Can you get a heat pump if you have a combi boiler?
Yes, but most homes need a hot water cylinder: about 35 to 45 litres per person. Where it can go, what it costs and the options if you have no space.
Numbers checked against official sources on 7 October 2026
In 30 seconds
- Yes, you can replace a combi boiler with a heat pump. Most homes will need a hot water cylinder, because a heat pump stores hot water rather than heating it instantly.
- A common rule of thumb is 35 to 45 litres of cylinder per person, so about 140 to 180 litres for a family of four.
- The cylinder is normally part of the heat pump quote, and the £7,500 grant applies to the whole installation.
- Heat batteries, a compact alternative, are due to get a £2,500 grant, but it was not yet available when we checked on 7 October 2026.
Millions of British homes heat their water with a combi boiler, which warms water the moment you turn on a tap and needs no tank. That is the main practical difference when you switch to a heat pump: you will almost always need somewhere to store hot water. This guide explains why, how big the cylinder needs to be, where it can go, and the alternatives if you have no space.
On this page
Why a heat pump usually needs a cylinder
A combi boiler burns gas at a high rate for a few minutes to heat water instantly. A heat pump works the other way round: it is most efficient producing heat steadily, at a lower temperature, over a longer time. It can’t produce enough heat instantly to run a hot shower straight from the mains.
So a heat pump heats a hot water cylinder in advance, and you draw water from that store. A well-insulated modern cylinder keeps the water hot for hours. Most heat pump cylinders are “unvented”, which means they run at mains pressure: showers can feel as strong as, or stronger than, a combi.
How big a cylinder do you need?
The Hot Water Association, the UK trade body for hot water cylinders, suggests allowing 35 to 45 litres per person in a typical household, and notes that a four-person household typically uses around 200 litres of hot water a day (Hot Water Association).
| People in the home | Rule-of-thumb cylinder size |
|---|---|
| 1 to 2 | 35 to 90 litres |
| 3 | 105 to 135 litres |
| 4 | 140 to 180 litres |
| 5 | 175 to 225 litres |
Long showers, a big bath or two bathrooms used at once push the size up. Installers also have to account for the heat pump’s lower water temperature, so they may recommend a slightly larger cylinder than you would have had with a boiler.
Where the cylinder can go
A typical cylinder is a tall cylinder roughly the footprint of a small fridge. Common places are:
- The old airing cupboard, if the house once had a tank before the combi was fitted;
- A utility room, garage or under-stairs cupboard;
- The loft, if it is boarded, accessible and strong enough, though this makes servicing harder;
- A bedroom cupboard built for the purpose.
Ask installers about slimline or horizontal cylinders if space is tight. Some systems combine the indoor heat pump parts and the cylinder in one unit, which saves wall space.
If you really have no space
For some flats and small terraces, a cylinder simply won’t fit. The options are narrower, and each has trade-offs:
| Option | How it works | Grant |
|---|---|---|
| Heat battery | A compact store that charges with electricity, often overnight, and releases heat on demand | £2,500 announced, not yet available (see below) |
| Air-to-air heat pump plus separate electric water heating | Heats rooms with warm air; hot water comes from an electric heater or immersion | £2,500 for the air-to-air heat pump |
| Keep a gas boiler alongside a heat pump (hybrid) | The boiler covers hot water and the coldest days | Not eligible for the Boiler Upgrade Scheme |
Heat battery grant: check before you sign. The Energy Saving Trust lists £2,500 off a heat battery among future funding updates (page updated 16 July 2026). Heat batteries do not appear in the government’s official list of grant values from 21 July 2026. If a company offers you “£2,500 off” a heat battery today, ask whether it is the government grant or the company’s own discount.
Does the grant cover the cylinder?
The Boiler Upgrade Scheme pays £7,500 off the cost of an air-to-water heat pump installation in England and Wales. In practice the cylinder is part of that installation and appears on the same quote; the grant comes off the total. To qualify you must be replacing your fossil fuel boiler, so you can’t keep the combi running alongside the heat pump and still claim (GOV.UK).
Installing a heat pump system, including its cylinder and radiators, is also VAT-free until 31 March 2027. What changes after that date.
What changes day to day
- Hot water is scheduled. The heat pump reheats the cylinder at set times, often when electricity is cheaper. If the whole household showers back to back, the cylinder can run low, which is why sizing matters.
- A regular high-temperature cycle. Many systems heat the cylinder to a higher temperature periodically, often with the immersion heater, to control bacteria. Ask your installer how often theirs runs, because it uses extra electricity.
- Stronger showers are possible with an unvented cylinder at mains pressure, if your water supply allows.
Will a heat pump cost more to run than your combi? Try it with your gas usage.
Open the calculatorFrequently asked questions
Can a heat pump work without a hot water cylinder?
Most air-to-water heat pumps need a cylinder for hot water. Without space for one, the options are a heat battery or an air-to-air heat pump with separate electric water heating.
What size cylinder do I need with a heat pump?
The Hot Water Association’s rule of thumb is 35 to 45 litres per person, so about 140 to 180 litres for four people. Your installer should size it for your household and the heat pump.
Can I keep my combi boiler and add a heat pump?
You can, but a hybrid system of a gas boiler plus a heat pump is not eligible for the Boiler Upgrade Scheme grant.
Is there a grant for heat batteries?
A £2,500 grant has been announced, but the Energy Saving Trust lists it as a future update and it was not in the official list of grant values when we checked on 7 October 2026.
Will I run out of hot water with a heat pump?
Not if the cylinder is sized correctly for your household. If you often run out, ask your installer to check the cylinder size and the reheat schedule.
