Air-to-air heat pumps

Most modern reversible air-conditioning systems are air-to-air heat pumps: they move heat rather than burn fuel — out of the building when cooling, and into it when heating. The "air-to-air" name describes what they work between: they take heat from the outside air and deliver it to the indoor air, which is what separates them from an air-to-water heat pump that feeds radiators or underfloor pipes.

Heating and cooling in one system

The same unit runs in both directions. In summer it removes heat from the room to cool it; in winter it reverses, drawing warmth from the outside air even on cold days and delivering it indoors. Because it moves existing heat rather than generating it from scratch, it can deliver several units of warmth for each unit of electricity it uses — which is why an air-to-air heat pump can be an efficient way to heat a room as well as cool it.

Which homes and rooms it suits

Pros and limitations

Running costs and grants

Running cost is driven by how much you heat and cool, the efficiency of the unit, and the unit price of electricity — which itself moves with the Ofgem energy price cap [4]. See running costs for what shapes the bill in practice.

On grants, the position changed in 2026 and most guidance online has not caught up. The Boiler Upgrade Scheme, which covers England and Wales, was for years aimed only at air-to-water heat pumps — the kind that replace a boiler and feed radiators or underfloor heating. It now also pays £2,500 towards an air-to-air heat pump, against £7,500 for air-to-water and ground source [2].

Two conditions decide most air-to-air enquiries, and neither is the one usually reported. The fuel rule is the same as for every technology in the scheme — the installation must replace either a fossil fuel or an electric heating system [6] — so having a gas boiler today does not disqualify you. What does disqualify you is keeping it: the property cannot be left with fossil fuel heating, any supplementary appliance must not be fossil-fuelled, and hybrid arrangements are excluded outright [6]. The other condition is flatter still: air-to-air can only be installed in residential properties under the scheme [6]. One practical consequence is easy to miss — an air-to-air unit cannot heat water, and the system must be capable of meeting the property's full space heating and hot water need alone or with a supplementary appliance, so a non-fossil route to hot water has to exist [6].

Two further practicalities. Both the installer and the specific unit need MCS certification, and certified air-to-air products were still working their way onto the MCS product directory during 2026 [1] — so a property can qualify on paper while no voucher can yet be issued for the equipment quoted. Treat a quote with £2,500 already deducted as something to verify, not assume. The scheme is installer-led: your installer applies, the grant comes off the quote, and Ofgem pays them directly, so you should never be asked to pay in full and reclaim [3]. Our heat pump grant eligibility guide works through the rules, amounts and the voucher process in full. Refrigerant work is legally restricted to F-gas certified engineers [5].

Costs and comparisons

For what a reversible system costs to fit and run, see air-to-air heat pump costs. If the terminology is the confusing part, air-to-air heat pump vs air conditioning explains why they usually describe the same box.

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