Replacing a Gas Boiler With a Heat Pump: What to Check First

Replacing a Gas Boiler With a Heat Pump: What Changes, What It Costs and What to Check First

For most UK homeowners, the gas boiler is the heating system they’ve never had to think about. It’s compact, it gives hot water within seconds, and it sits quietly in a cupboard or the kitchen. A heat pump is a different kind of machine, and swapping one for the other is less like changing an appliance and more like changing how your whole home is heated.

That’s why homeowners keep asking the same questions. Will it keep the house warm? What happens to my hot water? Do I need new radiators? What will it actually cost? This guide answers them for UK homes replacing a gas boiler, including the combi boilers found in millions of houses and flats. It covers what changes physically, what it costs, what the grant covers, and the checks that separate a smooth switch from an expensive disappointment.

The Short Answer

Yes, a heat pump can replace a gas boiler, including a combi, in most UK homes. But it isn’t a like-for-like swap. A heat pump heats at a lower temperature for longer, stores hot water in a cylinder, and sits partly outdoors. A typical installation costs somewhere between £8,000 and £15,000 or more before the grant, and the Boiler Upgrade Scheme can take £7,500 off that in England and Wales. The jobs that go well are the ones where the heat loss, radiators, pipework, cylinder space and electrics were checked before anyone signed.

How a Heat Pump Heats Differently From a Gas Boiler

The biggest adjustment isn’t the hardware. It’s how the heat is delivered.

A gas boiler heats water to a high temperature, pushes it through the radiators for an hour or two, then switches off. The house warms quickly, then cools until the next burst. A heat pump heats the water to a much lower temperature and runs for long periods at low output, holding the house at a steady temperature. Radiators feel warm rather than hot, and the system often seems to be running all day. That’s normal. Our guide on why a heat pump runs constantly explains when it is and isn’t.

What Physically Changes in Your Home

When the boiler goes, these are the parts of the house that change. Each one is worth checking before you accept a quote.

1. The outdoor unit. The heat pump itself sits outside, so you need a sensible spot for it with clear airflow, a firm base and sensible distance from windows and neighbours. Our guide on where an air source heat pump outdoor unit should go covers the positioning rules, and our article on planning permission explains when you do and don’t need it. Always check the current rules for your property.

2. A hot water cylinder. If you have a combi, you have no cylinder today, so one has to be found a home. That means space, usually in an airing cupboard, utility room, loft or garage, plus pipework to reach it. This is one of the most common surprises, and our guide on whether you need a hot water cylinder with a heat pump goes into it.

3. Radiators. Because the water is cooler, radiators need to be larger or better matched to the heat each room needs. Some homes can keep most of their radiators, and others need several replaced. Which category you’re in comes from a room-by-room calculation, not a guess. See why radiators can feel lukewarm for what happens when it’s wrong.

4. Pipework. A heat pump moves more water at a lower temperature than a boiler. Narrow or long pipework, particularly the 8mm or 10mm microbore found in some older homes, can restrict the flow. In a 1930s house in Leeds, the homeowner was told the microbore pipework needed replacing at a cost of £8,000 or more. Measurements showed that re-piping two radiators was enough.

5. The electrical supply. A heat pump needs a dedicated circuit and sometimes a consumer unit upgrade. Many homes need no supply upgrade at all, but it has to be checked, not assumed. Our article on whether your house has enough power for a heat pump explains how.

6. Controls and thermostats. The old boiler thermostat and timer often don’t suit a heat pump. The system should run on weather compensation, which adjusts the flow temperature to the weather. Our guide on how to set weather compensation shows what that looks like.

7. The old boiler and gas supply. The boiler is removed, the gas supply is capped, and any flue is made good. The space it leaves behind is often where the cylinder goes.

If You Have a Combi Boiler: What Happens to Your Hot Water?

This is the question combi owners ask most, and it deserves a straight answer. A combi gives you hot water instantly, with no tank. A heat pump can’t do that. It heats a cylinder of water in advance and stores it, so you have a tank of hot water ready when you need it.

For most households, this works well, with one condition: the cylinder has to be the right size. A typical family home often uses a cylinder somewhere in the region of 150 to 300 litres, but the right size depends on how many people live there, how many bathrooms you have and how you use hot water. Too small, and you run out. Our article on why a heat pump can run out of hot water quickly covers what happens when it’s undersized.

Hot water also affects running costs. Heating water to a usable temperature takes more effort than heating radiators, and a boost setting or immersion heater left on can quietly add a lot of electricity. Our guide on what temperature your heat pump hot water should be set to covers the sensible settings.

Is Your Home Suitable?

Most UK homes can have a heat pump, but suitability isn’t automatic. Three questions matter most.

Is there space? You need room for an outdoor unit and an indoor cylinder. Some small flats and terraced homes struggle with one or the other.

Can it heat the house at a low temperature? This is where heat loss comes in. A well-insulated home needs less heat, but a perfectly insulated home isn’t a requirement. Older and poorly insulated houses can work too, as long as the heat loss is calculated properly and the radiators are matched to it. Our articles on heat pumps in poorly insulated houses and older homes go through what that looks like.

Does the numbers case stack up? The upfront cost is higher than a new boiler, and the running cost depends on how efficiently the system runs and how electricity prices compare with gas.

Where a full heat pump doesn’t fit, there are middle routes. Our article on what a hybrid heat pump system is explains the option of pairing a smaller heat pump with an existing boiler.

What Does It Cost to Replace a Gas Boiler With a Heat Pump?

A typical air source heat pump installation in the UK costs roughly £8,000 to £15,000 or more before any grant. That’s the range we use throughout this site, and our guide on what heat pump installation costs in the UK breaks it down in detail. Where the price lands depends on:

• The size and make of the heat pump

• Whether you need a new cylinder, and where it has to go

• How many radiators need replacing

• Any pipework changes

• Electrical work, including a consumer unit upgrade if needed

• The groundwork for the outdoor unit

• Labour, commissioning and certification

The grant. In England and Wales, the Boiler Upgrade Scheme currently pays £7,500 towards an air source heat pump when it replaces a gas boiler. The installer applies for it and deducts it from your quote, and the installer must be MCS certified. As an illustration, a £12,000 quote would leave you paying about £4,500. Scotland and Northern Ireland have different support, and the rules and amounts change, so check gov.uk before you plan around it. The higher £9,000 rate applies only to homes replacing oil or LPG, not mains gas.

Running costs. Whether a heat pump is cheaper to run than gas depends on two things: how efficiently it runs and how much electricity costs compared with gas. As a rough guide, if electricity costs about 3.5 times as much as gas per unit, a heat pump needs a seasonal efficiency of around 3.2 to match a 90% efficient gas boiler. Above that it costs less, below it costs more. Our guide on heat pump efficiency explains what’s realistic, and our running costs versus fossil fuels guide goes through the comparison.

What to Check Before You Commit

This is the part that decides how the project turns out. Before you sign anything, make sure you can say yes to each of these.

1. A room-by-room heat loss calculation has been done. Not an estimate from floor area, and not a figure borrowed from your old boiler. See heat loss calculations explained.

2. The heat pump isn’t sized from your boiler’s output. A combi might be rated at 24 to 35 kW to cope with hot water demand, while a typical home’s heat loss may be a fraction of that, often somewhere around 5 to 12 kW. Sizing a heat pump from the boiler is a classic route to an oversized unit. Our guide on what happens if a heat pump is oversized shows why that costs you.

3. The quote lists which radiators change and why. “Existing radiators are fine” with no calculation behind it is a red flag.

4. There’s a plan for the pipework and its flow rate.

5. The cylinder has a confirmed size and location.

6. The outdoor unit position is agreed and sensible for noise and airflow.

7. The electrical capacity has been checked.

8. The design flow temperature is stated, and weather compensation will be switched on at handover.

9. You’ll receive proper handover documents, including the settings and how to use the controls.

10. There’s an aftercare and first-year review, because most tuning happens in the first winter.

If you’re comparing quotes, our guide on how to compare heat pump quotes lists what a good quote contains and what to watch for. And if you’d like a second pair of eyes, our Pre-Installation Design & Heat Loss Review checks the sizing, flow temperature and radiators of any quote before you commit. We’ve seen what happens when it’s skipped: one homeowner’s heat loss figure was wrong and the radiators undersized, and another needed to know which of two quotes was the better design.

How Long Does the Replacement Take?

The process usually runs in this order:

1. Survey and heat loss calculation, which is where the design starts

2. Design and quote

3. Grant application, made by the installer

4. Installation, often a few days for a straightforward job and longer for a complex one

5. Commissioning and handover

6. First winter tuning

Installation itself is the shortest stage. Most of the work that decides how the system performs happens before it and after it. Our article on what actually happens during an air source heat pump installation walks through the install itself.

What Living With It Is Like

Most people notice three things in the first few weeks.

The house feels steadier. There are no hot blasts, no cold dips, just an even temperature. Radiators feel warm instead of hot, and that’s normal.

It stays on. The system runs for long periods at low output. Turning it off and on, or setting big temperature changes, tends to work against it. Our guide on the best heat pump thermostat settings explains how to set it up so it can run efficiently.

It needs a first-year tune. Settings that were fine at handover often need adjusting once the weather turns. In one of our cases, a family’s bills doubled because the system was running at 55°C in mild weather. In another, a new build with nine underfloor zones was running at a COP of 1.2 until the zones were cut to two. Neither problem was a faulty unit. Both were settings and design, and both are the kind of thing a review picks up.

If your new system feels weaker than your old boiler, that’s a common reaction, and our article on why a heat pump can feel less powerful than your old boiler explains what’s going on.

The Most Common Mistakes When Replacing a Gas Boiler

• Sizing the heat pump from the old boiler. Boiler output and home heat loss are different things.

• Assuming the radiators will be fine. Some will, some won’t. Only a calculation tells you which.

• Forgetting the cylinder. Combi owners often don’t realise they’ll need the space until the survey.

• Skipping the heat loss calculation, or accepting a rough estimate.

• Accepting the default settings. Factory settings are rarely right for your home.

• Turning the system on and off. Heat pumps do best when they run steadily.

• Not asking for handover documents and the settings that were chosen.

• Not planning a first-year check. Tuning after the first cold spell usually pays for itself.

Frequently Asked Questions

Can a heat pump replace a combi boiler?
Yes, in most homes. The main difference is that you’ll need a hot water cylinder, because a heat pump stores hot water rather than heating it instantly. You’ll also need space for an outdoor unit, and the radiators and pipework need to suit the lower temperature.

How much does it cost to replace a gas boiler with a heat pump?
Typically £8,000 to £15,000 or more before any grant. In England and Wales the Boiler Upgrade Scheme currently takes £7,500 off an air source heat pump replacing a gas boiler, so many homeowners end up paying a few thousand pounds, and more where a lot has to change.

Is a heat pump cheaper to run than a gas boiler?
It depends on how efficiently the heat pump runs and the price of electricity compared with gas. A well-tuned system can cost less than gas, and a poorly set up one can cost more.

Do I need to replace all my radiators?
Not necessarily. Some homes keep most of them and others need several replaced. A room-by-room heat loss calculation shows which, and a good quote should list them.

Do I need a hot water cylinder?
If you have a combi, yes. A heat pump stores hot water in a cylinder, so you’ll need space for one and the right size for your household.

Can I keep my gas boiler as a backup?
Sometimes. A hybrid system pairs a smaller heat pump with an existing boiler. It can suit homes where a full replacement is difficult, though it isn’t right for everyone.

How long does the installation take?
Often a few days for a straightforward home, and longer when there’s a lot of pipework, radiator or electrical work. The survey and design beforehand take longer than the installation itself.

Will my house be cold in winter?
Not if the system is designed correctly. A heat pump sized from a proper heat loss calculation, with suitable radiators, keeps the house at a steady temperature all winter.