Should we install a ground source or an air source heat pump?

Our current project has underfloor heating in an openplan area of approximately 120m2. We also have 120mm thick Celotex insulation in the roof, 140mm in the walls and 100mm between the concrete slab and screed, so it’s a well-insulated structure. We have the space to install a ground source heat pump (GSHP), but not quite the budget. We are considering going for an air source heat pump (ASHP), but have seen mixed reviews of both.

With this amount of insulation, and as the system would not need to run all the time, is it worth going for a GSHP or would an ASHP be more cost effective? We would be happy to consider other systems, but there is no gas on site, only electricity. What do you suggest?

A low heat demand means the environmental impact of installing any system would be relatively high in comparison to running it. This suggests an AHSP may be your best bet, as it won’t involve so much work to install. These units are not as efficient in midwinter as GSHPS, but in theory, your heat demand is so low that the extra cost and environmental impact of a ground source unit would not be justified in this case. You could consider supplementing your ASHP with a small woodburner in the colder periods, or you could hook this up to a hot water cylinder, if it could take additional input.