Air Source Heat Pumps Compared to Ground Source Heat Pumps
ASHP vs GSHP
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 and air source heat pumps have been progressively evolving and advancing, most dramatically over the last 10 years, with the net result being that their seasonal Coefficient of Performance (sCoP) figures are now very close and are both in the 350% efficiency and upwards category! Both systems will produce a lower “grade” of heat which will require the upsizing/uprating of heat emitters to ensure no reduction in comfort level.
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!
Average sCoP of an ASHP is 3:1 and 4:1 on a GSHP.
Beyond the technical implications there are also two quite considerable installation differences:
- With GSHP you need to have a ground “collector” using either horizontal loops, vertical bore holes or a combination of the two in larger projects, whereas with ASHP we only need an absorber or outdoor unit (similar to an air conditioning outdoor unit) and in either these “collectors” or absorbers must be rated based on the heat loss of the building (as determined by a BS EN 12831 heat loss calculation, as per the standards). So with one you need land and the practical application is limited and with the other the limitation is only the access to an outdoor space.
- The other important consideration is the most important for most clients and that is the upfront supply and installation costs. The cost of an 8kW air source heat pump (big enough for a 2/3 bed house) is approximately £7,500 (excluding radiator upgrades or other heat emitters) where 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 is on bore hole(s) that need to be drilled, which could cost as much as £9,000 for a couple of (albeit very deep) holes in your back garden – assuming you’ve got a garden!
The renewable heat incentive takes into account the differences in upfront cost and the RHI formula is calculated as follows:
- Eligible Heat Demand = Total Annual Energy Demand x (1 – 1 / SPF)
- Annual RHI = Eligible Heat Demand x RHI Tariff
However the RHI tariff for the GSHP is 20.46p / kWh, while for the ASHP it is just 10.49p / kWh and they have calculated that so by and large the RHI (at the initial rates – they will be reviewed downward on a quarterly basis) will cover the cost of the installation, however this still favours the ASHP as it earns 50% of the GSHP tariff rate while only costing 30% of the upfront cost!
Heat pumps are a very worthwhile financial investment especially when you consider the renewable heat incentive and are an essential part of the battle to reduce carbon emissions.
Whether you opt for a ground source or air source heat pump is dependent on your situation, but as the technology improves there is very little in it and it really depends on whether you can afford the additional expense of the GSHP for the increased efficiency – bearing in mind that the RHI at its current level should largely cover the install cost of both of the technologies and this is where assignment of rights is aimed.