Air Source Heat Pumps are virtually identical in operation to ground source heat pumps but as their name suggests they use the surrounding air as a fuel source, which means they are subject to wider differences in temperature and as a result greater differences in performance levels.
As an example between the temperature range of 5°C and 35°C there is very little difference in their performance capability, in fact at these temperatures some air source pumps are more efficient than ground source pumps, but outside this range performance can be affected resulting in reduced efficiency. Air source heat pumps are less expensive to buy and install than their ground source equivalents but are slightly more expensive to operate over time, however, even with these differences they retain their cost advantages over oil or gas equivalents.