Air to Air Heat Pump (AAHP)
This is another type of heat pump which works in a similar manner to the GSHP described earlier, albeit using different components.
The same type of refrigerant is used as in the GSHP (see page 1 for details).
In this case the evaporator is a set of coils in which the refrigerant circulates. Hot air from outside the house is drawn in and blown across the coils by a fan. This causes the refrigerant to evaporate flashing off to a gas.
This device is fitted on the return line between the condenser and the evaporator. It fully expands the returning refrigerant to a liquid state.
The compressor compresses the gas raising its temperature further
This is another set of coils through which the gas now passes through and a fan blows across the hot coils forcing the heated air into the house. The gas has now lost most of its heat and has been condensed to a liquid that returns to the evaporator and the cycle starts once again.
This is fitted outside the house and controls the heat pump, turning it off when the ambient air temperature becomes too low to operate the system.
There are two of these, one being used to draw in the outside air to heat the refrigerant and, the other to blow the heated refrigerant gas into the house. Both the coils and the fans are contained within sheet steel enclosures, known as HVAC ducting. (See sketch).