-the-wall
to heat homes in cooler U.S. climates.
Compressor
Fan
Liquid Gas
evaporates condenses
into gas into a liquid
Expansion
Valve
Outdoor Indoor
Coils Coils
Fig. 1 A split-system heat pump heating cycle
2
Compressor
Gas Liquid
condenses Fan evaporates
into a into gas
liquid
Expansion
Valve
Indoor
Coils
Outdoor
Coils
Fig. 2 A split-system heat pump cooling cycle
installations, however, sometimes aren't The Seasonal Energy Efficiency Ratio
well insulated from inside to outside and (SEER) rates a heat pump's cooling effi-
can have infiltration problems. When ciency. In general, the higher the SEER, the
When selecting a new
used, mini-split systems can solve these higher the cost. However, the energy sav-
problems. ings can return the higher initial invest-
heat pump,
ment several times during the heat
Selecting a Heat Pump
pump s life. Replacing a 1970s vintage,
it s important that
When selecting an air-source heat pump,
central heat pump (SEER = 6) with a new
consider the following three characteris-
unit (SEER=12) will use half the energy to
you determine the
tics carefully: the energy efficiency rating,
provide the same amount of cooling, cut-
sizing, and the system's components.
ting air-conditioning costs in half. The
proper size needed for
most efficient heat pumps have SEERs of
Energy efficiency rating
between 14 and 18.
your home. In the United States, we rate a heat
pump's energy efficiency by how many You'll find the Energy Star® label spon-
British thermal units (Btu) of heat it moves sored by the U.S Department of Energy
for each watt-hour of electrical energy it (DOE) and the U.S. Environmental Protec-
consumes. Every residential heat pump tion Agency (EPA) on heat pumps with
sold in this country has an EnergyGuide an HSPF of at least 7 and a SEER of at
Label, which features the heat pump's least 12. Many new heat pumps exceed
heating and cooling efficiency perfor- these ratings, but looking for this label is a
mance rating, comparing it to other avail- good way to start shopping for one.
able makes and models.
The Heating Seasonal Performance Factor
(HSPF) rates both the efficiency of the
compressor and the electric-resistance ele-
ments. The HSPF gives the number of Btu
harvested per watt-hour used. The most
efficient heat pumps have an HSPF of
between 8 and 10.
3
efficiency not only depend on the selection
Sizing
New Energy-Efficient Heat Pumps and planning of the equipment but also on
When selecting a new heat
careful installation.
pump, it's important that you
The efficiency and performance of
determine the proper size
Consumers and home builders alike tend
today s air-source heat pumps is one-
needed for your home. Bigger
to accept the lowest bid for heating and
and-a-half to two times greater than
is not better. Oversizing
those available 30 years ago. This air-conditioning work. This unfortunate
causes the heat pump to start
improvement in efficiency has resulted
choice can often leave a system lacking 10
and stop more frequently,
from technical advances and options
to 30 percent in the materials and labor
such as: which is less efficient and
necessary to optimize heat-pump
" Thermostatic expansion valves for
harder on the components
performance. Rather than just accepting
more precise control of the refriger-
than letting it run for longer
the lowest bid, it's best to research the
ant flow to the indoor coil
cycles. A properly sized heat
performance records of local contractors,
" Variable speed blowers, which are
pump also will provide you
and get involved in the planning and
more efficient and can compensate
with better comfort and
decision-making about your new heat
for some of the adverse effects of
humidity control than an
pump system.
restricted ducts, dirty filters, and
oversized one.
dirty coils
You can avoid most of the common com-
The heating and cooling
" Improved coil design
fort and performance problems from
capacity of heat pumps is
" Improved electric motor and two-
improper installation by following these
speed compressor designs measured in Btu per hour. The
guidelines:
" Copper tubing, grooved inside to cooling capacity is commonly
" Make your home as energy-efficient as
increase surface area.
expressed in "tons" of cooling
you can with proper insulation, energy-
capacity each ton equaling
efficient windows, and an effective air
12,000 Btu per hour. Correct
barrier, etc. Then your contractor can
sizing procedures involve complex calcu-
install a smaller pump system with
lations, which are best performed by an
shorter duct lengths. In an energy-
experienced contractor, who uses sizing
efficient home, it isn t necessary to run
methods accepted by the heat pump
ducts all the way out to exterior walls to
industry. Don't employ a contractor who
install registers near the exterior walls.
guesses the size of the heat pump needed.
" Install the ducts inside your home s
Rule-of-thumb sizing techniques are gen-
insulation and air barrier, if possible.
erally inaccurate, often resulting in higher
Research shows that this strategy is a
than necessary purchase and annual
major energy saver.
energy costs.
" Insulate your ducts to R-8 if they
must be located in an attic or crawl
System components
space beyond the home s air barrier
You and your contractor should discuss
and insulation.
options that will help improve your
You and your
" Locate the outdoor unit on the northside
home's comfort and the economy of your
of your home if possible. If not, pick a
heat pump. Regarding ducts, for example,
contractor should dis-
shady spot. There should be no obstruc-
it's important to carefully consider their
tions within 10 feet of the sides with
design and materials, as well as the proper
cuss options
openings and the top.
amount of space they require. Check your
" Specify that the measured air leakage
home's blueprints to see if the architect
that will help improve
through your new ducts be less than
and builder have planned adequate space
10 percent of your system s airflow.
for ducts and fans. Heating and cooling
your home s comfort
Air leakage of 5 percent or less is possi-
contractors complain that they often have
ble with careful workmanship.
to squeeze heating and cooling systems
and the economy of
" Tell your contractor that you want a
into spaces that are too small, resulting in
return register in every room.
constricted ducts and inadequate airflow.
your heat pump.
" Don t use building cavities as ducts.
Except for packaged systems, you'll also Building-cavity return ducts are notori-
need to select the proper type of indoor ously leaky and often cause comfort,
coil for adequate summer moisture energy, and moisture problems.
removal. " Pull on ductwork after installation to
make sure it is fastened and sealed well.
Installing a New Heat Pump
(Seal duct joints with mastic.)
A heat pump's performance and energy
4
Air-sealing ducts
Improving Performance
Measurements of heat pump performance
Poor installation, duct losses, and inade-
indicate that duct leakage wastes 10 to 30
quate maintenance are more of a problem
percent of the heating and/or cooling
for heat pumps than for combustion fur-
energy in a typical home. It's one of the
naces. A growing body of evidence sug-
most severe energy problems commonly
gests that most heat pumps have
found in homes because the leaking air is
significant installation or service problems
20° to 70°F warmer than indoor air in win-
that reduce performance and efficiency.
ter and 15° to 30°F cooler in the summer.
According to a report on research funded
by Energy Star more than 50 percent of all
Duct leakage may cause some minor com-
heat pumps have significant problems
fort problems when ducts are located in
with low airflow, leaky ducts, and incor-
conditioned areas. But when leaky ducts
rect refrigerant charge.
are located in an attic or crawl space, the
energy loss is often large. Some of the
Increasing airflow in central
worst duct leakage occurs at joints
heat pumps
Measurements
between the air handler, and the main
The capacity and the efficiency of a heat
supply and return air ducts.
of heat pump
pump depend upon adequate airflow.
Some main return ducts use plywood or
There should be about 400 to 500 cubic
performance
fiberglass duct-board boxes. These boxes
feet per minute (cfm) airflow for each ton
of the heat pump s air-conditioning capac- frequently leak because their joints are
indicate that duct
exposed to the duct system s highest air
ity. Efficiency and performance deteriorate
pressures. Heating and air-conditioning
if airflow is much less than 350 cfm per
leakage waste 10 to 30
contractors often use wall, floor, and ceil-
ton.
ing cavities as return ducts. These build-
An ideal duct system has both a supply
percent of the
ing-cavity return ducts are often
register and a return register for every
accidentally connected to an attic, crawl
room. Most homes, however, have only
heating and/or cool-
space, or even the outdoors, creating seri-
one or two return registers for the entire
ous air leakage. Fiberglass ducts and flex
house. Air from other rooms must find its
ing energy in
ducts are often installed improperly. These
way back to these registers to be reheated
ducts may also deteriorate with age, lead-
or re-cooled. Obstructions in return air are
a typical home.
ing to significant supply-duct leakage.
a common air circulation problem, partic-
ularly from closed interior doors to rooms
The best heating and cooling contractors
with no return-air register.
have equipment to test for duct leakage.
Testing helps locate duct leaks and indi-
Blockage of supply or return air ducts and
cates how much duct sealing is necessary.
registers can pressurize or depressurize
Do not use duct tape for sealing its lifes-
portions of the home, resulting in poor
pan is very short, often less than 6 months.
performance and increased air leakage
through the building envelope. Restric-
Adjusting refrigerant charge
tions to airflow have the greatest impact
Room heat pumps and packaged heat
on the return-air side of the system, so
pumps are charged with refrigerant at the
repairs should start with the return ducts.
factory. They are seldom incorrectly
Air from every supply register must have
charged. Split-system heat pumps, on the
an unobstructed pathway back to a return
other hand, are charged in the field, which
register. You can install louvered grilles
can sometimes result in either too much or
through walls or doors, ducts between
too little refrigerant.
rooms, and/or additional return ducts and
registers to improve air circulation.
Split-system heat pumps that have the
correct refrigerant charge and airflow usu-
Technicians can increase the airflow by
ally perform very close to manufacturer s
cleaning the evaporator coil, increasing fan
listed SEER and HSPF. Too much or too
speed, or enlarging the ducts especially
little refrigerant, however, reduces heat-
return ducts. Enlarging ducts may seem
pump performance and efficiency.
drastic but in some cases, might be the
only remedy for poor comfort and high
energy costs.
5
For satisfactory performance and effi- A defrost control tells the reversing valve
ciency, a split-system heat pump should when to send hot refrigerant outdoors to
be within a few ounces of the correct thaw the outdoor coil during the winter.
charge, specified by the manufacturer. During the 2-to-10-minute defrost cycle,
When the charge is correct, specific refrig- auxiliary heat takes over, reducing the
erant temperatures and pressures listed by heat pump s overall efficiency up to 10
the manufacturer will match temperatures percent. The two most common types of
and pressures measured by your service defrost controls are time-temperature
technician. Verify these measurements and demand-defrost. Time-temperature
with the technician. defrost controls activate defrost at regular
time intervals for set time periods,
If the manufacturer s temperatures and
whether there is ice on the outdoor coil or
pressure s don t match the measured ones,
not. A demand-defrost control senses coil
refrigerant should be added or with-
temperature or airflow through the coil,
drawn, according to standards specified
and only activates defrost if it detects the
by the EPA. Refrigeration systems should
presence of ice. Obviously, choosing a heat
be leak-checked at installation and during
pump with demand-defrost will pay a sig-
each service call.
nificant efficiency dividend.
Manufacturer s say that a technician
For greater efficiency, don't locate a ther-
must measure airflow prior to checking
mostat near a heat source or cold draft
refrigerant charge because the refrigerant
The difference
because they can cause a heat pump to
measurements aren t accurate unless air-
operate erratically. This includes shading
flow is correct.
between the energy
thermostats from direct sunlight. Also, do
not turn the thermostat beyond the
Operating a heat pump
consumption of a
desired temperature. It will not make the
Like combustion heating systems, you
heat pump heat or cool your home any
control heat pumps using thermostats.
well-maintained heat
faster. It will only waste energy. Residents
If you leave and return at regular times
who duel one another over the thermostat
pump and a neglected everyday, you'll save money by using
settings, moving it up and down to suit
automatic thermostats, which minimize
their different comfort levels, cause heat
one ranges from 10 energy use during the times the home is
pumps to operate erratically and ineffi-
unoccupied. However, choosing an auto-
ciently.
to 25 percent. matic thermostat s reactivation time
Maintaining and Servicing
requires considering the duration of heat-
Heat-pump performance will deteriorate
pump operation necessary to restore a
comfortable temperature. During the heat- without regular maintenance and service.
The difference between the energy con-
ing season, some homeowners also set
sumption of a well-maintained heat pump
their thermostats back 10°F, manually or
and a severely neglected one ranges from
automatically, when they leave home or
10 to 25 percent.
go to bed.
A two-stage thermostat controls the heat- Regular Maintenance
ing. The first stage activates the refrigera-
Either the homeowner or service techni-
tion system. If it s too cold outside for the
cian can perform the following routine
refrigeration system to counteract the
maintenance tasks:
home s heat loss, then the thermostat s
second stage activates the electric resis- " Clean or replace filters regularly (every
2 to 6 months, depending on operating
tance coils. An outdoor thermostat will
time and amount of dust in the environ-
prevent the less efficient electric resistance
ment).
heat from coming on until the outdoor
temperature falls below 40°F. An outdoor
" Clean outdoor coils as often as
thermostat also will prevent auxiliary heat
necessary (when dirt is visible on the
from activating when an automatic ther-
outside of the coil).
mostat is warming the house after a set-
" Remove plant life and debris from
back period. Use setback thermostats that
around the outdoor unit.
are only for heat pumps.
6
" Clean evaporator coil and condensate " Verify correct refrigerant charge by mea-
pan every 2 to 4 years. surement.
" Clean the blower s fan blades. " Check for refrigerant leaks.
" Clean supply and return registers and " Inspect electric terminals, and if neces-
straighten their fins. sary, clean and tighten connections, and
apply nonconductive coating.
Professional Service
" Lubricate motors, and inspect belts for
You should have a professional technician
tightness and wear.
service your heat pump at least every year.
The technician can: " Verify correct electric control, making
sure that heating is locked out when the
" Inspect ducts, filters, blower, and indoor
thermostat calls for cooling and vice
coil for dirt and other obstructions.
versa.
" Diagnose and seal duct leakage.
" Verify correct thermostat operation.
" Verify adequate airflow by measurement.
Fax: [phone]
Resources
E-mail: [e-mail]
The following are sources of additional information on
Web site: www.acca.org/
heat pumps. This list is not exhaustive, nor does the
A national, trade association of heating, ventilation,
mention of any resource constitute a recommendation
air-conditioning, and refrigeration contractors.
or endorsement.
Air-Conditioning and Refrigeration Institute (ARI)
Ask an Energy Expert
4301 N. Fairfax Drive, Suite 425
DOE s Energy Efficiency and Renewable Energy
Arlington, VA 22203
Clearinghouse (EREC)
Phone: [phone]
P.O. Box 3048
Fax: [phone]
Merrifield, VA 22116
E-mail: [e-mail]
1-800-DOE-EREC (363-3732)
Web site: www.ari.org/
E-mail: [e-mail]
A national trade association representing manufacturers of
Online submittal form:
U.S. produced central air-conditioning and commercial refriger-
www.eren.doe.gov/menus/energyex.html
ation equipment.
Consumer Energy Information Web site:
www.eren.doe.gov/consumerinfo/
Consortium for Energy Efficiency (CEE)
Energy experts at EREC provide free general and
One State Street, Suite 1400
technical information to the public on many topics
Boston, MA 02109-3507
and technologies pertaining to energy efficiency and
Phone: [phone]
renewable energy.
Fax: [phone]
Web site: www.ceeformt.org/
DOE s Energy Efficiency and Renewable Energy
Network (EREN) A national, nonprofit, benefits corporation that promotes
the manufacture and purchase of energy-efficient products
Web site: www.eren.doe.gov
and services.
Your comprehensive online resource for DOE s energy
efficiency and renewable energy information.
Organizations
Air Conditioning Contractors of America (ACCA)
2800 Shirlington Rd., Suite 300
Arlington, VA 22206
Phone: [phone] (Continued on page 8)
7
(Continued from page 7)
Further Reading
Consumer Guide to Home Energy Savings, Wilson A.,
Eastern Heating & Cooling Council (EH-CC)
Thorne J., Morrill J, American Council for an Energy-
20,000 Horizon Way, Suite 260 Efficient Economy (ACE3), Washington, DC.
Mt. Laurel, NJ 08054 Available from ACE3 at [phone],
E-mail: [e-mail] [e-mail], or online at www.aceee.org.
Web site: www.eh-cc.org/
Energy Savers: Tips on Saving Energy and Money
Educates consumers and contractors on properly designed
at Home, DOE. Available from EREC (see Ask an Energy
and installed high-efficiency heating and cooling systems.
Expert above) or online in PDF and HTML at
www.eren.doe.gov/consumerinfo/energy_savers/.
Energy Star®
DOE and EPA
Geothermal Heat Pumps Make Sense for Homeowners,
Phone: (888) STAR-YES (782-7937)
DOE Office of Geothermal Technologies, 1998.
E-mail: [e-mail]/
Available from EREC (see Ask an Energy Expert above)
Web site: www.energystar.gov/
or online in PDF at www.eren.doe.gov/
geothermal/pdf/26161b.pdf or in HTML at
Provides lists of energy-efficient, Energy Star®-qualified
www.eren.doe.gov/erec/factsheets/
products, including heat pumps.
ghp_homeowners.html.
Home Energy Magazine
Residential Energy: Cost Savings and Comfort for
2124 Kittredge Street, #95
Existing Buildings , Krigger J., Saturn Resource
Berkeley, CA 94704
Management, Helena, MT 2000. Available from Saturn
Phone: [phone]
at [phone].
E-mail: [e-mail],
Web site: www.homeenergy.org/
A source of information on reducing energy
consumption in the home.
Web Sites
Heat Pumps
The Energy Outlet
Web site: www.energyoutlet.com/res/heatpump/
Heat Pumps for Heating and Cooling
DOE Office of Building Technology, State and
Community Programs
Web site: www.eren.doe.gov/buildings/
ee_heatpump.html
International Energy Agency Heat Pump Centre
Web site: www.heatpumpcentre.org/
Top-Rated Energy-Efficient Appliances
American Council for an Energy-Efficient Economy
Web site: www.aceee.org/consumerguide/2000enef.htm
Why Buy Energy-Efficient Appliances?
DOE Office of Codes and Standards
Web site: www.eren.doe.gov/buildings/
consumer_information/
8