Heat Pumps
Generating Heat from the Air, Ground or Water...
Heat pumps typically draw approximately 1/3 to 1/4 of the electricity of a standard resistance heater for the same amount of heating, reducing utility bills and with few moving parts, reducing maintenance requirements.
However, it should be ensured that the outdoor heat exchanger and fan is kept free from leaves and debris. Moreover, it must be borne in mind that a heat pump will have significantly more moving parts than an equivalent electric resistance heater or fuel burning heater.
As an electric system, no flammable or potentially asphyxiating fuel is used at the point of heating, reducing the potential danger to users, and removing the need to obtain gas or fuel supplies (except for electricity).
Heat pumps may be used to heat air, or water and have lower running costs, the compressor being the most power intensive component - when in comparison with traditional electrical resistance heaters.
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Heating and cooling is accomplished by moving a refrigerant through the heat pump's various indoor and outdoor coils and components. A compressor, condenser, expansion valve and evaporator are used to change states of the refrigerant from a liquid to hot gas and from a gas to a cold liquid. 
The refrigerant is used to heat or cool coils in a building or room and fans pull the room air over the coils. An external outdoor heat exchanger is used to heat or cool the refrigerant. This use of outside air has led to the term "Air Source" Heat Pump. The overall operation uses the concepts described in classic vapour compression refrigeration.
When the liquid refrigerant at a low temperature passes through the outdoor evaporator coils, the temperature of the outside air causes the liquid to boil. This change of state from liquid to a vapour requires a considerable amount of energy or "latent heat" which is provided by outside air passing over the coils.
This vapour is then drawn into the compressor where the temperature of the vapour is boosted to well over 100 degrees Celsius. At this point we have used heat from the outside air to change the liquid refrigerant to a gas and added an amount of compression "work" to raise the temperature of the vapour. The vapour now enters the condenser heat exchanger coils where it begins to transfer heat to the air being drawn across the coils. As the vapour cools, it condenses back to a liquid and in so doing releases and transfers considerable latent heat to the air passing over the condenser unit coils.
We have used the heat energy of outside air to change the phase of the refrigerant and then released this heat for heating, a typical heat pump operation.
At this stage we now have a very cold liquid refrigerant compressed to a high pressure. The refrigerant is next passed through an expansion valve which turns it back to a low pressure cold liquid ready to re-enter the evaporator to begin a new cycle.
The heat pump can also operate in a cooling mode where the cold refrigerant is moved through the indoor coils to cool the room air.
How it Works...
Outside air, necessarily exists above absolute zero, and is therefore a heat container. An air-source heat pump moves ("pumps") some of this heat to provide hot water or household 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:
A heat exchanger, over which outside air is blown, to extract the heat from the air
A compressor, which acts like a refrigerator but in reverse and raises the temperature from the outside air
A way to transfer the heat into a hot water tank or heating system, such as radiators or under-floor heating tubes