Air-to-Air vs Air-to-Water Heat Pumps

It surprises a lot of people, but the air conditioner Tempest installs is a heat pump. In technical terms it is an air-to-air heat pump: it uses a refrigerant cycle to move heat from one place to another, and because it is reverse-cycle it can run in both directions, cooling your rooms in summer and warming them in winter.

An air-to-water heat pump uses the same underlying physics, but instead of blowing warm or cool air directly into the room it heats (or in some cases cools) water. That water is then circulated through radiators, underfloor heating and a hot-water cylinder, much like a gas boiler would do.

So the simplest way to think about it: air-to-air heats and cools the air in your rooms; air-to-water heats the water in your central heating. If you want to understand the wider family of systems, our heat pumps guide and system types overview set out the options.

The biggest practical difference is what each system can and cannot do.

• Air-to-air (air conditioning): excellent at cooling in summer, and very efficient, fast heating in winter, room by room. It does not produce hot water for taps or showers, and it does not connect to radiators.
• Air-to-water: provides whole-home heating through radiators or underfloor pipes and heats your hot-water cylinder. Most models do not cool, so it offers no summer relief.

If a Surrey summer heatwave is your main concern, only air-to-air solves it. If you are replacing a gas boiler and want a like-for-like heating and hot-water system, air-to-water is the closer match. Some homes end up with both: an air-to-water system for heating and hot water, plus air conditioning for cooling. See our installation page for how we assess which is right for a property.

Air-to-air systems are generally cheaper to install because they require no pipework changes to your radiators or cylinder. A single split air conditioner typically runs from £1,500 to £3,000, and a multi-split covering three to four rooms from £3,500 to £7,000. In Surrey and the South East, expect roughly 10 to 20% above the UK average.

Air-to-water systems are usually a larger project once you include the cylinder, controls and any radiator upgrades, and installation typically runs into five figures. Running costs for both depend on the efficiency (SCOP) of the unit and your electricity tariff. Air conditioning in heating mode is very efficient on inverter models, often costing in the region of 10 to 30p per hour per unit. For full figures see our cost guide and running costs page.

This is where a lot of online information is misleading, so we want to be clear. The government's Boiler Upgrade Scheme (BUS) offers grants towards low-carbon heating, but it only covers air-to-water heat pumps, ground-source heat pumps and biomass boilers.

Air-to-air heat pumps (air conditioning) are not eligible for the Boiler Upgrade Scheme. Anyone telling you there is a government grant for air conditioning is mistaken. We explain what genuinely is and is not available in our heat pumps content. The fair counterpoint is that air conditioning is a very efficient way to add heating, and we offer FCA-regulated finance through Phoenix Financial (subject to status) so you can spread the cost. See finance options.

Choose air-to-air (air conditioning) if you want summer cooling as well as winter heating, you are keeping your existing boiler or hot-water source, or you want a lower-cost, less disruptive install that works room by room.

Choose air-to-water if you are replacing a fossil-fuel boiler, want one system for all your heating and hot water, and want to access the Boiler Upgrade Scheme grant.

Every home is different, which is why we always recommend a proper survey before deciding. If you are in Surrey, our team in Guildford and across the county can talk through the options honestly and size a system that fits your home and budget.