If you have read the heat pumps explained guide, you know the basic trick: a heat pump gathers warmth from outside and concentrates it to heat your home, delivering several units of heat for each unit of electricity. The two main types, ground source and air source, do the same job but gather that warmth from different places, and that difference shapes the cost, the efficiency and how much your garden gets dug up.
Same idea, different source
An air source heat pump takes heat from the outside air using a unit that sits outside the house, much like an air conditioner working in reverse. A ground source heat pump takes heat from the ground instead, through pipes buried in trenches or sunk down boreholes, where the temperature stays fairly constant all year. That last point is the crux: the ground a metre or two down barely changes temperature between summer and winter, while the air can be freezing on the very days you most need heat.
Efficiency
Because the ground is a warmer, steadier source in winter than cold air, a ground source system tends to run more efficiently and more consistently, especially in the depths of a cold snap when an air source unit has to work hardest against the lowest temperatures. Air source pumps have improved a great deal and perform well in most British winters, but on the coldest days their efficiency dips while a ground source system carries on much as before. Over a year, ground source usually edges it on running cost.
Installation cost and disruption
This is where the picture flips. An air source unit is comparatively cheap and quick to install, needing little more than a spot outside for the unit and the connections indoors, which is why the great majority of installations are air source. Ground source is a bigger undertaking: you need either enough land for long trenches or the budget for deep boreholes, plus the groundworks to install the pipe loops, all of which costs substantially more and makes more mess. The hardware lasts a long time, but the upfront figure and the disruption are real barriers.
Which suits you
Air source suits most homes, particularly where space is limited, budgets are tighter, or you want a simpler job, and it pairs well with the same insulate-first approach every heating upgrade needs. Ground source comes into its own where you have the land and the budget, want the best long-term efficiency, or are building or deeply renovating anyway so the groundworks are less of an imposition. Both reward a well-insulated, draught-proofed home that can be kept warm with gentle, steady heat rather than short hot blasts.
The bottom line
Ground source heat pumps are generally the more efficient and steadier performers, but they cost more and need land and groundworks, while air source pumps are cheaper, simpler and good enough for most homes, at a small efficiency cost on the coldest days. For the average household the practical choice is usually air source; ground source makes most sense with the space, the budget and a long horizon. Either way, do the cheap fabric improvements first so whichever pump you fit has less work to do.