Air to Water Heat Pump: How It Works and What It Needs

Air to Water Heat Pump: How It Works and What Your Home Needs

An air to water heat pump is the type most people mean when they talk about replacing a boiler with a heat pump. It takes heat from the outside air and uses it to warm water, which then runs through your radiators or underfloor heating and fills your hot water cylinder.

That’s the short version. The longer version matters more, because how well one works in your house depends less on the heat pump itself than on everything around it: the radiators, the pipework, the cylinder, the controls and, above all, whether the unit was sized from a proper heat loss calculation. This guide explains how an air to water heat pump works, how it differs from an air to air one, where the real downsides are and what to check before you ask anyone for a quote.

What is an air to water heat pump?

“Air source heat pump” is the umbrella term, and it covers two different machines. An air to water heat pump moves heat from the air into water, so it connects to the same kind of wet central heating system a gas or oil boiler does. An air to air heat pump moves heat from outside air into the air inside your home through indoor units, much like a reversible air conditioner.

When someone in the UK says they’re getting a heat pump to replace their boiler, they almost always mean the air to water type. It heats your radiators or underfloor heating, and it heats your hot water too, as long as there’s a suitable cylinder to store it in.

How does an air to water heat pump work?

The principle is the same as a fridge running in reverse. A fan pulls outside air across a coil called the evaporator. Inside that coil is a refrigerant that boils at a very low temperature, so even cold air has enough heat in it to turn the refrigerant into a gas. A compressor then squeezes that gas, which raises its temperature sharply. The hot gas passes through a heat exchanger, where it hands its heat over to the water in your heating system. Having given up its heat, the refrigerant turns back into a liquid, passes through an expansion valve that drops its pressure and temperature, and the cycle starts again.

The electricity in the system powers the compressor, the fan and the pumps. It isn’t creating heat, it’s moving heat that’s already there, which is why a well-designed system can deliver around three to four units of heat for every unit of electricity. That ratio changes through the year. It’s higher on a mild day and lower on a freezing one, and it’s higher when the heat pump only has to warm the water to a low temperature. In cold, damp weather the outdoor unit can also ice up and needs to run a short defrost cycle, which is normal.

Monobloc or split: the two main layouts

Most air to water heat pumps come in one of two layouts. In a monobloc, all the refrigerant parts are sealed inside the outdoor unit, and only water pipes run into the house. That makes installation simpler, but those outdoor water pipes need proper insulation and frost protection, and the installer has to follow the manufacturer’s guidance on how to handle that.

In a split system, the refrigerant circuit is divided between an outdoor unit and an indoor unit, usually called a hydrobox, with refrigerant pipes running between them. The water circuit stays inside the house, away from the frost, but the refrigerant pipework has to be installed by an engineer qualified to handle refrigerants. Neither layout is better in every home. The right choice depends on where the units can go, how far apart they need to be and what suits the property.

What your home needs for an air to water heat pump to work well

This is the part that decides whether you end up happy or frustrated, and it’s where most poor installations go wrong.

Radiators and flow temperature. A boiler typically heats radiator water to 70°C or more. A heat pump is most efficient when it heats water to a much lower temperature, often somewhere between 35°C and 50°C. A radiator running at those temperatures gives out roughly a third to a half of the heat it gives at boiler temperature, depending on the exact figures. So in many homes some radiators need to be larger, or some rooms need underfloor heating, to heat the room on the coldest days without pushing the flow temperature up and the efficiency down. If your radiators already feel lukewarm, our guide to heat pump radiators that feel lukewarm explains what’s normal and what isn’t.

A suitable hot water cylinder. An air to water heat pump needs somewhere to store hot water. If you have a combi boiler and no cylinder, you’ll need space for one. If you already have a cylinder, it may or may not be suitable, because heat pumps need a cylinder with a coil large enough to transfer heat at lower temperatures. Our article on what temperature your heat pump hot water should be set to covers how that balance works day to day.

Pipework that can carry the flow. A heat pump moves a lot more water at a lower temperature than a boiler does, so the pipes between the heat pump and the emitters have to be big enough. Old or undersized pipework, including microbore, can restrict that flow and hurt performance, however good the heat pump is.

Space and noise. The outdoor unit needs clear space around it for airflow and access, and it makes some noise. Where you put it matters for your comfort and your neighbours’. We’ve written about why heat pumps can be noisy and what to do about it.

Electrical supply. The installer should check that your electrical supply and consumer unit can handle the unit. Most homes manage, but it’s worth confirming before the day the installers turn up.

Controls. A heat pump runs best when it heats gently and steadily, adjusting its output to the weather. That’s what weather compensation does, and getting it set up properly makes a real difference to running costs. See our guide to setting weather compensation on a heat pump.

Air to water vs air to air heat pump

The two sound similar and do quite different jobs. An air to air heat pump heats (and usually cools) the air in a room directly, using an indoor unit mounted on a wall or ceiling. It doesn’t connect to radiators and it doesn’t make hot water, so you’d still need a separate way of heating your water, and in most UK homes you’d still be keeping a boiler or another heat source for the rest of the system.

An air to water heat pump connects to a wet heating system. It can replace the boiler entirely, work with your radiators or underfloor heating, and heat your hot water. That makes it more involved to design and install, but it’s the one that actually replaces a boiler.

If you want cooling as well, some air to water systems can provide it, and our guides on adding cooling to an existing heat pump and whether a heat pump can cool the whole house explain what’s realistic. Grant rules for the two types have differed and change from time to time, so check the current scheme conditions before assuming either type qualifies.

What are the downsides of an air to water heat pump?

There are real ones, and it’s better to know them up front.

The upfront cost is higher than a like-for-like boiler replacement. Installed costs commonly run from around £8,000 to £15,000 or more, depending on the property and what needs upgrading, and our heat pump installation cost guide breaks down where that money goes. Your radiators or pipework may need work, which adds to it. The outdoor unit needs space and makes some noise. The house warms up differently, with the heat pump running gently for long periods instead of blasting hot water through for short bursts, which feels different if you’re used to a boiler. And running costs depend heavily on your flow temperature and your electricity tariff, as we explain in how much a heat pump should cost to run in winter.

What’s worth saying is that most of the complaints we see aren’t about the technology. They come from a heat pump that was sized by guesswork, or an installation that didn’t allow for the radiators, or controls that were never set up properly.

Are air to water heat pumps any good?

Yes, when the system around them is designed properly. They’re mature, widely used technology, and a well-designed one can heat a UK home efficiently through the winter.

The catch is that “designed properly” starts with the heat pump’s size. An oversized unit switches on and off too often, which wastes energy and wears the equipment, as we explain in what happens if a heat pump is oversized. An undersized one struggles on cold days, which is why we also wrote about the signs your heat pump is undersized. Getting the size right depends on a room-by-room calculation of how much heat your home actually loses, not a rule of thumb based on floor area. Our guides on what size heat pump you need and how a heat loss calculation works go through how it’s done.

What to do before you ask for quotes

Before you collect quotes, it helps to pin down four things: the heat loss of each room, the flow temperature you’re aiming for, whether your radiators, cylinder and pipework can work with it, and what the system should look like. Without that, every installer fills the gaps differently, and you end up with quotes you can’t compare.

That’s what our independent heat pump design service is for. We work out the heat loss, size and specify the system, and give you a design you can send to any installer, so every quote is for the same system. If you’ve already got a design or quote and want it checked before you commit, our Pre-Installation Design Review does that. If you’d rather have an engineer at the property, we also offer a Heat Loss & Plumbing Survey.

Frequently asked questions

How efficient is an air to water heat pump?

A well-designed system commonly delivers around three to four units of heat for every unit of electricity over a year. Efficiency drops as the flow temperature goes up and as the weather gets colder, which is why low flow temperatures and correct sizing matter so much.

Does an air to water heat pump work in cold weather?

Yes. Modern units keep heating well below freezing. Output and efficiency do fall as it gets colder, and the unit runs occasional defrost cycles in cold, damp conditions. A correctly sized system allows for this.

Can I use my existing radiators with an air to water heat pump?

Sometimes. It depends on the heat loss of each room and the flow temperature you’re designing for. Some radiators can stay, some need replacing with larger ones, and some rooms suit underfloor heating better. That’s worked out room by room as part of the design.

Do I need a hot water cylinder?

Yes, if you want the heat pump to heat your hot water. A heat pump stores hot water in a cylinder rather than heating it instantly, so a combi boiler setup needs space for one. An existing cylinder may be reusable if it’s suitable.

How long does an air to water heat pump last?

Often quoted at around 15 to 20 years with regular servicing. Our guide on how long an air source heat pump lasts covers what affects that.

How much does an air to water heat pump cost?

Installed costs commonly run from around £8,000 to £15,000 or more, depending on the property, the size of the unit and any radiator, pipework or cylinder upgrades. Our installation cost guide goes into the detail.