Air to Water Heat Pumps Explained
Published: March 2026 | Reading time: 9 minutes
If you're looking at heat pumps for your UK home, the chances are you'll end up with an air-to-water system. It's by far the most popular type in Britain, accounting for the vast majority of domestic heat pump installations. And for good reason — it's the most practical replacement for a gas boiler, connecting to your existing radiators and providing both space heating and hot water.
This guide explains exactly how air-to-water heat pumps work, what they cost, how they integrate with your home, and why they've become the default choice for UK homeowners making the switch from gas.
What Is an Air-to-Water Heat Pump?
An air-to-water heat pump extracts heat from the outdoor air and transfers it to water. That heated water is then circulated through your radiators or underfloor heating to warm your rooms, and it also heats your domestic hot water cylinder.
In practical terms, it replaces your gas boiler. The outdoor unit sits in your garden. The indoor components (which may include a hydraulic module, buffer tank, and hot water cylinder) connect to your existing central heating pipework. Your radiators, thermostats, and room controls work much as they did before.
For a broader introduction to the technology, see our complete guide to air source heat pumps.
How Air-to-Water Heat Pumps Work
The process happens in four stages:
1. Heat Absorption (Outdoor Unit)
A large fan draws outdoor air across an evaporator coil containing cold refrigerant. Even when outdoor air is just a few degrees above freezing, the refrigerant (which has a boiling point of around -25°C to -40°C) absorbs heat from it and evaporates into a gas.
2. Compression
The gaseous refrigerant passes through a compressor, which increases its pressure and, crucially, its temperature. This is the key step that transforms low-grade outdoor heat into high-grade heat suitable for your home. The compressor is the component that uses the most electricity.
3. Heat Transfer (Indoor Unit)
The hot, compressed refrigerant passes through a heat exchanger (condenser) where it transfers its heat to water. This water is then pumped to your radiators, underfloor heating, and hot water cylinder. As the refrigerant gives up its heat, it condenses back into a liquid.
4. Expansion
The liquid refrigerant passes through an expansion valve, which drops its pressure and temperature back down, ready to absorb heat from the outdoor air again. The cycle repeats continuously.
The entire process is powered by electricity, but because the system is moving existing heat rather than generating it, you get 2.5-4.0 units of heat for every unit of electricity consumed.
How They Work with Radiators
This is one of the most common questions: can an air-to-water heat pump work with my existing radiators? The answer is usually yes, but with some important caveats.
Flow Temperature
A gas boiler typically heats water to 60-80°C before sending it to your radiators. A heat pump works most efficiently at much lower flow temperatures — ideally 35-45°C for underfloor heating, or 45-55°C for radiators.
Lower flow temperatures mean higher efficiency (better COP), which means lower electricity bills. But lower water temperature also means each radiator produces less heat. To compensate, you may need to:
- Upsize some radiators: Larger radiators emit more heat at lower temperatures. You might not need to replace all of them — often just the ones in rooms with the highest heat demand (typically living rooms and bedrooms).
- Add radiators: In some rooms, adding a second radiator may be more practical than replacing the existing one with a very large unit.
- Accept slightly higher flow temperatures: Running at 50-55°C instead of 40-45°C reduces efficiency slightly but may avoid the need for new radiators. It's a trade-off your installer can help you assess.
For a detailed look at this topic, read our guide on whether you need new radiators for a heat pump.
How They Work with Underfloor Heating
Underfloor heating (UFH) is the ideal partner for an air-to-water heat pump. Here's why:
- Low flow temperatures: UFH works optimally at 30-40°C, which is exactly where heat pumps are most efficient. COP with UFH is typically 0.5-1.0 higher than with radiators.
- Large surface area: The entire floor acts as a heat emitter, providing even warmth throughout the room.
- Thermal mass: UFH heats the floor slab, which acts as a heat store. This smooths out temperature fluctuations and allows the heat pump to run at steady, efficient speeds.
If you're building an extension, renovating, or doing significant work to your home, installing UFH in conjunction with a heat pump is well worth considering.
Hot Water: How It Works
Unlike an air-to-air system, an air-to-water heat pump heats your domestic hot water. It does this by heating water in a cylinder (similar to the hot water tanks many homes already have).
Key Points About Hot Water
- Cylinder required: You'll need a hot water cylinder, typically 150-250 litres depending on household size. If you currently have a combi boiler (no cylinder), one will need to be installed.
- Temperature: Heat pumps typically heat water to 45-55°C. This is adequate for domestic use but lower than the 60-65°C a gas boiler might produce. Regular Legionella pasteurisation cycles (heating to 60°C+) are usually programmed automatically.
- Reheat time: A heat pump heats the cylinder more slowly than a gas boiler — typically 1-2 hours for a full reheat compared to 20-40 minutes with gas. This is rarely noticeable if the system is properly programmed.
- Priority: Most systems give hot water priority over space heating, ensuring you always have hot water when you need it.
- Immersion backup: Most cylinders include an electric immersion heater as backup. This kicks in only if the heat pump can't meet demand (very rare in normal operation).
Costs: What to Expect
Installation Costs
Typical installed costs for an air-to-water heat pump system in 2026:
| Property Type | System Size | Total Cost (before grant) | After BUS Grant (£7,500) |
|---|---|---|---|
| 2-bed flat/terrace | 5-7 kW | £7,000 – £10,000 | £0 – £2,500 |
| 3-bed semi | 7-10 kW | £8,000 – £12,000 | £500 – £4,500 |
| 4-bed detached | 10-16 kW | £10,000 – £15,000 | £2,500 – £7,500 |
| Large/older property | 14-20 kW | £12,000 – £18,000 | £4,500 – £10,500 |
These costs include the heat pump unit, hot water cylinder, installation labour, pipework modifications, and commissioning. Additional costs may apply if significant radiator upgrades or electrical work is needed.
Running Costs
Annual heating and hot water costs for a typical 3-bed semi with an air-to-water heat pump are approximately £600-900, compared to £900-1,200 for gas. Actual savings depend on your home's insulation, the system's efficiency, and your electricity tariff.
For detailed running cost analysis, visit our heat pump running costs guide.
Why Air-to-Water Is the UK's Default Choice
Several factors make air-to-water the dominant heat pump type in Britain:
- Direct boiler replacement: It connects to your existing central heating pipework, making it a straightforward swap from a gas or oil boiler.
- Handles heating and hot water: One system does everything, unlike air-to-air which only provides space heating.
- BUS grant eligible: The £7,500 Boiler Upgrade Scheme grant makes air-to-water significantly more affordable.
- Invisible heating: No indoor units on walls — your radiators and UFH work silently as before.
- Proven in UK conditions: Hundreds of thousands of installations, extensive monitoring data, and a mature installer network.
- Works with existing infrastructure: Most UK homes have wet central heating systems that are compatible with air-to-water heat pumps.
For a comparison with the alternative, see our guide to air-to-air heat pumps.
Installation: What's Involved
A typical air-to-water heat pump installation takes 2-5 days and involves:
Day 1: Preparation
- Laying the concrete base or installing ground screws for the outdoor unit
- Making penetrations through the external wall for pipe runs
- Removing the old boiler (if being replaced)
Days 2-3: Main Installation
- Positioning and securing the outdoor unit
- Installing the indoor hydraulic components
- Installing or relocating the hot water cylinder
- Connecting pipework between outdoor unit, indoor unit, and heating system
- Electrical connections and dedicated circuit installation
Days 3-5: Completion
- Filling, flushing, and pressurising the heating system
- Refrigerant charge check
- Commissioning — setting up weather compensation, timers, and controls
- Handover and user training
- Any radiator upgrades if needed
For a more detailed walkthrough, see our heat pump installation guide.
Choosing the Right Size
Correct sizing is critical. An undersized heat pump won't keep your home warm. An oversized one cycles on and off too frequently, wasting energy and increasing wear.
Sizing is based on your home's heat loss calculation, which considers:
- Property size and type
- Insulation levels (walls, roof, floor, glazing)
- Air tightness
- Number of occupants and hot water demand
- Local design temperature
As a very rough guide:
- Well-insulated 2-bed: 4-6 kW
- Average 3-bed semi: 7-10 kW
- 4-bed detached: 10-14 kW
- Large/older property: 14-20 kW
Always get a proper heat loss survey. Don't rely on rough estimates or "rule of thumb" sizing — the difference between a correctly and incorrectly sized system is the difference between comfort and frustration. Use our heat pump calculator for a starting estimate.
Popular Brands in the UK
The UK market has several well-established air-to-water heat pump brands:
- Vaillant (aroTHERM): One of the most popular brands in the UK, excellent efficiency ratings, quiet operation
- Daikin (Altherma): Long track record, wide range of sizes, good installer support
- Mitsubishi Electric (Ecodan): Very popular with installers, reliable, good cold-weather performance
- Samsung (EHS): Competitive pricing, good smart features, strong efficiency
- Bosch: Growing presence in the UK market, solid reputation from their boiler heritage
- Grant (Aerona3): Particularly popular in rural areas and for oil boiler replacements
- NIBE: Swedish brand with an excellent track record in cold climates
Frequently Asked Questions
Can an air-to-water heat pump replace my combi boiler?
Yes, but you'll need to add a hot water cylinder since combi boilers heat water on demand without a tank. You'll need space for the cylinder — typically a cupboard or utility room. The cylinder is usually 150-200 litres for a standard household.
Will I need to change my radiators?
Not necessarily. Many existing radiators work fine with a heat pump, particularly if they were slightly oversized for the room (which is common). Your installer will calculate whether each radiator can deliver enough heat at the lower flow temperatures a heat pump uses. You may need to upsize radiators in some rooms — typically 2-4 out of the total.
How long does an air-to-water heat pump last?
With proper maintenance, 15-20 years for the main unit and compressor. The hot water cylinder can last even longer. This is comparable to a gas boiler, though heat pumps tend to require fewer repairs due to having no combustion components.
Is an air-to-water heat pump noisy?
The outdoor unit produces 40-50 dB at one metre — similar to a quiet conversation or a humming fridge. Indoors, the system is effectively silent as heat is delivered through radiators or UFH with no moving parts in the room.
Can I keep my existing hot water cylinder?
Sometimes. If your existing cylinder is in good condition, well-insulated, and has a suitable coil arrangement, it may be compatible. However, many installers recommend a new cylinder designed specifically for heat pump temperatures and with a larger coil for efficient heat transfer.
What electricity supply do I need?
Most domestic air-to-water heat pumps run on a standard single-phase electricity supply. Larger systems (above about 16 kW) may require a three-phase supply. Your installer will check your electrical capacity as part of the survey.
The Bottom Line
Air-to-water heat pumps are the standard choice for UK homeowners switching from gas or oil boilers, and for good reason. They provide both heating and hot water, work with your existing radiators (with possible upgrades), qualify for the £7,500 BUS grant, and deliver genuine running cost savings when properly installed.
The technology is mature, reliable, and proven in hundreds of thousands of UK homes. The key to success is correct sizing, quality installation by an experienced MCS-certified engineer, and appropriate radiator/emitter setup for low flow temperatures.
Ready to explore whether an air-to-water heat pump suits your home? Use our suitability checker for a quick assessment, or get free quotes from vetted local installers.