Air-to-Water Heat Pumps: What They Heat and What Your Home Needs
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The short answer: what an air-to-water heat pump heats
An air-to-water heat pump takes heat from outdoor air and uses it to warm water. That water then does the same jobs your boiler does today. It can heat:
- Radiators, usually larger or better-matched ones than a boiler needs
- Underfloor heating, which suits heat pumps particularly well
- Fan coil units, which are radiator-like units with a fan that push warmed air into a room
- Stored hot water, in a hot water cylinder
The key point is that it heats water, which then flows round a wet central-heating system. It does not blow warm air through your home by default.
Whether it works well in your house comes down to five things: how much heat the building loses, whether your heat emitters can do their job with cooler water, whether you have room for a hot water cylinder, whether there is a sensible spot outdoors for the unit, and whether your electricity supply can cope. The checklist later in this guide turns each into a pass, maybe or fix-first result.
As a beginner, treat that checklist as a way to prepare for a proper survey. It cannot replace a survey by a qualified installer.
How it works with a wet central-heating system
A wet central-heating system is one that circulates hot water through pipework to radiators or underfloor heating. If your home has a gas, oil or electric boiler feeding radiators, you already have one. The alternative is an air-based or ducted system, which moves warm air instead of water.
An air-to-water heat pump slots into the heat-source part of that wet system:
- A fan moves outdoor air across a heat exchanger in the outdoor unit. A refrigerant absorbs heat from that air, even when it feels cold to you.
- A compressor raises the temperature of that heat.
- The heat is passed to water, which is pumped to your emitters and to the hot water cylinder.
In short, the pump moves heat rather than creating it by burning something. That is why it can deliver several units of heat for each unit of electricity it uses, as long as conditions suit it.
The pipework, radiators and underfloor loops you already have may be reusable. Often some parts need changing, which is what the rest of this guide helps you judge.
Hot water works differently. A heat pump normally heats a stored tank of water rather than producing it instantly like a combi boiler. That is why a cylinder is part of the picture.
What it does not do by default
Because the output is warm water, a few things are worth knowing:
- It does not heat the air directly. Rooms warm up through radiators, floor loops or fan coils.
- It does not remove the need for a wet system. If your home has no pipework or emitters, adding them is a bigger job than swapping a heat source.
- It does not usually give instant hot water. Plan for a cylinder.
- It is not the same as an air-to-air heat pump. Those heat and cool rooms by blowing air from indoor units and do not feed radiators or a cylinder. They are a different product with their own trade-offs, so look at a dedicated comparison if you are weighing the two.
For a home replacing a wet central-heating boiler, air-to-water is generally the configuration to focus on, because it works with the system you already have.
Why flow temperature matters for efficiency and radiators
Flow temperature is the temperature of the water sent out to your emitters. Boilers often run it quite hot. Air source heat pumps work best as low-temperature systems, sending out cooler water for longer periods.
This matters because the harder a heat pump has to work to raise the water temperature, the more electricity it uses per unit of heat. Efficiency is described with two terms:
- COP (coefficient of performance): how much heat you get per unit of electricity at a particular moment.
- SCOP (seasonal COP): the same idea averaged over a whole heating season, which is a fairer guide to real life.
Lower flow temperatures generally support a better COP and SCOP. That is the reason emitters matter. Cooler water gives off less heat from the same radiator, so the radiator needs more surface area to deliver the same warmth.
| Emitter | How it copes with low flow temperatures |
|---|---|
| Underfloor heating | Very well, because a large floor area gives off gentle heat |
| Low-temperature or larger radiators | Well, if sized for the room's heat loss |
| Fan coil units | Well, because the fan helps release heat |
| Existing smaller radiators | Depends on the room; some may be too small and need upgrading |
You do not necessarily need underfloor heating, and you do not necessarily have to replace every radiator. A room-by-room calculation shows which emitters are already big enough and which are not. Radiators designed for low flow temperatures exist for exactly this reason, and steady, gentle heat is what a well-matched system delivers.
Will it work in my house? The five-gate checklist
Work through each gate and note whether your home is a pass, a maybe, or fix first. A fix-first result is not a verdict against heat pumps. It tells you what to sort out before or alongside installation.
| Gate | Pass | Maybe | Fix first |
|---|---|---|---|
| 1. Insulation and heat loss | Loft and walls are insulated, windows and doors are reasonably draught-free, and a survey shows modest heat demand | Some insulation gaps; rooms that stay cold or have large windows | Obvious draughts, uninsulated loft or walls, or an installer flags very high heat loss |
| 2. Emitters | Underfloor heating, fan coils, or radiators sized generously for each room | A mix of radiator sizes; some rooms look undersized | Many small radiators, or none in key rooms |
| 3. Hot water cylinder | You already have a cylinder and the installer confirms it can work with a heat pump | Old or small cylinder, or one that needs checking or replacing | Combi boiler with no cylinder and nowhere obvious to put one |
| 4. Outdoor space | A clear wall or ground area with open airflow, away from boundaries and neighbours' windows | A tight spot, or one close to bedrooms or a boundary | No practical location without a layout change |
| 5. Electrical supply | Installer confirms your supply and consumer unit have capacity | Not sure; older fuse board or other large electrical loads | Installer says the supply or board needs upgrading |
Notes on each gate
Insulation and heat loss. A well-insulated home is crucial for good heat pump performance. The lower the heat loss, the smaller the heat pump and emitters you need, and the lower the flow temperature you can use. A qualified installer should carry out a room-by-room heat loss calculation and use it to size both the heat pump and the emitters. Be cautious of anyone who sizes a system from floor area alone.
Emitters. Look at each radiator and ask whether it is large for its room. Single-panel radiators in big or cold rooms are the usual suspects. Replacing a few radiators is often far cheaper than redoing a whole system.
Cylinder. Check where a cylinder could go, such as an airing cupboard, utility room or garage. Ask whether the ceiling height and floor loading are suitable. Cylinders take up more space than many people expect.
Outdoor space. Look for somewhere with room for airflow and for service access. Rules on siting outdoor units, including planning or permitted development in the UK and Ireland, vary by location and property type. Check them with your local planning authority or your installer before committing.
Electrical supply. A heat pump is an additional electrical load. Your installer should check the incoming supply and consumer unit, and tell you whether any upgrade is needed. Do not assume either way.
Reading your result
- Mostly passes: your home is a strong candidate. Get survey-based quotes.
- A mix of passes and maybes: a survey will tell you which upgrades are needed and how much they add to the project.
- Any fix-first on insulation or emitters: deal with it first, because it affects efficiency and running costs for the life of the system.
Cold weather, noise and siting: setting expectations
Cold weather. Heat pumps are designed to work through winter, and some manufacturers state that their systems operate in outdoor temperatures as low as around -15°C. Efficiency does fall as the air gets colder, because the pump has to work harder. A system sized from a proper heat loss survey should still keep the house warm in cold spells. An undersized unit can struggle in cold spells. Some systems also include electric backup heat by design for low outdoor temperatures or rapid recovery; because resistance heat is less efficient than heat-pump operation, ask when the controls are expected to use it. If a quote includes backup heating, ask when and how often it is expected to come on.
Noise. The outdoor unit has a fan and a compressor, so it makes some sound. How noticeable it is depends on the model and, importantly, where it is placed. Avoid positions right next to bedroom windows, patios you use often, or neighbours' boundaries, and ask your installer about expected sound levels at the location they propose.
Siting. Beyond noise, think about:
- Clear airflow around the unit, with no tight corners or dense planting
- Easy access for servicing
- The distance of pipework to the cylinder and into the house, since longer runs lose more heat
- Drainage for condensate that the unit produces in cold, damp weather
Costs and installation. Costs depend on factors such as heat pump capacity, property type and the existing heating system. Heat pump installations are project-specific, so published price ranges are only rough guides. Always get quotes based on a survey of your own home. A home-specific estimate, such as the comparison calculator on this site, can help you frame what to expect, but it is not a quote.
What to do next
If your checklist shows mostly passes and maybes, you have enough to start a sensible conversation with installers.
- Note your results for each of the five gates, including anything you are unsure about.
- Ask for a room-by-room heat loss calculation, and ask to see the numbers behind the proposed heat pump size and emitter changes.
- Ask what will change: which radiators, the cylinder, pipework, electrical work and the outdoor unit location.
- Ask about performance in cold weather and whether any electric backup is included.
- Check local rules for siting the outdoor unit, and any current grants or incentives for your country, directly with the relevant authority. These change over time.
- Compare quotes like for like so that the scope of work is the same across them.
If an insulation or emitter gate came back as fix first, sort that out before you make a final decision. It often improves comfort and cuts running costs whatever heating you end up with.
