As the UK moves toward cleaner energy and low-carbon heating, heat pumps have taken centre stage as a viable alternative to gas boilers. Two of the most common types, wet air source heat pumps and air-to-air systems. In this blog, we’ll explain what each system is, how they differ, and what you can expect in terms of running costs.
What is a Wet Air Source Heat Pump System?
A wet air source heat pump extracts heat from the outdoor air and transfers it into water, which is circulated through your home’s radiators, underfloor heating, or a hot water cylinder.
Key Features:
- Works with standard central heating systems.
- Provides both space heating and domestic hot water.
- Best for well-insulated homes and low-temperature heating systems (e.g., underfloor or large radiators).
- Typically includes a hot water cylinder.
These systems are designed as a full replacement for a gas or oil boiler, making them ideal for homes upgrading their main heating system.
What is an Air-to-Air Heat Pump System?
An air-to-air heat pump works more like a reversible air conditioner — it absorbs heat from outside and delivers it as warm air indoors via fan units. In summer, it can be reversed to provide cooling.
Key Features:
- Directly heats (and cools) air indoors – no radiators or water involved.
- Uses one or more indoor fan coil units.
- Can provide air conditioning in hot weather.
- Does not heat domestic hot water.
Air-to-air systems are common in smaller homes, flats, or open-plan spaces, and are great for zoned heating.
Wet vs Air-to-Air: Key Differences
| Feature | Wet ASHP (Air-to-Water) | Air-to-Air ASHP | Air-to-Air + Electric Water Heating |
|---|---|---|---|
| Heating Method | Radiators or underfloor | Warm air via fan units | Warm air via fan units |
| Domestic Hot Water | ✅ Yes | ❌ No | ✅ Provided separately |
| Cooling | ⚪ Rare | ✅ Yes | ✅ Yes |
| Ideal For | Whole-home heating | Flats or zoned spaces | Smaller homes |
| Annual Running Cost (no solar) | ~£800–£1,200 | ~£400–£800 | ~£700–£1,400 |
| System Cost (approx.) | ~£10k–£15k | ~£4k–£8k | ~£5.5k–£10k |
Running costs will vary depending on the property, insulation, temperature settings, and electricity tariff.
What If You Need Electric Water Heating?
Because air-to-air systems don’t produce hot water, many households pair them with:
- An electric immersion heater, or
- A hot water cylinder with an electric element.
Heating water electrically can cost £300–£600 per year for 2–3 people, depending on use and tariff. That means total running costs for an air-to-air system plus electric water heating can reach £700–£1,400 per year — similar to, or sometimes higher than, a wet ASHP.
Which System is Right For You?
Choose a Wet ASHP if:
· You want a full replacement for a gas/oil boiler.
· You need hot water as well as heating.
· Your home is well insulated and already has or can support a radiator or underfloor system.
Choose an Air-to-Air ASHP if:
· You’re looking to heat (and cool) specific rooms efficiently.
· You have electric heating and want to reduce costs.
· You live in a smaller, open-plan home or flat.
Still Have Some Questions?
Both types of air source heat pumps are excellent low-carbon alternatives to traditional heating.
A wet ASHP offers full-home comfort and hot water, while an air-to-air system is often cheaper to install and great for zoned heating and cooling.
Before deciding, it’s worth booking a home energy survey to assess insulation, layout, and heating demand. And remember — as of 2025, the Boiler Upgrade Scheme can provide up to £7,500 toward installing a new air source heat pump until the 31st December 2027.
Still not sure? Give the Greener Homes in Lancashire team a call for some free impartial advice: 0800 058 4066