Summary

"Heat pump" gets used as if it describes one product, but air-to-air and air-to-water systems are different technologies serving different purposes, and confusing them with a customer leads to a genuinely wrong recommendation — not just a suboptimal one. An air-to-air system is, mechanically, very close to a reversible split or multi-split air conditioning system: an outdoor compressor unit connected by refrigerant pipework to one or more indoor units that blow heated or cooled air directly into a room. An air-to-water system extracts heat from outside air and transfers it, via a refrigerant circuit and heat exchanger, into water that then circulates through radiators or underfloor heating pipework — the same basic distribution infrastructure a gas boiler uses, just at a lower flow temperature.

This distinction matters commercially as much as technically. Air-to-water is the system covered by most heat pump policy discussion, most grant schemes, and most of the "should I switch from my gas boiler" conversations — see heat pump vs gas boiler comparison for that comparison in full. Air-to-air is a different proposition: often cheaper to install, faster to fit, genuinely effective for heating and cooling specific rooms or whole open-plan spaces, but it doesn't touch hot water and sits largely outside the current UK grant framework. Tradespeople fielding "I want a heat pump" from a customer need to establish which one is actually being asked about before scoping or pricing anything.

This article sets out the technical and commercial differences to guide that conversation. For running-cost and efficiency detail specific to air-to-air systems (which share testing standards with air conditioning), see ac energy efficiency seer ratings; for air-to-water sizing methodology, see heat pump sizing.

Key Facts

Quick Reference Table

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Factor Air-to-Air Air-to-Water
Heats water for taps/bath No Yes (via cylinder)
Distribution method Direct air, via indoor unit(s) Radiators or underfloor heating
Typical flow temperature concept N/A (air discharge temp) 35–45°C design flow
Cooling capability Yes, standard feature Not standard (some systems offer limited passive cooling)
BUS grant eligible Yes, £2,500 (since 28 April 2026), MCS-certified install Yes, £7,500+, MCS-certified install
MCS 3005 room-by-room sizing Not the standard sizing method used Yes, required for grant eligibility
Efficiency metric SEER (cooling) / SCOP (heating) CoP (point) / SCOP (seasonal)
Cylinder required No Yes (usually)
Existing radiators/pipework needed No Yes, reassessed/often upgraded
Typical install disruption Low Moderate–high (pipework, cylinder)
Best fit Supplementary heating/cooling, open-plan spaces, no wet system present Full central heating replacement or new-build
SAP/EPC treatment as primary heat source Not counted as primary — existing system stays recorded as primary heat source Well-established primary heating input

Detailed Guidance

What "Air-to-Air" Actually Means on Site

An air-to-air heat pump is, in hardware terms, the same core technology as a reversible split or multi-split air conditioning system: an outdoor unit containing the compressor and outdoor heat exchanger, connected by insulated refrigerant pipework to one or more indoor units (wall-mounted, ceiling cassette, or ducted) that condition the air in the room they serve. Running the refrigeration cycle in reverse lets the same unit heat in winter and cool in summer — this dual capability is one of air-to-air's strongest selling points, particularly as UK summers produce more genuine cooling demand than they once did.

Because there's no water circuit, there's no cylinder, no radiator work, and generally much less disruption to install than a wet-system heat pump — often a day or two per indoor unit rather than the multi-day pipework and cylinder work a full air-to-water conversion involves. The trade-off is scope: air-to-air heats the rooms with an indoor unit fitted, and only those rooms, and does nothing for hot water.

What "Air-to-Water" Actually Means on Site

An air-to-water heat pump uses the same outdoor-unit refrigeration principle, but the outdoor unit's heat exchanger transfers extracted heat into a water circuit via a heat exchanger (either within the outdoor unit as a monobloc, or via a separate indoor hydraulic unit in a split system). That heated water then feeds the property's existing (or upgraded) radiators or underfloor heating, and typically also charges a hot water cylinder through a secondary coil.

This is the system type that most directly substitutes for a gas boiler, because it reuses — with modification — the wet distribution infrastructure most UK homes already have. See heat pump sizing for MCS 3005 room-by-room sizing methodology, and radiator heat pump compatibility for what happens to existing radiator output at the lower flow temperatures an air-to-water system typically runs at.

When Air-to-Air Is the Right Recommendation

Air-to-air makes sense where the requirement is genuinely about heating and cooling specific rooms or open-plan living spaces rather than replacing whole-house central heating — a garden room or extension with no existing wet system, a loft conversion where running new pipework is impractical, an open-plan kitchen-diner that a customer wants cooled in summer as much as heated in winter, or as a supplementary system alongside existing gas or electric heating rather than a full replacement. It's also a sensible option where the property has no realistic space or budget for a cylinder, or where the customer's primary driver is summer cooling rather than winter heating cost.

It is not, on its own, generally a like-for-like whole-house central heating replacement — a customer expecting an air-to-air system to heat every room to the standard a wet central heating system does, including hot water, needs that expectation corrected before the job is scoped, not after installation when the shortfall becomes obvious.

When Air-to-Water Is the Right Recommendation

Air-to-water is the correct recommendation whenever the brief is a genuine central heating system replacement or new installation — replacing an ageing gas boiler, converting an off-grid oil-heated property, or specifying heating for a new build. It integrates with the property's existing (or planned) radiators/underfloor heating and delivers hot water through a cylinder, matching the functional role a boiler currently plays. This is also the system type that qualifies for the larger £7,500+ BUS grant band and sits within the MCS 3005 sizing and certification framework that most current UK heat pump policy and incentive structures are built around — though since 28 April 2026, air-to-air also qualifies for a smaller £2,500 BUS grant under the same MCS certification requirement (see Key Facts).

The Hot Water Gap With Air-to-Air

The most common scoping mistake with air-to-air is treating it as a drop-in boiler replacement without addressing hot water. Because air-to-air provides no water heating at all, a property converting to air-to-air alone needs a separate hot water strategy — retaining an existing boiler purely for hot water (an unusual and generally inefficient combination), fitting an electric immersion-only cylinder, or installing a standalone air-to-water unit or heat pump water heater dedicated to hot water. Flag this explicitly at quoting stage; it's a common source of customer disappointment when discovered after installation rather than before.

Efficiency and Running Cost Comparison

Because air-to-air systems are tested and labelled under the same ErP/Ecodesign framework as air conditioning (SEER for cooling, SCOP for heating — see ac energy efficiency seer ratings for the full methodology and worked running-cost examples), and air-to-water systems are typically quoted using CoP/SCOP under BS EN 14511/14825, direct like-for-like efficiency comparison between the two isn't straightforward from headline numbers alone. Both are genuinely efficient technologies (multiples of the energy input, unlike gas at ~90% or direct electric resistance heating at CoP 1.0), but the real-world running cost depends heavily on how well the system was sized and, for air-to-water, how well the flow temperature was matched to the radiators/emitters — see boiler vs heat pump running costs for the broader running-cost framework, which applies conceptually to both system types even though it's written primarily around air-to-water vs gas.

Combining Both Technologies

It's increasingly common to combine an air-to-water system for whole-house heating and hot water with one or two air-to-air units in specific rooms — a garden office, a loft room, or a space needing active summer cooling that a wet radiator system can't provide. This isn't a compromise; for a property with genuinely different heating/cooling needs by room, it can be the most cost-effective specification, avoiding the cost of extending wet pipework to a single awkward room while still delivering a proper central system everywhere else.

Frequently Asked Questions

A customer says they want "a heat pump" — how do I find out which one they actually need?

Ask what they're trying to solve: replacing a boiler and want hot water and whole-house heating (air-to-water), or heating/cooling a specific room or extension with no existing wet system (air-to-air). Also ask whether they've heard about the government grant, since that's frequently the reason "heat pump" comes up at all — both technologies are BUS-eligible via an MCS-certified installer, but the grant amount is very different (£7,500+ for air-to-water versus £2,500 for air-to-air), which is worth clarifying early so expectations on cost are realistic. Confirm the current grant category and value before quoting, as BUS categories and values are reviewed periodically.

Can an air-to-air system heat a whole house instead of a wet central heating system?

Technically, with enough indoor units on a multi-split or VRF system, yes — but this is a materially different (and often more expensive per room covered) approach than a wet central heating system, and it still doesn't solve hot water. For most UK houses with existing radiator pipework already in place, air-to-water reusing that infrastructure is the more practical and cost-effective whole-house solution. Air-to-air whole-house heating is more commonly seen in new-build or heavily renovated open-plan properties designed around it from the start.

Does an air-to-air system need planning permission the same as air-to-water?

Broadly similar considerations apply to the outdoor unit — permitted development rules, distance from boundaries, and any local Article 4 Direction restrictions — because both use an outdoor compressor unit of a similar type. Always check current permitted development rules for the specific property before assuming a certain siting is automatically allowed, particularly for flats and listed buildings, which generally have tighter restrictions regardless of heat pump type.

Is air-to-air cheaper to install than air-to-water?

Generally yes, for comparable coverage, because there's no wet pipework, cylinder, or radiator upgrade work involved — installation is largely refrigerant pipework and indoor/outdoor unit fitting. However, "comparable coverage" is doing a lot of work in that sentence: air-to-air pricing scales with the number of rooms/indoor units needed for full-house coverage, and doesn't include a hot water solution, so a true like-for-like whole-house comparison against an air-to-water system plus cylinder needs both quotes scoped to the same actual requirement before cost is compared directly.

Regulations & Standards