By Sarah Cooper, Technical Reviewer, MCS Certified Heat Pump Engineer · Last reviewed
Heat Pump in Summer: Cooling Your Home with a Reversible System
A Heating System That Also Cools
Most people in Britain buy a heat pump to replace a gas boiler, so they think of it purely as a winter machine. Yet the technology inside an air source heat pump is the same as the technology inside an air conditioner. It moves heat from one place to another using a refrigerant cycle, and a refrigerant cycle does not care which way it runs. Make it run in reverse and the unit that warmed your home all winter can pull heat out of your rooms and dump it outside instead.
That reversible behaviour is what turns a heating appliance into a year-round comfort system. As UK summers grow warmer and heat waves become more frequent, the question of whether a heat pump can cool as well as heat is no longer niche. The short answer is that many air source heat pumps can cool, but only if the unit, the controls, and the emitters are specified for it from the start. This guide explains how summer cooling works, what it costs to run, where it falls short, and how to decide whether active cooling is worth fitting.
How a Reversible Heat Pump Cools Your Home
A heat pump works by compressing a refrigerant so it gets hot, then letting it expand so it gets cold, and using a fan and a heat exchanger at each end of that cycle. In heating mode, the outdoor unit absorbs heat from the air outside, even when it feels cold to us, and the indoor side releases that heat into your water circuit. A component called a reversing valve can flip the direction of refrigerant flow. When it does, the indoor side becomes the cold side and the outdoor side becomes the hot side, so the system extracts warmth from indoors and rejects it outside.
This is identical in principle to how a fridge keeps its contents cold while warming the room behind it, and identical to a conventional air conditioning unit. The difference with a domestic heat pump is what happens to the cool it produces. In most British installations the heat pump heats water, and that water flows to radiators or underfloor pipes. So cooling has to be delivered through the same wet circuit, which has real consequences for how cold your home can get and how the system behaves.
There are two broad routes to cooling with a heat pump, and they perform very differently.
Cooling Through Underfloor Pipes
If you have underfloor heating fed by your heat pump, running chilled water through those same pipes in summer can take the edge off a hot room. The large surface area of a floor makes a gentle, even cooling effect. The catch is condensation. If the floor surface drops below the dew point of the room air, moisture condenses on it, which is uncomfortable and can damage finishes. For that reason, underfloor cooling is deliberately mild, typically holding the floor a few degrees cooler rather than chilling it hard, and it is best paired with humidity sensing that lifts the water temperature if condensation risk rises. Done properly it is pleasant and quiet, but it will not feel like air conditioning. Our guide to underfloor heating with a heat pump covers how that emitter behaves in both seasons.
Cooling Through Fan Coil Units
The more powerful option is a fan coil unit, sometimes called a fan convector. This is a small indoor heat exchanger with a fan that blows room air across chilled water. Because it actively moves air, it delivers far more cooling than a passive floor or radiator, and it manages condensation by collecting it in a tray and draining it away. Fan coils give a result much closer to conventional air conditioning, but they have to be plumbed and wired in during installation, and they are an extra cost on top of the heat pump itself. Standard radiators, by contrast, are almost useless for cooling, because they rely on rising warm air to circulate and that effect collapses when the surface is cold.
Which Heat Pumps Can Actually Cool
Not every air source heat pump is reversible, and not every reversible unit is set up to cool a home well. When you are specifying a system, this is one of the most important early decisions, because retrofitting cooling later is far more disruptive than building it in.
Many of the popular air source models sold in the UK offer cooling as a factory option, including units from Daikin, Mitsubishi Electric, Vaillant and Samsung, among others. The hardware is often capable, but the cooling function may be disabled in software, may require a specific controller, or may need the right emitters to be useful. Ground source heat pumps can also cool, and they have a particular advantage explored below, but the principles for the wet circuit are the same. If you are still weighing the two technologies, our comparison of air source versus ground source heat pumps sets out the wider trade-offs.
The practical checklist when you want summer cooling is short but firm. The heat pump must be a reversible model with cooling enabled. The emitters must be suitable, which in practice means fan coil units for meaningful cooling or underfloor pipes for gentle cooling, not standard radiators. The controls must support a cooling mode and, ideally, humidity management. And the installer must commission the cooling side, not just the heating side, because a system that was only ever tuned for winter will cool poorly or trip on condensation alarms.
What Cooling Costs to Run
Cooling is not free, and it is worth being clear-eyed about the running cost before you assume a heat pump will keep you cool all summer at no extra expense. In cooling mode the heat pump still consumes electricity to drive the compressor and fans, so every hour of cooling adds to your bill in the same way a portable air conditioner would.
The efficiency of cooling is described by a metric called the energy efficiency ratio, the cooling equivalent of the coefficient of performance used for heating. A typical reversible air source heat pump delivers several units of cooling for each unit of electricity, so it is far cheaper than resistive cooling, but the absolute cost still depends on how long you run it and how cold you try to make the house. If you understand how efficiency ratios drive heating bills, the same logic applies in reverse, and our explainer on heat pump COP lays out the underlying maths in plain terms.
In a typical British summer, the good news is that the demand is modest. Most homes need cooling only for a handful of genuinely hot weeks, often for a few hours in the afternoon and evening rather than around the clock. That intermittent pattern keeps total running costs low compared with the heating season. The figure to watch is not the headline rate but the hours of use, because a system run sensibly during the hottest part of the day costs far less than one left chilling an empty house. For a fuller picture of how electricity prices feed through to a heat pump bill, our breakdown of real heat pump running costs is a useful companion, and the same tariff thinking applies to cooling load.
The Ground Source Advantage: Passive Cooling
There is one cooling trick that ground source heat pumps can pull off and air source units cannot, and it is genuinely efficient. Because a ground source system circulates fluid through buried pipes, and the ground a metre or two down stays cool and stable through summer, the system can run in a passive or free cooling mode. Instead of running the compressor to make cold, it simply circulates the naturally cool ground fluid through the indoor emitters, using only the pumps. That uses a fraction of the electricity of active cooling, because the expensive compressor barely runs.
Passive ground cooling will not deliver the deep chill of a powerful air conditioner, but for taking the heat out of a home through a hot spell it is remarkably cheap to run. It is one of the underrated benefits of a ground array, and worth raising with an installer if cooling matters to you and a ground loop is feasible on your plot. Air source heat pumps have no such option, because they reject heat to the outside air, which in summer is the very thing making your home hot in the first place.
The Limits You Need to Plan Around
It is important to set expectations honestly, because the biggest cause of disappointment with heat pump cooling is assuming it behaves like a high-powered split air conditioning system. It usually does not, and understanding the limits up front avoids frustration.
The first limit is capacity. A heat pump is sized for your home's heat loss in winter, not for a peak cooling load in a heat wave. The cooling output it can deliver is constrained by the unit size and, more importantly, by the emitters. A system cooling through a couple of fan coils will struggle to chill a large open-plan space the way a dedicated air conditioner would. It is better thought of as taking the harsh edge off the heat than as recreating a chilled office.
The second limit is humidity. British summers can be muggy, and cooling a wet circuit always risks condensation. Underfloor cooling in particular must be run gently to keep floor surfaces above the dew point, which caps how much cooling you can extract. Fan coils handle this better because they actively dehumidify and drain the condensate, but they add cost and need a condensate route.
The third limit is the grant rules. The government's Boiler Upgrade Scheme, which offers grants towards heat pump installation in England and Wales, is aimed at heating and hot water. The Department for Energy Security and Net Zero sets the scheme's terms, and you can read the official detail on the Boiler Upgrade Scheme guidance at GOV.UK. The grant supports the heat pump as a heating system, so cooling capability is a feature you fit on top rather than something the grant is designed to fund. The Microgeneration Certification Scheme, which certifies installers and products for grant eligibility, publishes its standards through MCS, and the Energy Saving Trust offers independent, manufacturer-neutral advice on what air source systems can and cannot do.
The fourth limit is comfort style. Cooling through a floor or a fan coil is quieter and gentler than a blowing split unit, which many people prefer, but it is also slower to respond. If you want a room to drop several degrees in minutes, a wet heat pump circuit is not the right tool.
Is Cooling Worth Fitting?
Whether to specify cooling comes down to your home, your climate exposure and your budget, and there is no single right answer. For some households it is an obvious yes, and for others it is an avoidable cost.
Cooling tends to make sense when your home suffers badly from summer overheating, perhaps because of large south or west facing glazing, a top floor flat that bakes under the roof, or a well-sealed modern build that traps heat. It also makes sense when you are already installing underfloor heating or are open to fan coil emitters, because the marginal cost of adding cooling capability at that point is far lower than retrofitting it. And it appeals to anyone who would otherwise buy portable air conditioners or a separate split system, since a single reversible system avoids duplicate kit.
Cooling is harder to justify when your home stays comfortable through most summers with good shading, ventilation and thermal mass, when your only emitters are standard radiators that cannot cool, or when your budget is tight and the heating function is the priority. In those cases the sensible move is to specify a heat pump that is reversible and cooling-capable, even if you do not fit the cooling emitters immediately, so the option stays open for a future hot summer without ripping the system out.
The cost picture matters here too. Adding cooling means choosing the right unit and, crucially, fitting fan coils or cooling-capable underfloor, plus the controls and condensate handling. That is an addition to the core installation, so it is worth folding into your quotes from the outset rather than treating it as an afterthought. Our heat pump installation cost breakdown explains where the money goes on a typical job, which helps you judge where cooling sits in your priorities.
Getting the Specification Right
If you decide cooling matters, a few practical steps protect you from a system that heats well but cools poorly.
- Ask explicitly for a reversible, cooling-enabled model. Confirm in writing that cooling is active in the controller, not merely a latent hardware capability.
- Choose emitters that can cool. Fan coil units for meaningful cooling, or cooling-rated underfloor for gentle cooling. Standard radiators do not count.
- Insist on humidity and condensation management. Underfloor cooling needs a dew point control or humidity sensor. Fan coils need a reliable condensate drain.
- Have the cooling mode commissioned. Cooling needs its own setup and testing, not just the winter heating commission.
- Right-size expectations. Treat it as taking the edge off heat, not as a powerful air conditioner, unless you have specified fan coils generously.
A heat pump that has been thought through for both seasons gives you quiet warmth in winter and gentle, efficient cooling in summer from a single piece of kit. That is an appealing prospect as the climate shifts, and it removes the need for separate cooling appliances. If you are weighing up the whole proposition, our analysis of whether heat pumps are worth it puts the year-round value in context.
FAQ
Can any air source heat pump cool my home in summer?
No. Cooling needs a reversible unit with the cooling mode enabled in the controller, not just latent in the hardware. Many models are capable but ship with cooling switched off. Always confirm in writing that the heat pump is reversible and that cooling is actively commissioned, otherwise you have a winter-only heating machine.
Does a heat pump cool as powerfully as an air conditioner?
Usually not. Most UK heat pumps deliver cooling through a wet circuit feeding underfloor pipes or fan coils, which is gentler than a dedicated split air conditioner. Fan coils give meaningful cooling, underfloor gives gentle cooling, and standard radiators barely cool at all. Treat it as taking the edge off the heat unless you have specified fan coils generously.
Is heat pump cooling expensive to run?
Running costs are modest across a typical UK summer because cooling demand is intermittent rather than constant. The system still draws electricity, so it is not free, but the occasional warm spell keeps the bill low. Ground source systems can cool passively by circulating cool ground fluid, which is far cheaper than running the compressor for active cooling.
Should I plan for cooling even if I only want heating now?
Yes. Retrofitting cooling later is disruptive and costly, so the practical move is to choose a reversible, cooling-capable unit from the start even if you delay fitting the cooling emitters. Specify the cooling-ready model now and you keep the option open without tearing the system apart again later.
Do I need to worry about condensation with cooling?
Yes. Underfloor cooling needs dew point or humidity control to stop condensation forming on cold floors, and fan coil units need a reliable condensate drain. Insist on humidity and condensation management as part of the specification, and have the cooling mode commissioned and tested separately from the winter heating setup.
Key Takeaways
Summer cooling from a heat pump is real, but it rewards planning and punishes assumptions.
- Reversible heat pumps can cool by running the refrigerant cycle backwards, pulling heat out of your home and rejecting it outside.
- Emitters decide the result. Fan coils give meaningful cooling, underfloor gives gentle cooling, and standard radiators barely cool at all.
- Running costs are modest in a UK summer because demand is intermittent, though cooling still uses electricity and is not free.
- Ground source systems can cool passively by circulating cool ground fluid, which is far cheaper to run than active cooling.
- Plan for limits. Capacity, humidity and response speed mean a wet circuit will not match a powerful split air conditioner.
- Specify cooling from the start. Retrofitting is disruptive, so even if you delay the cooling emitters, choose a reversible, cooling-capable unit now.
Fit it thoughtfully and a heat pump becomes a true all-season comfort system, keeping your home warm through a British winter and pleasantly cool through the hottest weeks of summer, all from one efficient machine.