Air source heat pumps use an external unit mounted on a concrete base 9sometimes wall mounted). The unit draws in air passing it over a compressor which extracts the heat in the air (even at -7°C!) much like a fridge/freezer does in reverse. This gives you hot water with which you can heat HW cylinders or space heating emitters (UFH/rads).
They require electricity to function and the efficiency of the unit will depend on how much heat (in kW) you can extract from each unit of electricity (in kW) you put in. Typically Air source pumps work on around 300% – 400% efficiency – again they get more efficient when the flow temperature is reduced so ideal for new and renovated homes where UFH or over-sized radiators are fitted
Because of the fact that the air temperature changes throughout the year, the efficiency actually varies considerably and it is important to get an accurate idea of how it will perform in realistic situations. They are usually very well designed and can offer good economy. They do have a reputation for being a bit noisy but part of the proper design considerations concerns confirming the location of the pump unit to minimise any impact. Additionally, the more modern premium products (StiebelEltron) are now extraordinarily quiet under normal operation.
Air source pumps are relatively cost effective compared to other technologies as they require perhaps the minimum space and cost to install. They are perhaps not as adaptable as other forms and not suited to all types of heating but are very popular for new builds. RHI rates have recently risen for this technology making them a great choice for domestic users. Again bear in mind that the RHI is dependent on the efficiency of the pump so it really benefits from underfloor systems and low heat loss houses.
If you have applied in the past with no success you may be accepted now as the qualifying criteria was changed dramatically at the beginning of the year (2017) to help more households pass for free funding.