A heat pump doesn’t burn anything to keep your home warm. It extracts thermal energy that’s already present in the air (or the ground) outside your house and concentrates it up to a useful temperature, which is why a well-installed system can deliver 300-400% efficiency compared with the 90-95% you’ll get from even the best modern gas boiler. A gas boiler generates heat by converting fuel into energy, and you can never get more energy out of that process than you put in. A heat pump moves existing heat from one place to another, and moving heat costs far less energy than creating it from scratch.
That single distinction, generating heat versus moving heat, is the whole story of why heat pumps outperform boilers on running efficiency, and it’s worth understanding before you commit to a system for your home.
How can a heat pump extract heat from cold outside air?
A heat pump can extract heat from outside air because even air at 0°C or -5°C still holds usable thermal energy. Air only becomes truly free of heat at absolute zero, roughly -273°C, and everything above that contains molecules in motion that carry energy. It feels counterintuitive, but a heat pump doesn’t need the air to feel warm to your skin. It needs the air to be warmer than the refrigerant circulating inside the system, and refrigerants are specifically chosen because they can be made colder than a British winter’s day.
Heat naturally flows from warm places to cold ones, never the other way round on its own. That’s basic thermodynamics, and it’s why a cup of tea left on the side goes cold rather than getting hotter. A heat pump’s job is to reverse that natural direction, pulling heat out of cold outdoor air and pushing it into your already-warmer home. Doing that against the natural flow takes work, and that’s precisely what the electrically driven compressor at the heart of the system provides.
How does a heat pump’s refrigerant cycle work?
A heat pump’s refrigerant cycle works as a closed loop that runs through four repeating stages (evaporation, compression, condensation and expansion), cycling a refrigerant between liquid and gas to carry heat into your home. Understanding these four stages makes the rest of this article, and any conversation with an installer, easier to follow.
| Stage | What happens | Where |
|---|---|---|
| 1. Evaporation | Cold liquid refrigerant absorbs heat from outdoor air and boils into a low-pressure gas, even when the air feels cold to you | Outdoor unit |
| 2. Compression | An electrically driven compressor squeezes the gas, sharply raising its pressure and temperature. This is the one stage that consumes significant electricity | Compressor (usually outdoor unit) |
| 3. Condensation | The hot, high-pressure gas passes through a heat exchanger, releasing its heat into your central heating water or air, and condenses back to a liquid as it cools | Indoor unit / heat exchanger |
| 4. Expansion | The liquid passes through an expansion valve, dropping its pressure and temperature back down so it’s ready to absorb heat again | Expansion valve |
The cycle then repeats, continuously, for as long as your heating calls for warmth. The only point in that whole loop where you’re paying for electricity in any meaningful quantity is the compression stage. Evaporation, condensation and expansion happen because of the refrigerant’s own physical properties and the pressure differences the compressor has created. They’re essentially free once the compressor has done its work. That’s the mechanical reason a heat pump can move three or four units of heat energy into your home for every one unit of electrical energy you put in: you’re paying to drive the compressor, not to manufacture the heat itself.
Why does frost build up on a heat pump’s outdoor unit in winter?
Frost builds up because the outdoor coil runs colder than the surrounding air so it can absorb heat, and when that coil temperature drops below freezing while there’s moisture in the air, ice forms on the fins in the same way frost forms on a car windscreen overnight. If you’ve seen a heat pump’s outdoor unit with a thin layer of frost on it during a cold, damp spell, that’s normal and not a sign of a fault.
A build-up of ice reduces airflow across the coil and makes the system less effective at absorbing heat, so every heat pump has a defrost cycle to deal with it. Periodically, the system briefly reverses the refrigerant flow, effectively running as an air conditioner for a few minutes, which sends warm gas through the outdoor coil and melts the ice off. You might notice what looks like steam rising from the unit, or a brief pause in heat output. It costs a small amount of efficiency because that energy isn’t going into heating your home during the defrost, but it’s a normal, automatic part of how the system copes with UK winter conditions, not something that needs a callout.
What is COP and what does it tell you about a heat pump’s efficiency?
The Coefficient of Performance, or COP, is the standard measure of a heat pump’s efficiency at a given moment: the ratio of heat energy delivered to electrical energy consumed. A COP of 3.5 means that for every 1 kWh of electricity the compressor uses, the system delivers 3.5 kWh of heat into your home. Compare that with a gas boiler, which effectively has an “efficiency” of around 0.9-0.95. For every 1 kWh of gas burned, you get roughly 0.9-0.95 kWh of usable heat, because some energy is inevitably lost as flue gases and casing losses.
COP isn’t fixed. It changes with outdoor temperature, the flow temperature your system runs at, and how well the system has been designed and commissioned. We’ve written a dedicated article on what drives COP and how to get the best figure out of your system, so the short version here is all you need for this comparison: higher COP means more free heat extracted from the environment for every unit of electricity you buy, and it’s the figure that ultimately determines your running costs.
Is a heat pump cheaper to run than a gas boiler?
A heat pump can be cheaper to run than a gas boiler once its COP is high enough to offset the fact that electricity costs more per kWh than gas in the UK, though at the lower end of the COP range it can cost about the same or slightly more. Because of that price gap, people often assume a heat pump must be more expensive to run; COP is what flips that assumption on its head. Ofgem’s price cap moves every quarter and regional tariffs vary, so treat the figures below as illustrative rather than today’s exact rates. The relationship between them is what matters, and it holds broadly true across most of the plausible price range.
| System | Illustrative unit rate | Efficiency / COP | Effective cost per kWh of heat delivered |
|---|---|---|---|
| Gas boiler | ~6-7p/kWh | 90-95% efficient | ~6.7-7.4p per kWh of heat |
| Heat pump, COP 2.5 (poor conditions) | ~25-30p/kWh | COP 2.5 | ~10-12p per kWh of heat |
| Heat pump, COP 3.0 (typical) | ~25-30p/kWh | COP 3.0 | ~8.3-10p per kWh of heat |
| Heat pump, COP 4.0 (well-optimised) | ~25-30p/kWh | COP 4.0 | ~6.3-7.5p per kWh of heat |
What this shows is that at the lower end of the COP range, a heat pump can cost a similar amount to run as a gas boiler, or even slightly more, purely because of the gap between electricity and gas unit prices. But once a system reaches a COP in the 3.5-4.0 region (entirely realistic for a well-sized, well-installed air source heat pump with sensible flow temperatures), running cost per kWh of delivered heat comes in at or below what a gas boiler costs, before even accounting for solar generation, time-of-use tariffs or battery storage, all of which push the effective cost lower still. Ground source systems, which benefit from stable year-round ground temperatures, typically achieve higher COPs again.
The takeaway is that the running-cost case for heat pumps depends heavily on getting the COP right, which is why sizing, flow temperature and installation quality matter so much more with a heat pump than they ever did with a boiler. A gas boiler is fairly forgiving of a mediocre installation; a heat pump is not.
What’s the difference between air source and ground source heat pumps?
The key difference is where they draw their heat from: air source heat pumps extract it from outdoor air, while ground source heat pumps extract it from pipework buried in the garden. Most UK installations are air source, extracting heat from outdoor air via an external unit similar in size to an air conditioning condenser, and they suit the vast majority of homes. Ground source heat pumps instead extract heat from pipework buried in the garden (horizontal trenches or vertical boreholes), tapping into ground temperatures that stay far more stable across the seasons than air temperature does. That stability tends to deliver a higher, steadier COP, but it comes with a significantly higher installation cost and the need for garden space or a borehole. We’ve covered the full comparison in a dedicated article. For most homeowners weighing up their first heat pump, air source is the natural starting point.
What COP can you realistically expect from a heat pump in the UK?
Most UK heat pumps realistically achieve a seasonal COP (SCOP) of roughly 2.5 to 4.2 across the year, depending on the system type, insulation standard and how well it’s sized and installed. See the table below for typical ranges. A heat pump’s COP isn’t constant through the year. It’s higher on a mild October day than during a hard January frost, because the temperature gap the compressor works against is smaller. What matters for your annual bill isn’t peak COP on the best day but the seasonal average across the whole heating period, sometimes called SCOP.
| System type / conditions | Typical seasonal COP range | Notes |
|---|---|---|
| Air source, older property, high flow temp | ~2.5-3.0 | Radiators sized for a gas boiler, minimal insulation upgrades |
| Air source, well-insulated property, correct sizing | ~3.0-3.8 | Typical for a properly designed retrofit with some radiator upgrades |
| Air source, new build or fabric-first retrofit | ~3.5-4.0 | Low flow temperatures, underfloor heating or generously sized radiators |
| Ground source, most properties | ~3.5-4.2 | Benefits from stable ground temperatures year-round |
These are broad ranges rather than guarantees. The actual figure depends on insulation standard, radiator or underfloor sizing, the flow temperature the installer sets, and how the controls are configured day to day. Even at the lower end, a heat pump is still moving more heat energy than it consumes electrically; at the upper end, the running-cost advantage over gas becomes clear-cut.
Is a heat pump worth it over its 15-20 year lifespan?
Yes, over a 15-20 year lifespan a heat pump’s structural efficiency advantage tends to work in its favour, even though running costs per kWh alone don’t tell the whole story. Over a system’s realistic working life, commonly 15 to 20 years for both a heat pump and a boiler, a few other factors shift the balance further.
- Gas prices carry more long-term uncertainty than most assume. The UK’s decarbonising grid and policy direction favouring electrified heat make it plausible the gap between electricity and gas unit rates narrows over the system’s lifetime, improving the heat pump’s relative running cost further.
- Maintenance tends to be simpler. No combustion process, no flue, and none of the servicing requirements tied to gas safety regulation. An annual service is still worth having, but the failure modes are generally more straightforward than a boiler’s.
- Upfront cost is higher, and that has to be weighed up. A heat pump installation costs more than a like-for-like boiler swap, even after current government grant support. That gap is the main reason payback varies so much between properties. A poorly insulated home with a low-COP installation takes longer to see a return than a well-prepared one running efficiently.
- The system pairs naturally with solar generation. Since a heat pump’s main running cost is electricity, generating some of it yourself via solar panels can meaningfully reduce what you need to buy in.
None of this amounts to a precise return-on-investment figure. That depends on your property, tariff and set-up, and anyone quoting an exact payback period without assessing your home is guessing. What can be said with confidence is that moving heat rather than generating it gives a heat pump a structural efficiency advantage that persists for the system’s whole life, not just year one.
What should you prioritise when choosing and installing a heat pump?
When choosing and installing a heat pump, insulation, flow temperature, correct sizing and commissioning are the priorities that determine whether you land at the efficient end of the COP range or the disappointing end. If you’re weighing up a heat pump for your home, the theory above translates into these practical priorities:
- Insulation matters more than it did with your boiler. A gas boiler can brute-force heat into a leaky house by burning more fuel at high temperature. A heat pump works best at lower, steadier flow temperatures, so reducing heat loss through insulation and draught-proofing has a direct, measurable effect on your achievable COP.
- Flow temperature is a decision, not a default. The lower the flow temperature the system needs to reach, the higher its COP. This is why radiator sizing (or underfloor heating) is assessed as part of a proper installation rather than reusing whatever’s already on the wall.
- Correct sizing avoids two problems. An undersized heat pump struggles on the coldest days; an oversized one short-cycles inefficiently. Sizing needs a proper heat loss calculation for your property, not a rule of thumb based on your old boiler’s rating. We cover this in a dedicated sizing guide.
- Commissioning and controls set-up affect performance as much as the hardware. Two identical heat pumps on the same street can deliver different COPs depending on how well flow temperature and weather compensation have been configured.
The underlying physics, a compressor moving existing heat rather than a burner creating new heat, is what makes 300-400% efficiency possible, and it isn’t dependent on any particular brand. Whether your installation actually reaches that efficiency comes down to how well the system is sized, installed and set up for your home.
