The efficiency of a heat pump is governed by the ‘source’ temperature, in this case either the air or the ground. The colder the source, the harder the heat pump must work. This means that when we compare ground source or air source heat pumps for efficiency, we need to consider where the house is located and what time of year it is.
Air source heat pumps are working with air temperatures that fluctuate between -5°C to 25°C for most of the year. There are of course some days where it can get much colder, but for most locations in the UK, this temperature bracket covers at least 95% of days in the year.
Ground source heat pumps extract heat from the soil, where the temperature doesn’t reach as high, but also doesn’t drop below freezing. In most circumstances, the soil temperature will stay above 5°C throughout the year, as long as the ground loops, which extract heat from the soil, have been designed correctly.
This means there are some times during the year when the ASHP will be more efficient than the GSHP, but when it gets really cold, the ground source heat pump will be more efficient than the air source heat pump.
In addition to being cheaper to install, another advantage of an ASHP is that it doesn’t need to use energy to pump fluid around the pipework outside, as happens with a GSHP. So, on days when the air is the same temperature as the ground, an ASHP can still be slightly more efficient.
On balance, however, GSHPs tend to be more efficient over the year, but the extra cost savings will depend on where you are in the country and the environmental conditions. It would be easier to justify the added cost of a GSHP the further North you live.
Air source heat pumps generally continue to work at temperatures of around -15°C, while some can work at much lower temperatures. However, if you see these sorts of temperatures regularly, you might find that either a GSHP or hybrid heat pump is a better option.