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Air-source Heat Pumps: Selection Considerations

Energy efficiency considerations
Other selection considerations
Sizing considerations
Installation considerations
Operational considerations
Cost considerations
Life expectancy and warranties

Energy Efficiency Considerations

The annual cooling efficiency (SEER) and heating seasonal performance factor (HSPF) of an air-source heat pump are affected by the manufacturer's choice of features. The SEER of air-source heat pumps ranges from a minimum of 9 to a maximum of about 16. Canada’s Energy Efficiency Regulations require a minimum HSPF of between 5.9 and 8.8 in a Region V climate (such as that of Ottawa).

Figure 1. Air-Source Heat Pump Efficiency (Region V)

Figure 1. Air-Source Heat Pump Efficiency (Region V)

Note: Indicated values represent the range of allavailable equipment.

Minimum efficiency levels are regulated in Canada. They represent an improvement of 5 to 10 percent over the efficiency of only a few years ago. More efficient compressors, larger heat exchanger surfaces, improved refrigerant flow, and other controls have been largely responsible for these gains.

Today, new developments in compressors, motors, and controls have pushed the limits of efficiency even higher. More advanced designs (advanced reciprocating, scroll, variable-speed or two-speed compressors combined with improved heat exchanger and control designs) can yield SEERs as high as 16 and HSPFs of up to 8.8 for Region V.

At the lower efficiency end of the range, air-source heat pumps often have single-speed reciprocating compressors. Higher efficiency units generally incorporate scroll or advanced reciprocating compressors. Heat pumps with the highest SEERs and HSPFs invariably use variable or two-speed scroll compressors.

Other Selection Considerations

Select a unit with as high an HSPF for Region V as practical. For units with comparable HSPF ratings, check their steady-state ratings at –8.3oC, the low temperature rating. Units with higher values will normally be the most efficient in most of Canada. Select a unit with a demand defrost control. This minimizes defrost cycles (system reversals are hard on the machine), and reduces supplementary and heat pump energy use.

The sound rating is a tone corrected, A-weighted sound power level, expressed in bels. [ link to terms] The lower the value, the lower the sound power emitted by the outdoor unit. Select a heat pump with an outdoor sound rating in the vicinity of 7.6 bels or lower if possible. These ratings are available from the manufacturer.

Sizing Considerations

To ascertain heating and cooling loads, use 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 all the heat required by a house, it is not generally a good idea to do so. 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 result in reduced performance, possibly limiting the unit's ability to provide dehumidification in the summer. However heating uses 8 to 10 times as much energy, so some oversizing for lower cost heating is justified.

In addition, the efficiency of an air-source heat pump drops along with the outdoor temperature. Consequently, it doesn't make economic sense to try to meet all your heating needs with an air-source heat pump.

As a rule of thumb, 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. A heat pump sized in this manner would have a balance point between 0oC and –5oC. This generally results in the best combination of cost and seasonal performance.

Installation Considerations

In installing any kind of heat pump, it is important for the contractor to follow the manufacturer’s instructions carefully. The following general guidelines should be taken into consideration when installing an air-source heat pump:

  • For any 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.
  • High winds may reduce efficiency by causing defrost problems, so outdoor units should be protected from wind.
  • To prevent snow from blocking air flow over the coil and to permit defrost water to drain, 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 can be mounted from the wall of the house on a suitably constructed frame.
  • It may be advisable to locate the heat pump outside the drip line of the house (the area where water drips off the roof). Ice and water falling on it could restrict airflow or cause fan or motor damage.
  • The pan under the inside coil must be connected to the interior house floor drain, to ensure proper drainage of the condensate forming on the coil.
  • The heat pump should be placed so that a serviceperson has enough room to work on the unit and to have access to the indoor coil for cleaning.
  • Refrigerant lines should be as short and straight as possible. It is a good idea to place them in an insulated conduit to minimize unwanted heat loss and to prevent condensation.
  • Fans and compressors make noise. Locate outdoor units away from windows and adjacent buildings. Some units also make noise when they vibrate. Minimize this by selecting quiet equipment or mounting the unit on a noise-absorbing base.
  • Unless a mini or multisplit is used, ductwork must be installed in homes that do not already have an air distribution system. 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 kW, or 400 to 450 cubic feet per minute (cfm) per ton of cooling capacity.

Electrical requirements Generally speaking, it is not necessary to upgrade the electrical service when installing an air-source add-on heat pump. However, the age of the service and the total electrical load of the house may make it necessary to upgrade.

Heat distribution systems Heat pumps require distribution systems that handle airflow rates of 50 to 60 litres per second (L/s) per kW, or 400 to 450 cubic feet per minute (cfm) per ton of cooling capacity. This is approximately 20 to 30 percent higher than the flow rates required by central, forced-air furnaces. Restricting airflow rates decreases efficiency, and damage to the compressor can result if they are severely reduced for extended periods of time.

New heat pump systems should be designed according to established practice. If the installation is an add-on or a conversion, the existing duct system should be carefully examined to ensure that it is adequate.

Operational Considerations

The indoor thermostat should be set at the desired comfort temperature (about 20oC) and not readjusted.

Continuous indoor fan operation can degrade heating system efficiency unless a high-efficiency variable-speed fan motor is used. Operate the system on the "auto" fan setting on the thermostat.

Heat pumps have longer operation times than conventional furnaces because their heating capacity output is considerably lower.

Cost Considerations

The cost of installing an air-source heat pump depends on the type of system and the existing heating equipment. Costs are higher when the ductwork has to be modified, or if you need to upgrade your electrical service to deal with the increased electrical load.

The energy costs of a heat pump can be lower than those of other heating systems, particularly electric 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 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 or fuel is relatively inexpensive, your operating costs may be higher. Depending on these factors, the payback period for investment in an air-source heat pump could be anywhere from a few years to a decade or more.

Heating energy cost comparisons for heat pumps and electric heating systems

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-year warranty on the compressor (for parts only). However, warranties vary between manufacturers, so be sure to check the fine print.