Can you replace a combi boiler with a heat pump?

Yes, you can replace a combi boiler with a heat pump, and in a region full of combi-heated semis it's the swap most people are asking about. The catch is hot water. A combi heats water the moment you turn a tap on. A heat pump can't do that, so it heats a stored cylinder instead, and that cylinder needs a home. What we fit and how a job runs is on the air source heat pumps page.

Everything else about the swap is the same as any other heat pump job. The heat loss survey, the radiators and the grant all work the same way. The cylinder is the part that's different, so that's what this covers.

Can you replace a combi boiler with a heat pump without a cylinder?

In almost every house, no.

A combi fires up hard for a few minutes to heat water as it flows through. A heat pump works best running gently for long periods at low temperatures. It can't jump to a high output the second you open a hot tap, so it builds up a store of hot water ahead of time and you draw from that.

There are a few products aimed at small flats that try to get round this, and there are heat batteries sold as a cylinder alternative. For a family house with a bath and a shower, a proper heat pump hot water cylinder is still the answer. I wouldn't design around anything else.

What a combi house loses, and what it gains

The house loses a bit of space. That's the honest cost of the swap.

What it gains surprises people. A modern unvented cylinder gives you hot water at mains pressure, the same feel a combi gives you. Better than a combi, you can run a shower and a hot tap at the same time without either one going cold, because you're drawing from a full tank rather than a boiler working flat out.

Things that stay the same:

• The radiators stay where they are. Some may be swapped for bigger ones if the design calls for it.
• The heating is controlled from a thermostat or an app, as before.
• If you keep a gas hob, you keep the gas supply and its standing charge. Take the hob out too and you can have the gas capped.

Where does the heat pump hot water cylinder go?

This is the question the survey spends the most time on.

A combi house usually had its airing cupboard ripped out when the combi went in. So we find somewhere else, and there is nearly always somewhere. These are the places that tend to work:

• A bedroom or landing cupboard. The most common answer. Often it's the space where the old airing cupboard was.
• The loft. A loft is fine as long as the cylinder fits through the hatch and has a solid base to stand on. Plenty of people prefer it, because it keeps the cylinder out of the way and saves a cupboard downstairs.
• The garage. Also fine. A modern cylinder is well insulated, so it holds its heat in a garage the same as it does indoors.
• A utility room. Good on space, with the pipe runs kept short and insulated.
• Under the stairs. It works in some houses. It depends on the height and on whether you can live without the storage.

The cylinder also needs a discharge pipe. An unvented cylinder has safety valves, and building regulations require the pipe from them to run somewhere safe and visible outside. That can decide the location on its own, so it's worth thinking about early. The engineer fitting an unvented cylinder needs a G3 qualification, which is the unvented hot water ticket.

How the rest of the house changes

Combi boilers are usually on a sealed system with no tanks in the loft. That's good news for a heat pump, because it's also a sealed system.

The pipework is the thing to check. Some combi installs from the 1990s and 2000s used microbore pipe to the radiators. A heat pump moves more water at a lower temperature, and thin pipe can restrict that flow. Sometimes it's fine. Sometimes part of it has to be replaced. The survey tells you which.

Radiators get the same check as any heat pump job. The design works out the temperature each room needs and whether each radiator can deliver it at a low flow temperature. Is my house suitable for a heat pump goes through that in more detail.

Combi boiler vs heat pump: day to day

The difference you notice is in how the heating runs, not in how warm the house is.

A combi blasts heat in short bursts and then switches off. A well-set-up heat pump runs low and steady for most of the day in winter. The radiators feel warm rather than hot, but the rooms hold their temperature. People who are used to the heating "coming on" in the morning find this odd for a week or two, then stop noticing it.

Hot water is heated at set times, usually when electricity is cheapest if you're on a heat pump tariff. Once a week or so the cylinder is taken up to a higher temperature to keep it clean, and the controls handle that for you. Smart energy tariffs covers which tariffs suit a heat pump.

What does the grant cover when you replace a combi?

The same as any other heat pump job. The Boiler Upgrade Scheme is £7,500 for an air source heat pump in England and Wales (gov.uk). A gas combi counts as a fossil fuel system, so replacing one qualifies.

The grant goes towards the whole job, including the cylinder. It's claimed by your MCS certified installer and taken off the price before you pay. Ours is NAP-75287.

The higher £9,000 rate isn't for a combi on mains gas. It's only for homes heated by oil or LPG with no mains gas connection, so an LPG combi in a village off the gas grid does qualify.

Heat pump installs are zero-rated for VAT until 31 March 2027. After that they're due to go back to the reduced rate.

How a combi swap runs with us

The same as every heat pump job. We visit first and give you a ballpark figure. If you're happy with us and with that number, we book the full survey, which is chargeable, between £250 and £450. That buys the room-by-room heat loss calculation and the design, including where the cylinder goes and what size it needs to be.

The design is what makes the quote real. Anyone giving you a fixed heat pump price without having worked out the heat loss of your house is guessing.

Octopus Cosy, the heat pump tariff

A heat pump runs on electricity, so the tariff matters. Cosy Octopus is built for heat pump homes. It has three cheap periods every day, from 4am to 7am, 1pm to 4pm and 10pm to midnight, which Octopus says are 51% cheaper than its day rate. The dear period is 4pm to 7pm (Octopus, November 2026).

A heat pump suits that well. The controls heat the cylinder and warm the house in the cheap periods, then coast through the dear one. You need a smart meter, and Octopus has to be your supplier.

Newpower is an Octopus Energy trusted partner. Anyone can switch to Octopus through our link, newpower.octopus.energy, even if you don't have a heat pump or solar panels, and you get £50 for switching. There is no obligation. Once your heat pump is in and running, you can move onto Cosy.

What I'd do if you're on a combi now

Don't let the cylinder put you off. We will find a space for it. That is our job on the survey, and there is nearly always somewhere, from a cupboard to the loft or the garage.

Don't wait on the money either. The grant is there today, but grants change and nothing is certain. The 0% VAT on a heat pump ends on 31 March 2027, and the rate is due to go back up after that. If you are going to do it, the time is now.

If your combi is on its last legs, get the survey done before it fails, not after. An emergency swap in January nearly always ends up as another boiler.

Next step

For a ballpark on your own house, Get a quote with your postcode, or ring 01709 278248.

If you want to read more first, our guide to heat pump grants explains what you pay once the grant comes off. And if you are worried about noise from the unit outside, how noisy is a heat pump answers that.