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Air-Source Heat Pumps
Air-source heat pumps draw heat from the outside air during the heating
season and reject heat outside during the summer cooling season.
There are two types of air-source heat pumps. The most common is the
air-to-air heat pump. It extracts heat from the air and then transfers
heat to either the inside or outside of your home depending on the season.
The other type is the air-to-water heat pump, which is used in homes
with hydronic heat distribution systems. During the heating season, the
heat pump takes heat from the outside air and then transfers it to the
water in the hydronic distribution system. If cooling is provided during
the summer, the process is reversed: the heat pump extracts heat from
the water in the home's distribution system and "pumps"
it outside to cool the house. These systems are rare, and many don't
provide cooling; therefore, most of the following discussion focuses on
air-to-air systems.
More recently, ductless mini-split heat pumps have been introduced to
the Canadian market. They are ideal for retrofit in homes with hydronic
or electric resistance baseboard heating. They are wall-mounted, free-air
delivery units that can be installed in individual rooms of a house. Up
to eight separate indoor wall-mounted units can be served by one outdoor
section.
Air-source heat pumps can be add-on, all-electric or bivalent. Add-on
heat pumps are designed to be used with another source of supplementary
heat, such as an oil, gas or electric furnace. All-electric air-source
heat pumps come equipped with their own supplementary heating system in
the form of electric-resistance heaters. Bivalent heat pumps are a special
type, developed in Canada, that use a gas or propane fired burner to increase
the temperature of the air entering the outdoor coil. This allows these
units to operate at lower outdoor temperatures.
Air-source heat pumps have also been used in some home ventilation systems
to recover heat from outgoing stale air and transfer it to incoming fresh
air or to domestic hot water.
How Does an Air-Source Heat Pump Work?
An air-source heat pump has three cycles: the heating cycle, the cooling
cycle and the defrost cycle.
The Heating Cycle
During the heating cycle, heat is taken from outdoor air and "pumped"
indoors.
- First, the liquid refrigerant passes through the expansion device,
changing to a low-pressure liquid/vapour mixture. It then goes to the
outdoor coil, which acts as the evaporator coil. The liquid refrigerant
absorbs heat from the outdoor air and boils, becoming a low-temperature
vapour.
- This vapour passes through the reversing valve to the accumulator,
which collects any remaining liquid before the vapour enters the compressor.
The vapour is then compressed, reducing its volume and causing it to
heat up.
- Finally, the reversing valve sends the gas, which is now hot, to
the indoor coil, which is the condenser. The heat from the hot gas is
transferred to the indoor air, causing the refrigerant to condense into
a liquid. This liquid returns to the expansion device and the cycle
is repeated. The indoor coil is located in the ductwork, close to the
furnace.
The ability of the heat pump to transfer heat from the outside air to
the house depends on the outdoor temperature. As this temperature drops,
the ability of the heat pump to absorb heat also drops.
At the outdoor ambient balance point temperature, the heat pump's
heating capacity is equal to the heat loss of the house.
Below this outdoor ambient temperature, the heat pump can supply only
part of the heat required to keep the living space comfortable, and supplementary
heat is required.
When the heat pump is operating in the heating mode without any supplementary
heat, the air leaving it will be cooler than air heated by a normal furnace.
Furnaces generally deliver air to the living space at between 55°C
and 60°C. Heat pumps provide air in larger quantities at about 25°C
to 45°C and tend to operate for longer periods.
The Cooling Cycle
The cycle described above is reversed to cool the house during the summer.
The unit takes heat out of the indoor air and rejects it outside.
- As in the heating cycle, the liquid refrigerant passes through the
expansion device, changing to a low-pressure liquid/vapour mixture.
It then goes to the indoor coil, which acts as the evaporator. The liquid
refrigerant absorbs heat from the indoor air and boils, becoming a low-temperature
vapour.
- This vapour passes through the reversing valve to the accumulator,
which collects any remaining liquid, and then to the compressor. The
vapour is then compressed, reducing its volume and causing it to heat
up.
- Finally, the gas, which is now hot, passes through the reversing
valve to the outdoor coil, which acts as the condenser. The heat from
the hot gas is transferred to the outdoor air, causing the refrigerant
to condense into a liquid. This liquid returns to the expansion device,
and the cycle is repeated.
During the cooling cycle, the heat pump also dehumidifies the indoor
air. Moisture in the air passing over the indoor coil condenses on the
coil's surface and is collected in a pan at the bottom of the coil.
A condensate drain connects this pan to the house drain.
The Defrost Cycle
If the outdoor temperature falls to near or below freezing when the heat
pump is operating in the heating mode, moisture in the air passing over
the outside coil will condense and freeze on it. The amount of frost buildup
depends on the outdoor temperature and the amount of moisture in the air.
This frost buildup decreases the efficiency of the coil by reducing its
ability to transfer heat to the refrigerant. At some point, the frost
must be removed. To do this, the heat pump will switch into the defrost
mode.
- First, the reversing valve switches the device to the cooling mode.
This sends hot gas to the outdoor coil to melt the frost. At the same
time the outdoor fan, which normally blows cold air over the coil, is
shut off in order to reduce the amount of heat needed to melt the frost.
- While this is happening, the heat pump is cooling the air in the
ductwork. The heating system would normally warm this air as it is distributed
throughout the house.
One of two methods is used to determine when the unit goes into defrost
mode. Demand-frost controls monitor airflow, refrigerant pressure, air
or coil temperature and pressure differential across the outdoor coil
to detect frost accumulation on the outdoor coil.
Time-temperature defrost is started and ended by a preset interval timer
or a temperature sensor located on the outside coil. The cycle can be
initiated every 30, 60 or 90 minutes, depending on the climate and the
design of the system.
Unnecessary defrost cycles reduce the seasonal performance of the heat
pump. As a result, the demand-frost method is generally more efficient
since it starts the defrost cycle only when it is required.
Figure 2a: Components of an Air-source
Heat Pump (Heating Cycle)
![Components of an Air-source Heat Pump (Heating Cycle)](/web/20061103101302im_/http://www.oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/images/fig2ae.gif)
Figure 2b: Components of an Air-source
Heat Pump (Cooling Cycle)
![Components of an Air-source Heat Pump (Cooling Cycle)](/web/20061103101302im_/http://www.oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/images/fig2be.gif)
Parts of the System
The components of an air-source heat pump are shown in Figure
2a and Figure 2b. In addition to the indoor and
outdoor coils, the reversing valve, the expansion device, the compressor,
and the piping, the system has fans that blow air over the coils and a
supplementary heat source. The compressor can be located indoors or outdoors.
If the heat pump is all-electric, supplementary heat will be supplied
by a series of resistance heaters located in the main air-circulation
space or plenum downstream of the heat pump indoor coil. If the heat pump
is an add-on unit (see Figure 3), the supplementary
heat will be supplied by a furnace. The furnace may be electric, oil,
natural gas or propane. The indoor coil of the heat pump is located in
the air plenum, usually just above the furnace. See the section titled
"Supplementary Heating Systems",
for a description of the operation of a heat pump and furnace combination.
In the case of a ductless mini-split heat pump, supplementary heat can
be provided by the existing hydronic or electric resistance baseboard
heaters.
Figure 3: Add-On Heat Pump
![Add-On Heat Pump](/web/20061103101302im_/http://www.oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/images/fig3e.gif)
Energy Efficiency Considerations
The annual cooling efficiency (SEER) and heating efficiency (HSPF) of
an air-source heat pump are affected by the manufacturer's choice
of features. At the time of this publication, the SEER of air-source heat
pumps ranged from a minimum of 10 to a maximum of about 17. The HSPF for
the same units ranged from a minimum of 5.9 to a maximum of 8.6, for a
Region V climate as required in CSA C656. Region V has a climate similar
to that of Ottawa.
The minimum efficiency levels above are currently regulated in a number
of jurisdictions. New minimum efficiency requirements are scheduled to
come into effect across Canada in 2006. The minimum SEER will likely be
13, and the minimum HSPF will be 6.7. These levels represent a significant
improvement over the average sales-weighted efficiency from only a few
years ago. More efficient compressors, larger heat exchanger surfaces,
improved refrigerant flow and other controls are largely responsible for
these gains. New developments in compressors, motors and controls will
push the limits of efficiency even higher.
More advanced compressor designs by different manufacturers (advanced
reciprocating, scroll, variable-speed or two-speed compressors combined
with current best heat exchanger and control designs) permit SEERs as
high as 17 and HSPFs of up to 8.6 for Region V.
Air-source heat pumps at the lower end of the efficiency range are characterized
as having single-speed reciprocating compressors. Higher efficiency units
generally incorporate scroll or advanced reciprocating compressors, with
no other apparent design differences. Heat pumps with the highest SEERs
and HSPFs invariably use variable- or two-speed scroll compressors.
Figure 4: Air-Source Heat Pump Efficiency
(Region V)
![Air-Source Heat Pump Efficiency (Region V)](/web/20061103101302im_/http://www.oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/images/fig4e.gif)
Note: Indicated values represent the range
of all available equipment.
The EnerGuide Ratings for Heat Pumps
Natural Resources Canada (NRCan) and the Heating, Refrigerating and
Air Conditioning Institute of Canada (HRAI) have established an industry-managed
energy efficiency rating system for furnaces, central air conditioners
and air-to-air heat pumps. The energy efficiency rating scale appears
under the EnerGuide logo on the manufacturers' brochures (see Figure
5). As with the EnerGuide label for room air conditioners, the inverted
triangle and graduated bar can be used to compare a particular model with
other model designs and types.
Figure 5: EnerGuide Rating for Central
Air Conditioners and Heat Pumps
![EnerGuide Rating for Central Air Conditioners and Heat Pumps](/web/20061103101302im_/http://www.oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/images/fig5e.gif)
ENERGY STAR®
![Energy Star](/web/20061103101302im_/http://www.oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/images/energystar.gif)
Today's ENERGY STAR® qualified air-to-air heat pumps
use up to 20 percent less energy than standard new models. The ENERGY
STAR specifications require that the EnerGuide SEER rating be 12.0 or
greater for a single package unit or 13.0 or greater for a split system.
By choosing to buy an ENERGY STAR qualified heat pump that is sized correctly
for your home, you can help to reduce emissions of GHGs and smog precursors,
realize substantial electrical savings and increase your household's
comfort.
Other Selection Considerations
Select a unit with as high an HSPF as practical. For units with comparable
HSPF ratings, check their steady-state ratings at –8.3°C, the
low temperature rating. The unit with the higher value will be the most
efficient one in most regions of Canada.
Select a unit with demand-defrost control. This minimizes defrost cycles
(system reversals are hard on the machine), which reduces supplementary
and heat pump energy use.
The sound rating is a tone-corrected, A-weighted sound power level, expressed
in bels. Select a heat pump with an outdoor sound rating in the vicinity
of 7.6 bels or lower if possible. The sound rating is an indicator of
the sound power level of the heat pump outdoor unit. The lower the value,
the lower the sound power emitted by the outdoor unit. These ratings are
available from the manufacturer and are published by the Air-Conditioning
and Refrigeration Institute (ARI), 4301 North Fairfax Drive, Arlington,
Virginia 22203, U.S.A.
Sizing Considerations
Heating and cooling loads should be determined by using a recognized
sizing method such as CSA F280-M90, "Determining the Required Capacity
of Residential Space Heating and Cooling Appliances."
While a heat pump can be sized to provide most of the heat required
by a house, this is not generally a good idea. In Canada, heating loads
are larger than cooling loads. If the heat pump is sized to match the
heating load, it will be too large for the cooling requirement, and will
operate only intermittently in the cooling mode. This may reduce performance
and the unit's ability to provide dehumidification in the summer.
Also, as the outdoor air temperature drops, so does the efficiency of
an air-source heat pump. Consequently, it doesn't make economic
sense to try to meet all your heating needs with an air-source heat pump.
As a rule, an air-source heat pump should be sized to provide no more
than 125 percent of the cooling load. A heat pump selected in this manner
would meet about 80 to 90 percent of the annual heating load, depending
on climate zone, and would have a balance point between 0°C and –5°C.
This generally results in the best combination of cost and seasonal performance.
Installation Considerations
In installing any kind of heat pump, it is most important that the contractor
follow manufacturers' instructions carefully. The following are
general guidelines that should be taken into consideration when installing
an air-source heat pump:
- In houses with a natural gas, oil or wood furnace, the heat pump
coil should be installed on the warm (downstream) side of the furnace.
- If a heat pump is added to an electric furnace, the heat pump coil
can usually be placed on the cold (upstream) side of the furnace for
greatest efficiency.
- The outdoor unit should be protected from high winds, which may reduce
efficiency by causing defrost problems. At the same time, it should
be placed in the open so that outdoor air is not recirculated through
the coil.
- To prevent snow from blocking airflow over the coil and to permit
defrost water drainage, the unit should be placed on a stand that raises
it 30 to 60 cm (12 to 24 in.) above the ground. The stand should be
anchored to a concrete pad, which in turn should sit on a bed of gravel
to enhance drainage. Alternatively, the unit might be mounted from the
wall of the house on a suitably constructed frame.
- It is advisable to locate the heat pump outside the drip-line of
the house (the area where water drips off the roof) to prevent ice and
water from falling on it, which could reduce airflow or cause fan or
motor damage.
- The pan under the inside coil must be connected to the house's
interior floor drain, to ensure that the condensate that forms on the
coil drains properly.
- The heat pump should be placed so that a serviceperson has enough
room to work on the unit.
- Refrigerant lines should be as short and straight as possible. It
is good practice to insulate the lines to minimize unwanted heat loss
and to prevent condensation.
- Fans and compressors make noise. Locate the outdoor unit away from
windows and adjacent buildings. Some units make additional noise when
they vibrate. You can reduce this by selecting quiet equipment or by
mounting the unit on a noise-absorbing base.
- Heat pump systems generally require larger duct sizes than other
central heating systems, so existing ducting may have to be modified.
For proper heat pump operation, airflow should be 50 to 60 litres per
second (L/s) per kilowatt, or 400 to 450 cubic feet per minute (cfm)
per ton, of cooling capacity.
The cost of installing an air-source heat pump varies depending on the
type of system and the existing heating equipment. Costs will be higher
if the ductwork has to be modified, or if you need to upgrade your electrical
service to deal with the increased electrical load.
Operation Considerations
The indoor thermostat should be set at the desired comfort temperature
(20°C is recommended) and not readjusted.
Continuous indoor fan operation can reduce the overall efficiency achieved
by a heat pump system, unless a high-efficiency variable-speed fan motor
is used. Operate this system with the "auto" fan setting on
the thermostat.
Heat pumps have longer operation times than conventional furnaces because
their heating capacity is considerably lower.
Major Benefits of Air-Source Heat Pumps
Efficiency
At 10°C, the coefficient of performance (COP) of air-source heat
pumps is typically about 3.3. This means that 3.3 kilowatt hours (kWh)
of heat are transferred for every kWh of electricity supplied to the heat
pump. At –8.3°C, the COP is typically 2.3.
The COP decreases with temperature because it is more difficult to extract
heat from cooler air. Figure 6 shows how the COP is
affected by cooler air temperature. Note, however, that the heat pump
compares favourably with electric resistance heating (COP of 1.0) even
when the temperature falls to –15°C.
Figure 6: Performance Characteristics
of a Typical Air-Source Heat Pump
![Performance Characteristics of a Typical Air-Source Heat Pump](/web/20061103101302im_/http://www.oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/images/fig6e.gif)
Air-source heat pumps will operate with heating seasonal performance
factors (HSPFs) that vary from 6.7 to 10.0, depending on their location
in Canada and their rated performance. Figure 7 shows
the range of performance of air-source heat pumps operating in various
regions in Canada. For this booklet, we have identified three regions
where it would be viable to use air-source heat pumps. The first region
is the West Coast, characterized as mild with high heat pump performance.
The second region – southern Ontario, Nova Scotia and interior British
Columbia – is colder, and requires a heat pump with higher performance.
The third region includes colder regions in British Columbia, Alberta,
Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island, and
Newfoundland and Labrador. Outside these regions, air-source heat pumps
are not as economically attractive.
Figure 7: Heating Seasonal Performance
Factors (HSPFs) for Air-Source Heat Pumps for various locations in Canada
![Heating Seasonal Performance Factors (HSPFs) for Air-Source Heat Pumps for various locations in Canada](/web/20061103101302im_/http://www.oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/images/fig7e.gif)
8.7 to 10.0
Chilliwack, B.C.
Nanaimo, B.C.
Richmond, B.C.
Vancouver, B.C.
Victoria, B.C. |
7.4 to 8.5
Kelowna, B.C.
Nelson, B.C.
Penticton, B.C.
Chatham, Ont.
Hamilton, Ont.
Niagara Falls, Ont.
Toronto, Ont.
Windsor, Ont.
Halifax, N.S.
Yarmouth, N.S. |
6.7 to 7.4
Kamloops, B.C.
Prince Rupert, B.C.
Lethbridge, Alta.
Medicine Hat, Alta.
Maple Creek, Sask.
Barrie, Ont.
Kingston, Ont.
Kitchener, Ont.
London, Ont.
North Bay, Ont.
Ottawa, Ont.
Sault Ste. Marie, Ont.
Sudbury, Ont.
Montréal, Que.
Québec, Que.
Sherbrooke, Que.
Moncton, N.B.
Saint John, N.B.
Amherst, N.S.
Sydney, N.S.
Charlottetown, P.E.I.
Grand Bank, N.L.
St. John's, N.L. |
Note: Indicated values represent the range
from "standard-efficiency" to "high-efficiency" equipment.
Energy Savings
You may be able to reduce your heating costs by up to 50 percent if
you convert from an electric furnace to an all-electric air-source heat
pump. Your actual savings will vary, depending on factors such as local
climate, the efficiency of your current heating system, the cost of fuel
and electricity, and the size and HSPF of the heat pump installed.
More advanced designs of air-source heat pumps can provide domestic water
heating. Such systems are called "integrated" units because
heating of domestic water has been integrated with a house space-conditioning
system. Water heating can be provided with high efficiency in this way.
Water heating bills can be reduced by 25 to 50 percent.
Maintenance
Proper maintenance is critical to ensure that your heat pump operates
efficiently and has a long service life. You can do some of the simple
maintenance yourself, but you may also want to have a competent service
contractor do an annual inspection of your unit. The best time to service
your unit is at the end of the cooling season, prior to the start of the
next heating season.
- Filter and coil maintenance has a dramatic impact on system performance
and service life. Dirty filters, coils and fans reduce airflow through
the system. This reduces system performance, and can lead to compressor
damage if it continues for extended periods of time.
Filters should be inspected monthly and cleaned or replaced as required
by the manufacturer's instructions. The coils should be vacuumed
or brushed clean at regular intervals as indicated in the manufacturer's
instruction booklet. The outdoor coil may be cleaned using a garden hose.
While cleaning filters and coils, look for symptoms of other potential
problems such as those described on the following page.
- The fan should be cleaned but the fan motor should only be lubricated
if the manufacturer instructions specify this. This should be done annually
to ensure that the fan provides the airflow required for proper operation.
The fan speed should be checked at the same time. Incorrect pulley settings,
loose fan belts, or incorrect motor speeds in the case of direct drive
fans can all contribute to poor performance.
- Ductwork should be inspected and cleaned as required to ensure that
airflow is not restricted by loose insulation, abnormal buildup of dust,
or any other obstacles that occasionally find their way through the
grilles.
- Be sure that vents and registers are not blocked by furniture, carpets
or other items that can block airflow. As noted earlier, extended periods
of inadequate airflow can lead to compressor damage.
You will need to hire a competent service contractor to do more difficult
maintenance such as checking the refrigerant level and making electrical
or mechanical adjustments.
Service contracts are similar to those for oil and gas furnaces. But
heat pumps are more sophisticated than conventional equipment and, therefore,
can have higher average service costs.
Operating Costs
The energy costs of a heat pump can be lower than those of other heating
systems, particularly electric or oil heating systems.
However, the relative savings will depend on whether you are currently
using electricity, oil, propane or natural gas, and on the relative costs
of different energy sources in your area. By running a heat pump, you
will use less gas or oil, but more electricity. If you live in an area
where electricity is expensive, your operating costs may be higher. Depending
on these factors, the payback period for investment in an air-source heat
pump rather than a central air conditioner could be anywhere from two
to seven years. Later in this booklet, heating energy cost comparisons
between air-source and ground-source heat pumps and electric and oil heating
systems will be made.
Life Expectancy and Warranties
Air-source heat pumps have a service life of between 15 and 20 years.
The compressor is the critical component of the system.
Most heat pumps are covered by a one-year warranty on parts and labour,
and an additional five- to ten-year warranty on the compressor (for parts
only). However, warranties vary between manufacturers, so check the fine
print.
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