Air-to-air heat pumps are designed to directly heat the air within the building. Heat is extracted from the external air via an externally located unit housing the evaporator. The internally located condenser heats the air where it is supplied to the building.
Where more than one area within the building is to be heated, or where the area is large, an air distribution system can be used to supply the air. Multiple internal units can also be used to heat separate areas.
An air-to-air heat pump may be a dedicated heat pump package providing heating only, but split, multi-split and variable refrigerant volume (VRF) comfort cooling systems can also be thought of as heat pumps if they have the ability to operate in reverse and provide heating.
Split systems are made up of one or more indoor room units and an outdoor unit that extracts heat from the exterior when the system is in heating mode. The indoor and outdoor units are linked by pipes which transport refrigerant between the units. VRF systems are essentially a more complex split system. The difference is the ability to provide heating or cooling from each of the indoor units on an individual basis.
Split systems are not capable of heating and cooling simultaneously, unless two systems are installed for each zone. VRF systems can provide heat recovery whereby heat rejected by units in cooling mode can provide heat to units in heating mode.
An advantage of air-to-air heat pumps over air-to-water heat pumps is the lower sink temperature (the temperature of the air passing over the condenser coil). This results in a higher COP and increased heat output. (COPs increase with reduced difference between source and sink temperatures.)
Air-to-water heat pumps use water as the heat sink. The heated water can be used for space heating or for domestic hot water.
A potential downside of using air as a heat source is the heat pump's coefficient of performance (COP). During the heating season the outside air temperature is often less than the ground temperature (at a depth at which heat is extracted by a ground-source heat pump). This lower temperature has the effect of reducing the COP.
Some manufacturers of air-source heat pumps quote COPs of four or more, but this data should be treated with caution. The relevant test standard for most packaged heat pumps is BS EN 14511. For an air-to-water heat pump the standard specifies test conditions of 7oC outdoor air temperature (source temperature). At external air temperatures lower than this, the COP will fall, as will the heating output of the heat pump. Depending on the application this reduction may be significant, such as during a cold winter morning when building pre-heat is needed.
A further factor influencing the COP of a heat pump is the sink temperature (the temperature of the supplied heated air or circulated water within the building). For an air-to-water heat pump BS EN 14511 specifies a return and flow temperature of 40oC and 45oC respectively.
At temperatures higher than these the COP (and heat output) will fall. This means that heat pumps, although potentially suited to low temperature heating systems (such as underfloor heating), have poor COPs when used with conventional hydronic heating systems with high circulation temperatures, such as 60oC or higher. High flow temperatures will result in a lower heat pump COP, while lower flow temperatures will require greater radiator surface area.
Test conditions (and hence manufacturers' quoted COP data) can therefore differ significantly from actual design and operating conditions.
Air-source heat pumps can have a number of environmental and operation advantages. For a start, more heat is supplied to the building than energy is consumed by the heat pump.
An air-source heat pump with a COP of three will supply three kilowatts of heat energy for the consumption of one kilowatt of electricity. If the heat pump is replacing (or being used as an alternative to) electric space heating, the use of the heat pump will offer significant carbon savings.
If a heat pump is used in place of a modern gas-condensing boiler, the carbon savings may be less clear-cut. Electrical energy from the National Grid is carbon inefficient when the low thermal efficiency of power stations is taken into consideration, along with distribution losses over the grid.
Other benefits of air-source heat pumps over conventional boilers include no combustion or explosive gases within the building, no need for flues or ventilation, no local pollution (although noise from the external unit fan may be a problem), long life expectancy, and low maintenance costs.