Air Source Heat Pumps Compared to Ground Source Heat Pumps

ASHP vs GSHP

Technology Basics

Both absorb heat by way of a refrigeration circuit that allows the transfer of energy to water, with the fundamental difference only being the source of heat. This is however where the similarity ends, as ground source heat pumps are still using technology developed 20 years ago, whereas air source heat pumps have been progressively evolving and advancing, most dramatically over the last 10 years. The result is that their seasonal Coefficient of Performance (sCoP) figures are now very close, with both technologies operating in the 350% efficiency and upwards category.

Both systems will produce a lower “grade” of heat, which will require the upsizing or uprating of radiators or other heat emitters to ensure no reduction in comfort levels.

The net result is that both systems have efficiency levels far greater than 100%, reaching as high as 500% or 5:1 when properly designed and installed, whereas the best gas boiler on the market claims an efficiency of 92% or 0.92:1.

The average sCoP of an ASHP is around 3:1, compared with approximately 4:1 for a GSHP.

Beyond the technical implications there are also two considerable installation differences:

With a GSHP you need to have a ground “collector” using either horizontal loops, vertical bore holes, or a combination of the two on larger projects. With an ASHP only an external absorber or outdoor unit is required, similar to an air conditioning outdoor unit. In both cases these “collectors” or absorbers must be correctly sized based on the heat loss of the building, as determined by a BS EN 12831 heat loss calculation.

Therefore, with one system you need sufficient land and the practical application is more limited, whereas with the other the main limitation is simply access to suitable outdoor space.

The other important consideration for most clients is the upfront supply and installation cost. The cost of an 8kW air source heat pump, suitable for a typical 2–3 bedroom house, is approximately £7,500 excluding radiator upgrades or other heat emitters. The equivalent ground source heat pump is approximately £23,000, again excluding the equally necessary radiator or heat emitter upgrades.

Much of the additional cost of a GSHP is due to the bore hole installation, which could cost as much as £9,000 for a couple of very deep holes in your back garden — assuming you have a suitable garden.

In Summary

Heat pumps are a very worthwhile investment and are an essential part of reducing both energy costs and carbon emissions.

Whether you opt for a ground source or air source heat pump depends on your property, available outdoor space, budget and long-term priorities. Ground source systems generally offer slightly higher efficiency, but air source heat pumps have improved significantly in recent years and usually provide a far more cost-effective solution for most homes.