An air source heat pump (ASHP) is a heating and cooling system that uses outside air as its heat source and heat sink. Under the principles of vapor compression refrigeration, an ASHP uses a refrigerant system involving a compressor and a condenser to absorb heat at one place and release it at another.
In domestic heating use, an ASHP absorbs heat from outside air and releases it inside during winter, and can often do the converse in summer. When correctly specified, an ASHP can offer a full central heating solution and domestic hot water up to 80°C.
Contents |
Outside air, at any temperature above absolute zero, contains some heat. An air-source heat pump moves ('pumps') some of this heat to provide hot water or space heating. This can be done in either direction, to cool or heat the interior of a building.
The main components of an air-source heat pump are:
Air source heat pumps can provide fairly low cost space heating. A high efficiency heat pump can provide up to four times as much heat as an electric heater using the same energy. In comparison to gas as a primary heat source, however, the lifetime cost of an air source heat pump may be affected by the high price of electricity versus gas (where available). Gas may cause higher carbon emissions, depending upon how the electricity is generated.
A "standard" air sourced heat pump found in most homes can extract useful heat down to about -5F or 0F (-15c). At colder outdoor temperatures the heat pump is inefficient; it could be switched off to run only on supplemental heat (or emergency heat) if the supplemental heat is sized large enough. There are specially designed heat pumps that, while giving up some performance in a/c mode, will provide useful heat extraction to even lower outdoor temperatures. An air source heat pump designed specifically for very cold climates can extract useful heat from ambient air as cold as -20F or even -25F (-30c), but these are uncommon in most homes.
Air source heat pumps can last for over 20 years with low maintenance requirements. There are numerous heat pumps from the 1970s and 1980s that are still in service as of 2011, even in places where winters are extremely cold. Few moving parts reduce maintenance requirements, however, the outdoor heat exchanger and fan must be kept free from leaves and debris. Heat pumps have significantly more moving parts than an equivalent electric resistance heater or fuel burning heater.
Air source heat pumps are used to provide interior space heating and cooling even in colder climates, and can be used efficiently for water heating in milder climates. A major advantage of ASHP's is that the same system may be used for air conditioning in summer and heating in winter. Though the cost of installation is generally high, it is less than the cost of a ground source heat pump, because a ground source heat pump requires excavation to install its ground loop.
ASHP's are often paired with auxiliary or emergency heat systems to provide backup heat when outside temperatures are too low for the pump to work efficiently, or in the event the pump malfunctions. Propane, natural gas, or oil furnaces can provide this supplementary heat. All-electric heat pump systems have an electric furnace or electric resistance heat, or strip heat, which typically consists of rows of electric coils that heat up. A fan blows over the heated coils and circulates warm air throughout the home. This serves as an adequate heating source, but as temperatures go down, electricity costs rise, and power outages pose an even greater threat.
The outdoor section on some units may 'frost up' when outdoor temperatures are between 0°C and 5°C (between 32°F and 41°F) and there is sufficient moisture in the air which restricts air flow across the outdoor coil. These units employ a defrost cycle where the system switches to "A/C" mode to move heat from the home to the condenser to melt the ice. This requires the supplementary heater (resistance electric or gas) in the indoor section to activate, to temper the cold air being distributed. The defrost cycle reduces the efficiency of the heat pump significantly, although the newer (demand) systems are more intelligent and need to defrost less. As temperatures drop below freezing the tendency for frosting of the outdoor section decreases due to reduced humidity in the air.
It is difficult to retrofit conventional heating systems that use radiators/radiant panels, hot water baseboard heaters, or even smaller diameter ducting, with ASHP-sourced heat. The lower heat pump output temperatures would mean radiators would have to be increased in size or a low temperature underfloor heating system be installed instead.
Heating and cooling is accomplished by moving a refrigerant through the heat pump's indoor and outdoor coils. Like in a refrigerator, a compressor, condenser, expansion valve and evaporator are used to change states of the refrigerant between a cold liquid and a hot gas.
When the liquid refrigerant at a low temperature passes through the outdoor evaporator heat exchanger coils, ambient heat is used to cause the liquid to boil. This boiling or change of state process amasses energy as latent heat. The vapor is then drawn into a compressor which further boosts the temperature of the vapor.
Passing into the building, the vapor enters the condenser heat exchanger coils where it transfers heat to indoor air, which is drawn across the coils by a fan. As the vapor cools, it condenses back into a liquid, and releases its latent heat to the air passing over the condenser unit.
Exiting the condenser, the cold liquid refrigerant is under high pressure. The refrigerant passes through an expansion valve which reduces the pressure, draws in heat and allows the refrigerant to re-enter the evaporator to begin a new cycle.
Most heat pumps can also operate in a cooling mode where the cold refrigerant is moved through the indoor coils to cool the room air.
The 'Efficiency' of air source heat pumps is measured by the Coefficient of performance (COP). A COP of 3 means the heat pump produces 3 units of heat energy for every 1 unit of electricity it consumes. Within temperature ranges of -3°C to 10°C, the COP for many machines is fairly stable at 3-3.5.
In mild weather, the COP of an air source heat pump can be up to 4. However, on a very cold winter day, it takes more work to move the same amount of heat indoors than on a mild day. The heat pump's performance is limited by the Carnot cycle and will approach 1.0 as the outdoor-to-indoor temperature difference increases, which for most air source heat pumps happens as outdoor temperatures approach −18 °C / 0 °F. Heat pump construction that enables carbon dioxide as a refrigerant may have a COP of greater than 2 even down to -20°C, pushing the break-even figure downward to -30 °C (-22 °F). A ground source heat pump has comparatively less of a change in COP as outdoor temperatures change, because the ground from which they extract heat has a more constant temperature than outdoor air.
The specific design of a heat pump has a considerable impact on its efficiency. Many air source heat pumps are designed primarily as air conditioning units, mainly for use in summer temperatures. Designing a heat pump specifically for the purpose of heat exchange can attain greater COP ratings and an extended life cycle. The principal changes are in the scale and type of compressor and evaporator.
Seasonally adjusted heating and cooling efficiencies are given by the heating seasonal performance factor (HSPF) and seasonal energy efficiency ratio (SEER) respectively.
In units charged with HFC refrigerants, the COP rating is reduced when heat pumps are used to heat domestic water to over 60°C or to heat conventional central heating systems that use radiators to distribute heat (instead of an underfloor heating array).
Units charged with HFC refrigerants are often marketed as low energy or a sustainable technology, however the HFCs have the potential to contribute to global warming, as measured in global warming potential (GWP) and ozone depletion potential (ODP). Recent government mandates have seen the phase-out of R-22 refrigerant and its replacement with more environmentally sound R410a refrigerant.
Summer, John A. (1976). Domestic Heat Pumps. PRISM Press. ISBN 0-904727-10-6.