An air-to-water heat pump heats radiators and hot water, while an air-to-air heat pump heats air blown into rooms via fan units and cannot provide hot water. According to the Energy Saving Trust, air-to-water systems typically achieve a COP of 2.5 to 3.0, meaning they produce 2.5 to 3 kWh of heat for every 1 kWh of electricity used (Energy Saving Trust, 2026).
The choice between these two heat pump types depends on your existing heating system and hot water needs. Air-to-water systems connect directly to a wet central heating system, so they work with existing radiators or underfloor heating. Air-to-air systems require fan units in each room and do not connect to a wet system, meaning you need a separate solution for hot water, such as an electric immersion heater or a solar thermal system. The UK government’s Boiler Upgrade Scheme currently offers a £7,500 grant for air-to-water heat pumps but does not cover air-to-air installations (GOV.UK, 2026).
Heating method and efficiency
An air-to-water heat pump circulates heated water through pipes to radiators or underfloor heating. This provides consistent, even warmth. An air-to-air heat pump uses refrigerant to heat indoor air directly, which is then blown into rooms by fan units. Efficiency varies significantly: air-to-water systems have a seasonal efficiency (SPF) of around 2.8 to 3.2, while air-to-air systems can achieve an SPF of 3.0 to 4.0 in well-insulated homes (MCS Certified, 2026). However, air-to-air systems lose efficiency in very cold weather because they rely on outdoor air temperature more directly.
Hot water capability
Air-to-water heat pumps can supply domestic hot water for taps, showers, and baths by storing heated water in a cylinder. This makes them suitable as a primary heating system for most UK homes. Air-to-air heat pumps cannot heat water at all, so you must install a separate system for hot water, typically an electric immersion heater or a gas boiler. This adds installation cost and ongoing energy use. For a typical three-bedroom home, an air-to-water system with a 200-litre hot water cylinder costs around £9,000 to £12,000 installed, while an air-to-air system costs £4,000 to £7,000 but requires an additional £1,500 to £2,500 for hot water (GOV.UK, 2026).
Installation and running costs
Air-to-water heat pumps require more extensive installation: an outdoor unit, pipework to radiators, a hot water cylinder, and often a larger buffer tank. Installation takes 2 to 4 days. Air-to-air systems are simpler to install, with an outdoor unit and indoor fan units mounted on walls, typically completed in 1 to 2 days. Running costs also differ: at current electricity prices of 24p per kWh, an air-to-water system costs roughly 8p per kWh of heat delivered, while an air-to-air system costs around 6p per kWh of heat delivered (Ofgem, 2026). However, air-to-water systems qualify for the £7,500 Boiler Upgrade Scheme grant, which reduces the upfront cost difference significantly.
A worked example
A typical 1930s semi-detached house in Manchester switching from a gas boiler to an air-to-water heat pump costs roughly £12,000 after the £7,500 Boiler Upgrade Scheme grant. This system heats radiators and provides hot water, saving around £395 per year on energy bills according to the Energy Saving Trust. The payback period is about 30 years at current energy prices, though this shortens to roughly 20 years if you also install solar panels. Over 25 years, total savings reach approximately £9,875, not accounting for future energy price rises. For the same house, an air-to-air heat pump costs around £5,000 installed, saves about £300 yearly on heating alone, but requires a separate electric immersion heater for hot water costing an extra £300 per year to run. The air-to-air system pays back in roughly 17 years, but total 25-year savings drop to around £1,000 after factoring in hot water costs.
| Item | Figure |
|---|---|
| Upfront cost after grants | £12,000 (air-to-water) / £5,000 (air-to-air) |
| Yearly savings | £395 (air-to-water) / £0 (air-to-air after hot water costs) |
| Payback period | 30 years (air-to-water) / 17 years (air-to-air) |
| 25-year lifetime savings | £9,875 (air-to-water) / £1,000 (air-to-air) |
What homeowners often get wrong
The most common mistake is assuming both heat pump types qualify for the same government grants. Here are three frequent errors to avoid.
- Believing air-to-air qualifies for the BUS grant Only air-to-water heat pumps are eligible for the £7,500 Boiler Upgrade Scheme. Installing an air-to-air system means you miss out on this grant entirely, leaving you to pay the full £5,000 cost yourself.
- Thinking air-to-air is always cheaper to run Air-to-air units have a higher SPF of 3.0-4.0, but you must factor in separate hot water costs. An electric immersion heater for hot water adds roughly £300 per year, wiping out any heating savings in most UK homes.
- Ignoring existing pipework compatibility Air-to-water systems need a wet central heating system with radiators or underfloor heating. Homes with microbore pipes often require expensive upgrades costing £2,000-£4,000, which many homeowners overlook when comparing upfront prices.
Quick reference
- Air-to-water heat pumps achieve a COP of 2.5-3.0, while air-to-air units reach an SPF of 3.0-4.0 in well-insulated homes.
- Only air-to-water systems qualify for the £7,500 Boiler Upgrade Scheme grant from the UK government.
- Air-to-air systems cannot provide hot water, so you need a separate solution like an electric immersion heater or solar thermal system.
- A typical air-to-water installation for a 1930s semi-detached house costs £12,000 after the BUS grant, with yearly savings of £395.
- Mixing system types by adding an air-to-air unit to an existing gas boiler can void manufacturer warranties and complicate your heating controls.
Frequently Asked Questions
Air-to-air heat pumps can achieve an SPF of 3.0 to 4.0, slightly higher than air-to-water's 2.8 to 3.2, according to MCS Certified data. However, air-to-water systems provide hot water, making them more versatile for UK homes.
No, an air-to-air heat pump cannot provide hot water. You would need a separate system, such as an electric immersion heater or solar thermal, for domestic hot water.
No, the Boiler Upgrade Scheme only covers air-to-water heat pumps, offering a £7,500 grant. Air-to-air installations are not eligible under current GOV.UK rules.
