Most heating-electrification content treats "heat pump" as one decision. It isn't. Air-to-water and air-to-air heat pumps are genuinely different technologies, with different running costs, different grant eligibility, and different limitations — and the choice between them, or gas, depends on a factor most guides skip entirely: how much of the running cost you can schedule onto cheap electricity.
Two technologies, not one
Air-to-water (A2W) connects to your existing wet heating system — radiators and hot water cylinder. It replaces your boiler's function directly: heating and hot water, one system.
Air-to-air (A2A) is a reversible heat pump delivered through wall or ceiling units, room by room — closer to what most people picture as "air conditioning" than a boiler replacement. It heats the rooms it's installed in, and it does not provide hot water at all. A household installing A2A typically keeps its existing hot water arrangement (usually gas) running alongside it. A2A also does something A2W doesn't: it cools in summer, at effectively no extra hardware cost once installed.
This distinction matters more than most comparisons let on, because it directly affects grant eligibility (below) and because "heat pump vs gas" is really two separate comparisons depending on which technology you mean.
The lever that decides almost everything: off-peak scheduling
A heat pump's real-world efficiency is measured as SPF (Seasonal Performance Factor) — not the lab-tested COP figure often quoted in marketing. Realistic retrofit SPF figures sit around 2.8–3.5 depending on radiator type and installation quality (A2W generally lower, A2A generally higher, since A2A avoids wet-system distribution losses).
Here's the part that actually determines whether either technology beats gas: running a heat pump on-demand, at whatever rate applies when heat is needed, is a very different calculation from running it predominantly on a cheap off-peak rate.
- On-demand, no scheduling: heat pumps frequently come out worse than gas on current tariffs, because daytime/peak electricity rates are high enough that even a good SPF doesn't close the gap.
- With meaningful off-peak scheduling (a wide peak/off-peak tariff spread, paired with thermal mass or a thermal store to shift running hours): the electricity cost per unit of heat can drop well below gas's effective cost.
This is the single biggest lever in the entire heat pump vs. gas decision — bigger than SPF, bigger than solar, bigger than which technology you pick. Any comparison that doesn't model a range of scheduling scenarios is giving you an incomplete answer.
Why solar helps less than you'd expect
Solar generation and heating demand are seasonally mismatched: heating need peaks in winter, solar generation peaks in summer. Roughly half of a typical heat pump's annual electricity use falls in the November–February window, while that same window produces a small fraction of annual solar output. Solar can meaningfully offset a heat pump's daytime running in shoulder seasons, but don't expect it to solve winter heating costs — that's a structural mismatch no amount of panel capacity fixes.
Air-to-air gets one genuine solar advantage A2W doesn't: cooling demand and solar generation both peak on the same sunny summer days. If you use A2A for cooling, that portion of its electricity use can be substantially covered by free solar — a real synergy, not a marketing claim.
Grants: what's actually current, and a catch worth knowing
A2W: £7,500 via the Boiler Upgrade Scheme (England and Wales). Well-established, straightforward eligibility for most gas-heated homes replacing their boiler with an air-to-water system.
A2A: £2,500, added to the Boiler Upgrade Scheme in 2026 — but with a real eligibility catch. The grant appears to require that the property has no retained fossil-fuel heating — meaning a household that installs A2A but keeps a gas boiler running for hot water (a very common setup, since A2A doesn't do hot water at all) is likely not eligible, even though the grant itself is genuinely live. This specific interaction — A2A's inability to cover hot water combined with the no-retained-fossil-fuel eligibility rule — catches out many of the households who'd naturally consider A2A. Confirm directly with an MCS installer or Ofgem before assuming this grant applies to your situation; don't take a generic "the grant exists" claim as confirmation you personally qualify.
Realistic 2026 cost ranges
- A2W: roughly £11,000–£16,000 installed for a typical UK home, before the £7,500 grant (net roughly £3,500–£8,500)
- A2A: roughly £1,500–£7,000+ depending on how many rooms/zones need coverage — a single-room unit sits at the low end, whole-home multi-split coverage at the high end
Both ranges vary meaningfully by installer and property — get multiple quotes rather than treating either range as a firm price.
Common mistakes worth avoiding
- Comparing against a heat pump's lab-tested COP instead of realistic retrofit SPF — overstates the savings case
- Assuming solar meaningfully cuts winter heating costs — the seasonal mismatch limits this more than most calculators admit
- Assuming the A2A grant applies without checking the retained-fossil-fuel rule — a common and costly assumption to get wrong
- Treating "heat pump" as one technology — A2W and A2A solve different problems and shouldn't be compared to gas using the same assumptions
Want to see the real numbers for your own home? Our free calculator models both A2W and A2A against your actual gas costs and tariff — including the grant eligibility check most tools skip — and shows the off-peak share at which each one beats gas — see the full side-by-side breakdown across every scheduling level in our Deep Dive report.