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Personal: Residential

Air-source Heat Pumps

Air-source heat pumps draw heat from the outside air during the heating season and dump heat outside during the summer cooling season.

The most common type of air-source heat pump is the air-to-air heat pump. It extracts heat from the air and then transfers it to either inside or outside your home depending on the season.

Air-to-water systems are rare, and many don't provide cooling; this discussion focuses on air-to-air systems.

Ductless, multisplit heat pumps have recently been introduced to the Canadian market. They are ideal for retrofitting in homes with hydronic or electric resistance baseboard heating. They are wall-mounted, free air delivery units which can be installed in individual rooms of a house. Up to 20 separate indoor wall-mounted units can be served by one outdoor section.

Air-source heat pumps can be either add-on, all-electric, or bi-valent.

  • 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 heat pumps come equipped with their own supplementary heating system in the form of electric-resistance heaters.
  • Bi-valent 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 are also used in some home ventilation systems to recover heat from outgoing stale air and to transfer it to incoming fresh air or to the domestic hot water.

How Does an Air-source Heat Pump Work?

The air-source heat pump has three cycles:

  • the heating cycle
  • the cooling cycle
  • the defrost cycle

The Heating Cycle

During the heating cycle, heat is extracted 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.

The reversing valve sends this vapour to the accumulator, which collects any remaining liquid before the vapour passes to 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 acts as 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 ability of the heat pump to transfer heat from the outside air to the house depends on the outdoor temperature. As this temperature drops, so does the ability of the heat pump to absorb heat (the unit's capacity).

At the outdoor ambient balance point temperature, the heat pump's capacity is equal to the heat loss of the house. Below this outdoor ambient temperature, the heat pump cannot supply all the heat required to keep the living space comfortable, and supplementary heaters must be used.

When the heat pump is operating in the heating mode without any supplementary heat, the air leaving it will be cooler than air leaving a 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 29°C to 43°C.

The Cooling Cycle

The heating cycle is reversed to cool the house during the summer. The unit takes heat out of the indoor air and transfers 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.

The reversing valve sends this vapour to the accumulator, which collects any remaining liquid, and then to the compressor. The vapour is then compressed. As its volume is reduced, it heats up.

Finally, the reversing valve sends the heated gas to the outdoor coil, which acts as the condenser. The heat from the 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 build-up depends on the outdoor temperature and the amount of moisture in the air.

This frost build-up 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 dumping cool air into the house. The supplementary heating system can be used to warm this air before 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 air flow, 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.

Parts of the System

Figure 1: Components of an Air-source Heat Pump (Heating Cycle)

Figure 1: Components of an Air-source Heat Pump (Heating Cycle)

Figure 2: Components of an Air-source Heat Pump (Cooling Cycle)

Figure 2: Components of an Air-source Heat Pump (Cooling Cycle)

Like other heating sources, air-source heat pumps have indoor and outdoor coils, a reversing valve, an expansion device, a compressor, and piping. In addition, however, the system has fans that blow air over the coils and a supplementary heat source. The compressor can be located indoors or outdoors.

Figure 3: Add-On Heat Pump

Figure 3: Add-On Heat Pump

If the heat pump is all-electric, supplementary heat will be supplied by a series of resistance heaters located in the main air-circulation plenum downstream of the heat pump indoor coil. If the heat pump is an add-on unit, 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. In the case of a ductless minisplit heat pump, supplementary heat can be provided by the existing hydronic or electric resistance baseboard heaters.

Supplementary Heating Systems

Most heat pump installations require a supplementary heating system. Air-source heat pumps are usually set to shut off at either the thermal or economic balance point. In an air-source heat pump, supplementary heat (also called backup or auxiliary heat) may also be required during the defrost cycle.

Supplementary heat can be supplied by any type of heating system, as long as it can be activated by the thermostat controlling the heat pump. However, most supplementary heating systems are central furnaces that use oil, gas, or electricity. Many new ground-source heat pumps use duct heaters to supply auxiliary heat.

Figure 4: Balance Point for a Typical Air-Source Heat Pump

Figure 4: Balance Point for a Typical Air-Source Heat Pump

The figure above shows the thermal balance point for a typical air-source heat pump. To the right of the thermal balance point, the heat pump is capable of satisfying all of the home's heating requirements. To the left of the thermal balance point, the house heat loss is greater than the heat pump's capacity; this is when supplementary heat is required.

In this shaded area, the heat pump can operate in two ways. If heat pump operation is unrestricted by outdoor temperature, it will operate to satisfy first stage heating requirements each time heat is called for by the thermostat. When second stage heat is called for, the heat pump shuts off if it is an add-on unit, or continues to operate if it is an all-electric heat pump system, and the supplementary heating system provides heat until all heating requirements have been satisfied.

If heat pump operation is restricted, an outdoor temperature sensor shuts the heat pump when the temperature falls below a preset limit. Below this temperature, only the supplementary heating system operates. The sensor is usually set to shut off at the temperature corresponding to the economic balance point, or at the outdoor temperature below which it is cheaper to heat with the supplementary heating system instead of the heat pump.

ENERGY STAR Qualified Air-to-air Heat Pumps

ENERGY STAR qualified air-to-air heat pumps meet the following requirements:

  • a seasonal energy efficiency ratio (SEER) of 14.5 for split systems
  • a heating seasonal performance factor (HSPF) of 7.1

List of models: air source heat pumps
Manufactures of ENERGY STAR qualified models

Manufacturers or retailers place the ENERGY STAR symbol on models shown to meet or exceed the ENERGY STAR energy-efficiency criteria. Today, most leading manufacturers of home heating and cooling equipment are producing high-efficiency systems that qualify for the ENERGY STAR symbol.

You can usually locate the ENERGY STAR symbol on the back of the manufacturer’s brochures, beside the EnerGuide rating box. Use the EnerGuide rating to determine the SEER and HSPF ratings and locate the ENERGY STAR symbol to find the most efficient product available for you.

A high-efficiency air-to-air heat pump will work best in conjunction with an ENERGY STAR programmable thermostat. These thermostats have four daily settings, weekend/day settings, and other energy comfort features. They automatically adjust the temperature to the comfort setting you choose, lowering it while you are away at work, raising it when you are at home and providing flexibility for weekend use.

Efficiency

In Canada, where air temperatures can go below –30°C, and where winter ground temperatures are generally in the range of –2°C to 4°C, earth-energy systems have a coefficient of performance (COP) of between 2.5 and 3.8.

The HSPFs in Figures 14 and 15 were calculated using a procedure very similar to that used for air-source heat pumps, but taking into account industry-sizing practice and regional ground water temperatures across Canada. Since earth-energy heat systems have both COP and EER standard performance ratings, it was necessary to calculate heating seasonal performance to compare operating costs with those of air-source heat pumps.

A ground water EES installation in southern Canada will have a heating seasonal performance factor (HSPF) of between 10.7 and 12.8, compared with an HSPF of 3.4 for electrical-resistance heating. Similarly, a closed-loop EES in southern Canada will have an HSPF of between 9.2 and 11.0, with the higher value achieved by the most efficient closed-loop heat pump available. Figure 5 shows the HSPFs of ground water earth energy systems operating in different climatic regions in Canada, while Figure 6 shows the same for closed-loop EESs.

Energy Savings

Earth-energy systems will reduce your heating and cooling costs substantially. Energy-cost savings compared with electric furnaces are around 65 percent.

On average, an EES will yield savings that are about 40 percent more than would be provided by an air-source heat pump. This is due to the fact that underground temperatures are higher in winter than air temperatures. As a result, an EES can provide more heat over the course of the winter than an air-source heat pump.

Figure 5: Heating Seasonal Performance Factors (HSPFs) for Ground Water or Open System EESs in Canada (left to right)

Heating Seasonal Performance Factors

green square
HSPF 10.8 to13
pale green square
HSPF 10.7 to 11.8
gray square
HSPF 10.1 to 12.0
light gray square
HSPF 9.9 to 11.7
Chilliwack, B.C.
Nanaimo, B.C.
Richmond, B.C.
Vancouver, B.C.
Victoria, B.C.
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.
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.
Prince George, B.C.
Banff, Alta.
Calgary, Alta.
Edmonton, Alta.
Peace River, Alta.
Prince Albert, Sask.
Regina, Sask.
Saskatoon, Sask.
Brandon, Man.
Winnipeg, Man.
Thunder Bay, Ont.
Timmins, Ont.
Chicoutimi, Que.
Rimouski, Que.
Shawinigan, Que.
Edmundston, N.B.

Note: Indicated values represent the range from "standard-efficiency" to "high-efficiency" equipment.

Figure 6: Heating Seasonal Performance Factors (HSPFs) for Closed-Loop EESs in Canada (left to right)

Heating Seasonal Performance Factors

green square
HSPF 9.3 to 11.1
pale green square
HSPF 9.2 to 11.0
gray square
HSPF 8.9 to 10.6
light gray square
HSPF 8.7 to 10.4
Chilliwack, B.C.
Nanaimo, B.C.
Richmond, B.C.
Vancouver, B.C.
Victoria, B.C.
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.
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.
Prince George, B.C.
Banff, Alta.
Calgary, Alta.
Edmonton, Alta.
Peace River, Alta.
Prince Albert, Sask.
Regina, Sask.
Saskatoon, Sask.
Brandon, Man.
Winnipeg, Man.
Thunder Bay, Ont.
Timmins, Ont.
Chicoutimi, Que.
Rimouski, Que.
Shawinigan, Que.
Edmundston, N.B.

Note: Indicated values represent the range from "standard-efficiency" to "high-efficiency" equipment.

Actual energy savings will vary depending on the local climate, the efficiency of the existing heating system, the costs of fuel and electricity, the size of the heat pump installed, and its coefficient of performance at CSA rating conditions. Later in this booklet, heating energy-cost comparisons will be made between earth-energy systems and electric heating systems, as well as air-source heat pumps.

Domestic Hot Water Heating

EESs also provide savings in domestic hot water costs. Some have a desuperheater that uses some of the heat collected to preheat hot water; newer designs can automatically switch over to heat hot water on demand. These features can reduce your water heating bill by 25 to 50 percent.

Heating Energy Cost Comparison: Heat Pump and Electric Heating Systems

Factors Affecting Heating Cost Comparisons

The relative savings you can expect from running a heat pump to provide heating in your home depend on a number of factors, including:

  • The cost of electricity and other fuels in your area.
  • Where your home is located – severity of winter climate.
  • The type and the efficiency of the heat pump you are considering – whether closer to the least energy-efficient or most energy-efficient HSPF or COP
  • How the heat pump is sized or matched to the home – the balance point below which supplementary heating is required.
How the heat pump is sized or matched to the home – the balance point below which supplementary heating is required.

Comparison Results

Table 2 shows estimated heating energy costs for eight different heat pumps, an electric furnace, and an oil furnace. Seven locations across Canada have been selected for the purposes of this comparison. Six of these locations are cities, while one, rural central Ontario, is a region. Each has unique electricity costs. Results in other cities in the same climate region may differ, due to variations in electricity costs.

A range of annual energy costs is provided by region for each heating system. This accounts for variations in equipment efficiency, size of house or annual heating requirements, and the ratio of heat pump to house heat loss. According to Table 2, the lowest operating costs for all systems are found in Vancouver, which has the warmest climate. The highest operating costs for most systems are found in rural central Ontario. In all of these estimated cases, heat pump systems have lower annual heating energy costs than electric or oil furnaces. Also note that in all locations, ground water EESs have lower operating costs than closed-loop EESs.

The comparisons shown in Table 2 include only energy costs for space heating. For some heat pumps equipped with a desuperheater, domestic water heating costs can be reduced by 25 to 50 percent. This would increase the savings and improve the payback on investment for these systems. Furthermore, there may be payback and energy savings for those heat pumps, which can be used to meet space cooling requirements.

Table 2. Heating Energy Cost Comparison for a Heat Pump and Conventional Heating Systems (Energy cost range in $/yr. Simple payback period ranges in years are in red)

Location Furnace with Air Conditioning Air-Source Add-on to Oil Furnace
  Electric 100% AFUE Oil 78% AFUE Standard Efficiency High Efficiency
Vancouver $405-$727 $441-$786 $139-$258
3.6-5.2 years
$125-$232
4.1-6.1 years
Calgary $1,128-$1,907 $930-$1,536 $634-$1,053
3.5-4.8 years
$597-$985
3.7-5.2 years
Winnipeg $1,057-$1,776 $1,290-$2,128 $867-$1,402
2.3-3.4 years
$837-$1,346
2.6-3.8 years
Rural Central
Ontario
(North Bay)
$1,509-$2,551 $1,072-$1,764 $806-$1,341
4.4-5.8 years
$758-$1,251
4.4-6.0 years
Toronto
(Etobicoke)
$1,082-$1,854 $803-$1,338 $490-$825
3.8-5.2 years
$452-$755
4.0-5.7 years
Montréal $832-$1,417 $716-$1,190 $462-$766
4.3-5.9 years
$433-$712
4.5-6.5 years
Halifax $1,068-$1,833 $836-$1,397 $452-$772
2.7-3.7 years
$414-$701
2.9-4.1 years

 

Location Air-Source with Electric
Resistance Backup
Ground Water
ESS
Closed-Loop
ESS
  Standard
Efficiency
High Efficiency Standard
Efficiency
High Efficiency Standard
Efficiency
High Efficiency
Vancouver $138-$258
4.0-5.9 years
$125-$232
4.6-6.9 years
$170-$339
14.6-17.1 years
$141-$282
14.2-17.0 years
$197-$394
29.7-33.8 years
$165-$329
26.5-31.2 years
Calgary $689-$1,137
2.2-3.2 years
$650-$1,063
2.4-3.6 years
$432-$863
4.9-5.3 years
$365-$730
4.9-5.3 years
$488-$975
9.6-10.3 years
$410-$820
8.8-9.4 years
Winnipeg $750-$1,225
3.1-4.6 years
$717-$1,162
3.3-5.1 years
$332-$665
4.6-5.0 years
$281-$562
4.7-5.2 years
$375-$751
8.7-9.3 years
$316-$632
8.3-9.1 years
Rural Central
Ontario
(North Bay)
$935-$1,531
1.8-2.7 years
$882-$1,430
2.0-3.0 years
$453-$905
3.4-3.8 years
$382-$763
3.5-3.9 years
$515-$1,030
6.4-7.0 years
$432-$864
6.2-6.8 years
Toronto
(Etobicoke)
$529-$873
2.0-3.0 years
$491-$801
2.2-3.4 years
$282-$493
4.3-5.2 years
$235-$411
4.6-5.5 years
$326-$571
8.0-9.6 years
$273-$477
7.9-9.6 years
Montréal $484-$796
2.9-4.3 years
$454-$738
3.2-4.9 years
$314-$627
6.9-7.4 years
$264-$528
6.8-7.6 years
$357-$713
13.5-14.1 years
$299-$599
12.3-13.5 years
Halifax $471-$791
1.6-2.4 years
$432-$719
1.8-2.7 years
$280-$490
3.8-4.6 years
$233-$409
4.0-4.8 years
$324-$567
7.0-8.5 years
$271-$474
7.0-8.4 years

Notes to Tables:

  1. Electricity prices are residential runoff rates as of November 2003, as supplied by local utilities. Rates varied from a low of 5.16¢ per kWh in Winnipeg to a high of 8.67¢ per kWh in Toronto.
  2. Oil prices are "typical" prices from local suppliers as of November 2003. Prices varied from 38.9¢ per litre in Montréal to 50¢ per litre in Halifax.
  3. AFUE: Annual Fuel Utilization Efficiency (seasonal efficiency).
  4. Simple payback period (shown in italics) is based on heating cost savings and initial cost. The initial cost for air-source heat pump systems is the additional cost from an air conditioner to the heat pump. The initial cost for EES heat pump systems is the full installed cost of the heat pump.
  5. HVAC Advisor 2.0 software, developed by NRCan, was used for all the cost simulations.
  6. The above costs are for space heating only. EESs are commonly equipped with a desuperheater to facilitate water heating. Desuperheaters can reduce electric water heating bills by $100 to $200 per year. Adding this savings to the space heating operating savings would reduce the payback period of an EES.
  7. The above costs are based on the HSPFs of Figures 7, 14 and 15 and house insulation values of RSI-3.5 (R-20) for walls, RSI-5.3 (R-30) for the roof, RSI-0.39 (R-2.2) for windows and RSI-1.8 (R-10) for the basement.

The cost of equipment for the payback period analysis was derived from the data of RSMeans and other sources. These costs were adjusted to reflect local costs according to the location factors supplied by RSMeans