How does a heat pump actually work?

The defining difference between a gas boiler and a heat pump is what each one does with electricity. A boiler burns gas to produce heat. The chemistry is one-way: the boiler turns ten units of fuel into roughly nine units of heat, and that ratio sets the ceiling. A heat pump doesn't burn anything. It picks up heat that already exists in the outdoor air, or in the ground, and moves it into your house. The electricity it uses runs the moving part, not the heating part. That is why a typical air-source heat pump in the UK produces three to four kilowatt-hours of heat for every kilowatt-hour of electricity it draws.

The ratio is called the Coefficient of Performance, or COP. A modern unit in mild winter weather sits at roughly 4.0, falling toward 2.5 in colder weather. Even at the bottom of that range, the heat coming out is more than the electricity going in, which is something a boiler, by definition, cannot do.

The mechanism is the same one your fridge uses, run in reverse. Inside the heat pump is a refrigerant (a fluid that boils at temperatures well below freezing) and a four-stage loop the diagram below traces. Cold outdoor air boils the refrigerant in the evaporator. A compressor squeezes the now-gaseous refrigerant, raising its temperature. The hot refrigerant releases that heat into the water heading to your radiators in the condenser. An expansion valve drops the pressure, the refrigerant cools right down, and the cycle starts again.

What the cycle implies day to day: the heat that comes out is warm, not very hot. A gas boiler produces water at 70 to 80°C. A heat pump produces water at 40 to 55°C, depending on the design. That lower temperature is the price you pay for the higher efficiency. The radiators in your house need to release the right amount of heat at that lower flow temperature, which usually means one or two specific radiators want enlarging or replacing during the install. The HeatPass property-suitability guide covers what that looks like in practice, and how to tell from your current radiators whether your home needs a lot of changes or only a couple.

In the UK, four words come up around heat pumps. Most domestic-heating conversations are really about one of them.

Type	Grant	Right answer for
Air-source (ASHP) Fridge-sized unit outside, takes heat from the air. £10,000–£14,000 install.	£7,500	Almost every UK home. Around 95% of installs.
Ground-source (GSHP) Heat from boreholes or trenches in your land. £18,000–£30,000 install.	£7,500	Larger detached homes with the land, and a long enough timeframe for the higher upfront to pay back.
Hybrid Small heat pump alongside a backup gas boiler.	none	Not the supported direction. Policy has settled on fully-electric heat pumps.
Air-to-air Warm air through indoor units rather than heated water through radiators.	£2,500	Flats where a wet system would be disruptive. Hot water from an immersion heater.

Air-source (ASHP)

Fridge-sized unit outside, takes heat from the air. £10,000–£14,000 install.

Grant

£7,500

Right for

Almost every UK home. Around 95% of installs.

Ground-source (GSHP)

Heat from boreholes or trenches in your land. £18,000–£30,000 install.

Grant

£7,500

Right for

Larger detached homes with the land, and a long enough timeframe for the higher upfront to pay back.

Hybrid

Small heat pump alongside a backup gas boiler.

Grant

none

Right for

Not the supported direction. Policy has settled on fully-electric heat pumps.

Air-to-air

Warm air through indoor units rather than heated water through radiators.

Grant

£2,500

Right for

Flats where a wet system would be disruptive. Hot water from an immersion heater.

For the rest of this guide, "heat pump" means an air-source heat pump unless stated otherwise.

This is the biggest single myth about heat pumps in the UK, and it deserves an accurate answer rather than a dismissal.

Heat pumps work less efficiently as outdoor air gets colder. That part is true, and the chart below shows it. What is not true is that they stop working in British weather. Modern monoblocs from the major manufacturers are rated to operate down to between −20°C and −28°C outdoor air. The CIBSE design temperature, which is the coldest single hour the heating system needs to handle, sits at around −2 to −4°C across most of England, and around −4 to −5°C in Scotland. The Met Office 1991–2020 UK January mean is 3.9°C. Whatever the worst hour of your worst winter looked like, it was well inside the operating range of any modern unit.

Two things to take from the chart. First, the curve sits comfortably above the COP-1.0 floor, which is effectively the resistive-heater equivalent, across the whole UK winter range. Even in the deep cold tail at −15°C, colder than most of England has seen in recent winters, a modern unit produces roughly twice as much heat as the electricity it draws. A boiler can never beat 1.0. A heat pump rarely falls below 2.0 in normal UK operation.

Second, the chart's flow-temperature assumption is conservative. In practice, a competently commissioned UK install runs a weather-compensation curve, where the flow temperature drops to 35–40°C in mild weather and only climbs toward 45–50°C in the coldest hours. That means the real-world COP at moderate UK winter temperatures is higher than the W45 curve plotted here. The same manufacturers report a 7°C outdoor COP of 4.5 to 5.0 on a 35°C flow curve.

The gap between datasheet figures and the running-cost figures elsewhere on this site is the commissioning-and-design gap. Energy Saving Trust's typical UK figure of 3.0 SCOP and the volunteer-monitored HeatpumpMonitor.org average around 3.9 bracket what UK installs actually deliver across a heating season. The HeatPass running-costs guide covers SCOP and what moves it, and the winter-performance guide tackles the "doesn't work in winter" worry head-on.

This is the question that stops a lot of UK homeowners. A meaningful share of the housing stock predates the cavity-wall era and is solid-walled. The standard impression is that heat pumps don't work in old houses. The reality is more useful than that.

Whether a heat pump works in a particular house depends on the heat-loss number, which is how many kilowatts of heat the house leaks per hour at the coldest design temperature, not on the year on the deeds. A well-insulated 1900 terrace can be easier to heat than a poorly insulated 1980 detached. Pre-1919 solid-wall housing is the hardest category, but plenty of individual examples sit comfortably inside the workable range. The MCS-certified installer surveying your property runs the heat-loss calculation, and that number, not the architectural style, decides what's possible.

What helps. Loft insulation matters most because warm air rises, and on a heat pump the system runs more hours per day than a boiler so the gain compounds. Draught-proofing is the cheapest single improvement, often a weekend's work in materials. Cavity-wall insulation, where the cavity exists, is a one-off £1,000 to £2,000 job that pays back inside three to seven years on Energy Saving Trust figures. Solid-wall insulation is the expensive one (£8,000 to £25,000 depending on whether it goes inside or outside) and is the prerequisite some Victorian and Edwardian homes need before a heat pump becomes sensible. The Victorian house guide walks that path on a 9-inch brick terrace, with the realised-SCOP picture. The 1930s house guide does the same for cavity-walled stock.

Sometimes the fabric work has to come first. If the heat-loss survey shows the house would need radiators larger than the rooms can sensibly accommodate, or a heat pump bigger than the available models, the system is being forced. Insulating the walls or roof first reduces the heat demand to a number a normal-sized heat pump can hit. The HeatPass property-suitability guide covers the six factors that decide it, and the cost-and-grants guide covers which insulation grants stack with the £7,500 Boiler Upgrade Scheme.

The second-most-common worry. Worth taking seriously, and worth bounding accurately.

A modern outdoor unit at one to two metres distance runs at roughly 40 to 50 decibels at the nameplate spec. That sits between a quiet fridge (around 40 dB) and a normal indoor conversation (around 50 dB), and comfortably below a typical dishwasher in operation (around 50 to 55 dB). On a still winter night, in a quiet rural garden, the unit is audible if you stand next to it. From inside the house, with the unit against an external wall, it is essentially inaudible.

UK permitted-development rules wrap this with a hard limit. To install an air-source heat pump under permitted development, without applying for full planning permission, the unit must:

• be at least one metre from the property boundary
• not exceed 42 decibels at the nearest neighbour's window, calculated using the MCS Planning Standard 020 method
• be the only heat pump on the property (a second unit needs full planning)
• not be installed on a flat above ground level (the rules for flats sit outside this checklist and are easing under the 2025 permitted-development reforms)

Listed buildings and conservation areas need full planning regardless, as do properties immediately bordering a designated site. Your MCS installer runs the noise calculation as part of the design and submits the planning application if needed. The homeowner doesn't deal with it directly.

Placement is the variable that matters. A unit positioned hard against a neighbour's bedroom window will cause complaints even at the rated dB. A unit positioned competently, facing your own garden with at least a metre of breathing space and away from a reflecting wall, almost never does. Any installer worth working with chooses placement deliberately and shows you the noise calculation before signing off.

This guide keeps to the broad outline. A separate guide will cover the install in detail once we have enough post-install material to publish. The pattern is consistent across most UK monobloc retrofits.

Four phases:

1. Heat-loss survey. A qualified surveyor (often the installer or someone working with them) measures the rooms, examines the construction, and computes how many kilowatts of heat the house demands at the coldest design temperature. This is the number that sets the heat-pump kilowatt rating. Half a day on site, plus desk time. Free with most installers as part of the quote.
2. Design. The installer produces a written system design covering the heat-pump model and size, the cylinder spec if you don't already have one, pipework routing, radiators that need changing, electrical work, and where the outdoor unit will sit. You see the design before committing money. The MCS scheme requires it.
3. Install. Two to five days on site for a standard property, longer for cylinder swaps or larger radiator-change lists. Most homeowners are without heating for one to three days during the cutover.
4. Commissioning. The installer balances the system, sets the weather-compensation curve, and runs you through the controls. The MCS sign-off and the £7,500 BUS grant claim happen at this point. The grant goes directly from Ofgem to the installer rather than passing through your account.

What doesn't change. The boiler comes out. The gas connection can be capped (most homeowners do this) or kept for a gas hob, in which case the standing charge stays. Hot water still arrives at the tap the same way, just from a cylinder rather than a combi where there wasn't one before. Radiators still warm rooms the same way, at a slightly lower flow temperature so they run a bit longer to reach the same room temperature. Most controls familiar from a gas system still work, with one small change: the off-overnight / on-from-7am pattern that suits a boiler is less efficient on a heat pump than a steady setpoint with gentle weather compensation.

The UK was a relative latecomer to domestic heat pumps compared to Northern Europe. Annual installs were below 30,000 as recently as 2021. Three forces are now turning that.

The first is gas exposure. UK domestic gas prices are tied to the international wholesale market, and that market shifted state in 2022. The Ofgem cap on gas in 2026 is materially higher than the 2020 cap, and the spike of late 2022 reset what most homeowners think of as normal. A heat pump doesn't insulate you from energy prices, but it shifts your exposure from a volatile gas market to a more diversified electricity grid where renewables already supply roughly 40% of generation.

The second is the £7,500 Boiler Upgrade Scheme grant. Before BUS, the upfront cost was the deal-breaker for most homeowners. A heat pump install ran two to three times the price of a boiler replacement, and the lifetime running-cost difference wasn't large enough to make the maths work. With the grant, an air-source install on a typical UK home lands at roughly £4,000 to £8,000 net, which is competitive with replacing a boiler outright. Scotland's Home Energy Scotland Grant + Loan provides the same effect north of the border. The HeatPass cost-and-grants guide covers the figures.

The third is the policy direction. The UK's plan is to phase out new gas boilers, with new-build homes off gas from 2025 onwards under the Future Homes Standard and a broader retrofit programme through the 2030s. The current install volume of around 130,000 in 2025 is still well below the 600,000-by-2028 target the government has staked policy on. Whether or not the target is reached, the trajectory is fixed and new-installation incentives will continue.

There is a counter-argument worth naming. UK electricity costs around four times as much per kilowatt-hour as gas at the current Ofgem cap, while a typical heat pump is roughly three times more efficient. The two numbers offset each other on paper. In practice the running-cost saving against gas is modest for many homes and meaningful for others, depending on the home's heat-loss, the install quality, and whether you can shift load to a heat-pump tariff. Nesta, the innovation foundation, argues the structural answer is to lower the cost of electricity relative to gas by reforming the levies. The HeatPass running-costs guide covers it in detail. The case works today, and strengthens as the policy reforms land. If you've heard heat pumps are unpopular here and want the full accounting, the guide to why heat pumps are still unpopular in the UK takes each reason in turn.