Here is what actually differs, without the advocacy in either direction.
The short answer
A gas boiler burns fuel to make heat. A heat pump moves heat that already exists from outside to inside, using electricity to do it. Because moving heat takes far less energy than creating it, a heat pump delivers several units of heat per unit of electricity, while a boiler can never deliver more energy than the fuel contains.
That is the physics, and it is not in dispute. What varies is what it means for a specific home, because electricity and gas are priced very differently in the UK, and because the two systems ask different things of the building.
How they heat a house differently
A boiler produces hot water on demand at a high temperature and pushes it round the radiators in bursts. Rooms heat up quickly, and the system is usually switched off for large parts of the day.
A heat pump produces water at a lower temperature and works best running gently for longer periods. Rooms are kept at temperature rather than repeatedly brought back up to it. It is a different rhythm, and it is the single most common thing people have to unlearn when they change over. Running a heat pump like a boiler, in short blasts at high demand, is the usual cause of disappointing results.
What changes indoors
| Consideration | Gas boiler | Heat pump (air-to-water) |
|---|---|---|
| Fuel | Mains gas | Electricity |
| Flow temperature | High, typically 60–75°C | Lower, commonly 35–50°C |
| Radiators | Existing sizes usually fine | Some usually need upsizing |
| Hot water | Instant with a combi, or a cylinder | Cylinder required |
| Indoor space | Boiler in a cupboard | Indoor unit plus cylinder |
| Outdoor equipment | A flue terminal | An outdoor unit needing clear airflow |
| Installation | Often a day or two for a like-for-like swap | Several days, with design work beforehand |
| Servicing | Annual service by a registered gas engineer | Annual check; no combustion, no flue gas |
| Grant support | None | £7,500 under the Boiler Upgrade Scheme, if eligible |
Running costs: how to think about it honestly
Anyone quoting a single national figure for what you will save is guessing. Running cost depends on four things, and three of them are specific to your house.
- The efficiency the system actually achieves. A heat pump’s seasonal performance depends on the design flow temperature. A system designed to run at 40°C behaves very differently from one running at 55°C.
- The price you pay per unit. Electricity costs more per kilowatt-hour than gas in the UK. A heat pump has to deliver enough units of heat per unit of electricity to overcome that gap, which good systems do and poor ones may not.
- Your tariff. Time-of-use electricity tariffs change the arithmetic considerably, particularly where hot water and some space heating can be shifted to cheaper periods.
- The building. Heat loss is heat loss whichever system you install, but it affects the two differently, because it determines the flow temperature the heat pump needs.
The break-even test, in one line
A heat pump costs less to run than a gas boiler when its seasonal efficiency is higher than the price of electricity divided by the price of gas, allowing for the boiler’s own losses. Our heat pump running costs guide shows how to work that out with the rates on your own bills.
If you are off the mains gas grid, the comparison is different again, and usually much more favourable to a heat pump, because you are comparing against oil or LPG rather than mains gas.
We do not publish a savings figure, because a defensible one would have to be calculated for your property and your tariff. Ask any installer who quotes you a saving to show how they worked it out.
Where the Boiler Upgrade Scheme fits
This is the point most people arrive at with a misconception, and it is worth stating plainly.
The scheme does not pay for a gas boiler
The Boiler Upgrade Scheme does not fund a conventional replacement gas boiler. Despite its name, it is a grant scheme for low-carbon heating: heat pumps, and biomass boilers in limited circumstances. If your boiler has failed and you want another boiler, the scheme is not relevant to that.
If you are weighing a heat pump against a boiler, though, the grant is a real part of the comparison, because it reduces the installed cost of the heat pump but not of the boiler.
Air-to-water or ground source
£7,500
The standard grant for an eligible heat pump replacing mains gas heating.
Air-to-air
£2,500
Residential properties only. Air-to-air systems are not eligible in non-residential buildings.
Every value is on our heat pump grants page, and the criteria on the eligibility page. Two related guides go further: getting a heat pump grant with a gas boiler and whether the scheme pays for a new boiler.
When a boiler is still the sensible answer
- The existing boiler has failed in winter and the house needs heat this week. A heat pump is a designed installation, not an emergency one.
- There is genuinely no space for a hot water cylinder and no way to create it.
- You are moving within a short time frame and will not see the benefit.
- The property is a rental or short-term holding where the design work is not practical.
None of these are permanent. A boiler installed now does not prevent a heat pump later, and choosing sensibly sized radiators when you replace a boiler makes that later change considerably easier.
When a heat pump is the stronger choice
- You are off the gas grid, heating with oil or LPG.
- Your boiler is old and you are choosing rather than reacting.
- The property already has a cylinder, or space for one.
- You are doing other work, such as an extension, new floors or a rewire, that the installation can be folded into.
- You expect to be in the property long enough for the design work to be worth doing properly.
Our guide to whether heat pumps are worth it sets out the advantages and drawbacks in more detail.
What to do next
Get a heat-loss survey before you decide. It costs relatively little, it is the basis of any credible heat pump quotation, and it tells you what your house actually needs, which is the only information that settles this comparison for you rather than in general.
If you have concluded you need a boiler, our boiler replacement page covers what that involves.
Check your property
Pick the grant you want to check. Three quick checks, then our team calls you back. It is free, with no obligation.
