Air Source vs Ground Source Heat Pumps

Both air source heat pumps (ASHP) and ground source heat pumps (GSHP) extract renewable heat to warm your home — but they work very differently, cost different amounts, and suit different properties. This guide gives you an honest comparison so you can decide which makes sense for your home.

How They Work

An air source heat pump absorbs heat from outside air — even when it feels cold to you. The unit sits outside your home and looks like an air conditioning unit. It works down to around -20°C, though efficiency drops as temperatures fall.

A ground source heat pump extracts heat from the ground via buried pipes (ground loops). Because soil temperature stays at a stable 8–12°C year-round in the UK, GSHP efficiency is remarkably consistent regardless of weather.

Cost Comparison

Efficiency: GSHP Wins, But Not by as Much as You Think

Ground source heat pumps have a higher COP (Coefficient of Performance) — typically 3.5 to 4.0 compared to 2.8 to 3.5 for air source. In practice, this means GSHP produces 3.5–4 units of heat for every 1 unit of electricity, while ASHP produces 2.8–3.5 units.

The difference is most noticeable in winter. When outside air drops to 0°C, an ASHP's COP falls to around 2.5. The ground stays at 8–12°C regardless, so a GSHP barely notices the season change. Over a full year, GSHP typically saves £150–£250 more on electricity than ASHP — meaningful, but not transformative given the much higher installation cost.

Space and Disruption

Air source needs minimal space. The outdoor unit is roughly the size of a washing machine and mounts on a wall or sits on a base outside. Installation takes 2–3 days with minimal disruption.

Ground source is a different story. Horizontal trenches need a garden area roughly 2–3 times the heated floor area of your home — for a 100m² house, you need around 200–300m² of diggable garden. Vertical boreholes need less surface area but require specialist drilling equipment. Either way, expect 2–3 weeks of installation work and significant garden disruption.

Noise

Ground source heat pumps are virtually silent because the compressor sits indoors and the ground loops make no noise. Air source units produce 42–55 dB at 1 metre — roughly equivalent to a fridge or quiet conversation. Modern inverter-driven ASHPs are significantly quieter than older models. For terraced houses or properties close to neighbours, noise is worth considering — see our full heat pump noise guide.

The BUS Grant

Both types qualify for the same £7,500 Boiler Upgrade Scheme grant. Since ASHP costs less to begin with, the grant covers a much larger proportion of the total cost — sometimes more than half. For GSHP, the grant is helpful but still leaves a significant amount to pay. Read our full BUS grant guide for eligibility details.

When Air Source Is the Better Choice

• Budget is a priority — ASHP costs roughly half the price of GSHP
• Limited outdoor space — no garden needed for ground loops
• You want minimal disruption — 2–3 days vs 2–3 weeks
• You're replacing a gas boiler — the payback period is faster with the lower upfront cost
• Terraced or semi-detached homes with small or no gardens

When Ground Source Is the Better Choice

• Large rural property with plenty of garden space
• Off the gas grid — replacing expensive oil or LPG heating
• Long-term investment — you plan to stay 15+ years and want the lowest running costs
• Noise is a concern — GSHP is virtually silent
• New build — ground loops can be installed during construction at lower cost

The Verdict

For most UK homeowners, air source is the practical choice. It's cheaper, quicker to install, doesn't need garden space, and qualifies for the same grant. ASHP accounts for over 90% of UK heat pump installations for good reason.

Ground source makes sense for larger rural properties with available land, especially those off the gas grid where running cost savings are greatest. If you can afford the higher upfront cost and plan to stay long-term, GSHP's superior efficiency and longer lifespan deliver better lifetime value.

Frequently Asked Questions