Air source heat pumps
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An air source heat pump (ASHP) is a system which transfers heat from outside to inside a building, or vice versa. 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. They can be used as a space heater or cooler, and are sometimes called "reverse-cycle air conditioners".
In domestic heating use, an ASHP absorbs heat from outside air and releases it inside the building, as hot air, hot water-filled radiators, underfloor heating and/or domestic hot water supply. The same system can often do the reverse in summer, cooling the inside of the house. When correctly specified, an ASHP can offer a full central heating solution and domestic hot water up to 80°C.
Air, at any temperature above absolute zero contains some heat. An air-source heat pump transfers ('pumps') some of this heat from one place to another, for example between the outside and inside of a building. This can provide space heating and/or hot water. A single system can be designed to transfer heat in either direction, to heat or cool the interior of the building in winter and summer respectively. For simplicity, the description below focuses on use for interior heating.
The technology is similar to a refrigerator or freezer or air conditioning unit: the different effect is due to the physical location of the different system components. Just as the pipes on the back of a refrigerator become warm as the interior cools, so an ASHP warms the inside of a building whilst cooling the outside air.
The main components of an air-source heat pump are:
- An outdoor heat exchanger coil, which extracts heat from ambient air
- An indoor heat exchanger coil, which transfers the heat into hot air ducts, an indoor heating system such as water-filled radiators or underfloor circuits and/or a domestic hot water tank
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 price of electricity compared to gas (where available). Use of gas may be associated with higher carbon emissions, depending upon how the electricity is generated.
A "standard" domestic air source heat pump can extract useful heat down to about -5F or 0F (-18C). At colder outdoor temperatures the heat pump is less efficient; it could be switched off and the premises heated using only supplemental heat (or emergency heat) if the supplemental heating system is large enough. There are specially designed heat pumps that, while giving up some performance in cooling 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 in the United States that are still in service in 2012, 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 more moving parts than an equivalent electric resistance heater or fuel burning heater. Ground source heat pumps have fewer moving parts than air source heat pumps as they do not need fans or defrosting mechanisms.
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 some ASHPs is that the same system may be used for heating in winter and cooling in summer, though it is not true air conditioning without a facility to adjust the humidity of the inside air. 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. The advantage of a ground source heat pump is that it has access to the thermal storage capacity of the ground which allows it to produce more heat for less electricity in cold conditions.
ASHPs 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 there is sufficient moisture in the air and outdoor temperature is between 0°C and 5°C (32°F to 41°F). This restricts air flow across the outdoor coil. These units employ a defrost cycle where the system switches temporarily to 'cooling' mode to move heat from the home to the outdoor coil to melt the ice. This requires the supplementary heater (resistance electric or gas) to activate. 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 pumping 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 colder liquid and hotter gas states.
When the liquid refrigerant at a low temperature and low pressure passes through the outdoor heat exchanger coils, ambient heat causes the liquid to boil (change to gas or vapor): heat energy from the outside air has been absorbed and stored in the refrigerant as latent heat. The gas is then compressed using an electic pump; the compression increases the temperature of the gas.
Inside the building, the gas passes through a pressure valve into heat exchanger coils. There, the hot refrigerant gas condenses back to a liquid and transfers the stored latent heat to the indoor air, water heating or hot water system. The indoor air or heating water is pumped across the heat exchanger by an electric pump or fan.
The cool liquid refrigerant then re-enter the outdoor heat exchanger coils 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.
Efficiency Ratings 
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.
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).
Risks and Precautions 
- Many air source heat pumps can heat water up to 70°C without difficulty when the air temperature is 5°C or greater. However, to minimize the risk from Legionellosis it is advised that hot water is heated to above 60°C, which may not be possible for some units if outside temperature dips.
- Most air source heat pumps lose their efficiency as the external temperatures fall below 5 degrees Celsius (about 41 degrees Fahrenheit). In colder climates, the system needs to be installed with an auxiliary source of heat to supplement the heat pump in the event of extremely cold temperatures or when it is simply too cold for the heat pump to work at all.
- An Auxiliary Heat/Emergency Heat system, for example a traditional furnace, is also important if the heat pump is malfunctioning or being repaired. In Northern climates, split-system heat pumps matched with gas or oil furnaces will work even in extremely cold temperatures.
Units charged with HFC refrigerants are often marketed as low energy or a sustainable technology, however if the HFC leaks out from the system, there is potential to contribute to global warming, as measured in global warming potential (GWP) and ozone depletion potential (ODP). Recent government mandates[where?] 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.