How Much Is It to Install a Combi Boiler
26 September 2026
How Much Is It to Install a Combi Boiler?
Installing a combi boiler typically costs between £1,900 and £3,200 in the UK. This price applies to a straightforward replacement where the new combi boiler is fitted in the same position as the old one.
The price can rise to between £2,900 and £4,600 if you are converting from a system or conventional boiler to a combi. Removing a back boiler and installing a new combi can cost between £3,000 and £4,800.
The type of installation makes a major difference. A simple replacement may take one working day, while a conversion or relocation can require two or three days of labour, new pipework and changes to the flue.
Quick answer: Budget £1,900 to £3,200 for a normal combi boiler replacement. Allow £2,900 to £4,800 if tanks, a hot-water cylinder or a back boiler must be removed.
Combi Boiler Installation Costs in 2026
The following prices are based on InstantCost's current UK boiler installation data. They include the boiler, standard installation labour and the usual certification requirements. Your final quotation will depend on the boiler model and the work required at your property.
Installation type | Typical UK cost | Typical time |
|---|---|---|
Combi to combi replacement in the same position | £1,900 to £3,200 | 4 to 8 hours |
System boiler to combi conversion | £2,900 to £4,600 | 2 to 3 days |
Boiler moved to another room | £2,600 to £4,200 | Around 2 days |
Back boiler removed and replaced with a combi | £3,000 to £4,800 | 2 to 3 days |
A quotation near the lower end normally means that the engineer can install the new boiler in the existing position and use a similar pipework layout. Prices towards the top of the range normally involve a premium boiler, difficult access, a longer flue or additional heating-system work.
How Much Is the Boiler Itself?
A 24kW to 30kW combi boiler unit typically costs between £750 and £1,700 before installation. The brand, output, controls and warranty length all affect the price.
Boiler category | Typical unit price | Typical warranty |
|---|---|---|
Budget models, including Ideal and Baxi | £750 to £1,150 | 7 to 10 years |
Mid-range models, including Vaillant | £950 to £1,450 | Around 10 years |
Premium Worcester Bosch models | £1,100 to £1,700 | 10 to 12 years |
Premium Viessmann models | £1,050 to £1,650 | Around 10 years |
A longer warranty can offer useful protection, but check its conditions. Many manufacturers require annual servicing by a qualified engineer. Missing a service may affect the warranty.
How Much Is the Labour?
Labour for a standard combi boiler installation usually costs between £450 and £800. This normally covers removing the existing boiler, mounting the new unit, connecting the pipework and flue, testing the system and completing the necessary paperwork.
Labour can rise to between £1,000 and £1,700 for a system-to-combi conversion. This is because the engineer must remove or disconnect the hot-water cylinder and any storage tanks, alter the pipework and commission a different type of heating system.
Boiler and labour figures do not always add up exactly to the complete fitted price. A full quotation can also contain a flue kit, valves, controls, filter, chemicals, fittings, waste disposal and certification.
Why Does a Combi Conversion Cost More?
A like-for-like replacement involves exchanging one combi boiler for another. Most of the heating layout is already suitable, so fewer changes are required.
A conversion is more involved. If your home currently has a conventional boiler, the installer may need to remove a cold-water tank from the loft and a hot-water cylinder from an airing cupboard. The old pipes must then be capped or rerouted before the combi boiler can provide heating and hot water directly.
The engineer must also check whether the incoming water flow is strong enough for a combi. A combi boiler may not provide satisfactory results in a property with weak mains pressure or high simultaneous hot-water demand.
Additional Combi Boiler Installation Costs
Several additional jobs can increase the quotation. These should be listed separately so you can compare installers properly.
Additional work | Typical cost | Why it may be required |
|---|---|---|
Power flush | £350 to £650 | Removes sludge and debris from an older heating system |
Magnetic system filter | £120 to £200 | Collects metal particles before they enter the boiler |
Smart thermostat | £200 to £350 | Provides improved heating control and scheduling |
Flue relocation | £150 to £400 | Needed when the existing flue position is unsuitable |
Gas pipe upgrade | £150 to £350 | May be necessary when an older gas pipe is too small |
Scaffolding for a high flue | £200 to £500 | Provides safe access to an elevated external wall |
Moving the boiler | £600 to £1,200 extra | Requires extended pipework, drainage and a new flue route |
Ask whether these items are included before accepting a quotation. A low initial price can become expensive if important fittings and compliance work are later treated as extras.
Calculate Your Combi Boiler Cost
What Size Combi Boiler Do You Need?
Choosing the correct boiler output is important. A boiler that is too small may struggle to heat the home or provide enough hot water. An oversized boiler can cost more and may cycle on and off more frequently than necessary.
A smaller flat with one bathroom may need a different boiler from a detached house with several bedrooms. The installer should consider the number of radiators, bathrooms, occupants and the property's heat loss.
The hot-water flow rate is particularly important. Combi boilers heat water directly from the mains, so two showers or taps being used together can reduce the available flow. Homes with several bathrooms and high simultaneous demand may be better suited to a system boiler with a hot-water cylinder.
What Should a Combi Boiler Installation Quote Include?
A clear written quotation should explain exactly what you are paying for. Look for the following:
The boiler manufacturer, model and output
The length and conditions of the manufacturer's warranty
Removal and disposal of the old boiler
The flue and any flue extensions
Standard fittings, valves and pipework
A magnetic filter where specified
System cleaning and corrosion inhibitor
Heating controls or a thermostat
Testing and commissioning
Gas Safe and Building Regulations paperwork
Making good around the old and new boiler positions
VAT and the final amount payable
Check whether the installer is offering a fixed price or an estimate. A fixed price should identify the circumstances that could cause it to change.
Get a Combi Boiler Cost Estimate
How Long Does It Take to Install a Combi Boiler?
A straightforward combi-to-combi replacement normally takes between four and eight hours. The heating and hot water will usually be unavailable for most of that working day.
A system-to-combi conversion generally takes two to three days. Moving a boiler to another room usually takes around two days because new water, gas and heating pipes may be needed.
Power flushing can add three to five hours. Scaffolding and difficult access may also affect the schedule. You can find a full breakdown in InstantCost's guide to how long a new boiler installation takes.
Does the Price Include VAT and Certification?
The InstantCost fitted-price ranges include VAT, Gas Safe certification and Building Regulations notification for a standard installation. Still, every quotation should confirm these points in writing.
Once the work is finished, the boiler should be tested and commissioned. The installer should explain the controls, provide the warranty information and arrange the required notification. Keep the installation records because they may be needed for the warranty or when selling the property.
Regional Price Differences
Installation prices vary across the UK. London and the South East can be around 10% to 15% above the national average. Prices in the North East, Scotland and Northern Ireland may be around 10% to 18% lower.
Location should not be considered on its own. A simple replacement in London can still cost less than a complicated conversion elsewhere. Compare quotations based on the complete scope of work rather than the total alone.
Is a Combi Boiler Suitable for Your Home?
A combi boiler is often suitable for a flat or small to medium house with one main bathroom. It does not require a separate hot-water cylinder, which can free space in an airing cupboard.
It may be less suitable for a large household that frequently uses several showers and hot taps at once. Because the water is heated directly from the mains, performance depends on the incoming flow rate and pressure.
Ask the installer to test the water flow before recommending a combi. Do not choose a boiler only because removing the cylinder would create more storage space.
How to Reduce the Installation Cost
Keep the boiler in the same place: Relocation can add £600 to £1,200.
Choose the correct output: Paying for a larger boiler than the property needs offers little benefit.
Get detailed written quotations: Compare the boiler model, controls, filter, flush, warranty and certification.
Plan the replacement before a breakdown: An emergency during cold weather gives you less time to compare installers.
Check what is already reusable: Existing controls or recently updated pipework may affect the required work, subject to the engineer's assessment.
Protect the new boiler: A clean system and magnetic filter can help protect internal components and support warranty conditions.
Common Combi Boiler Installation Mistakes
Choosing a boiler based only on price
The cheapest boiler is not automatically the best value. Consider output, warranty, parts availability and whether local engineers regularly service that model.
Accepting a quotation without a model number
A quote that simply says “new combi boiler” is too vague. The exact model and output are needed to compare two prices properly.
Assuming every home is suitable for a combi
Low mains flow or several bathrooms can make a combi unsuitable. The water supply and household demand should be assessed first.
Forgetting the cost of moving the boiler
A new location requires more pipework and possibly a different flue route. It may also require floorboards, walls or cupboards to be disturbed.
Not checking what the warranty requires
Many warranties depend on correct installation, product registration and annual servicing. Read the conditions rather than relying only on the advertised number of years.
Using an unregistered installer
A lower quotation is not worth the risk if the person cannot legally work on gas appliances. Always verify the engineer before work begins.
Combi Boiler Installation Safety
A gas combi boiler must be installed by an engineer who is legally registered and qualified for the relevant type of work. Check both sides of the engineer's Gas Safe ID card and confirm the registration through the official Gas Safe Register.
Do not disconnect an old gas boiler or attempt to connect the gas supply yourself. Boiler installation also involves combustion safety, ventilation, flue positioning, electrical connections and condensate drainage.
If you smell gas, avoid flames and electrical switches. Open doors and windows if it is safe, leave the property and call the National Gas Emergency Service on 0800 111 999.
Frequently Asked Questions
How much does it cost to install a combi boiler in a three-bedroom house?
A straightforward combi replacement generally falls within the £1,900 to £3,200 InstantCost range. The number of bedrooms alone does not determine the price. The required output, number of radiators, boiler location, flue and condition of the existing system also matter.
How much is a combi boiler conversion?
Converting a system or conventional boiler to a combi typically costs between £2,900 and £4,600. The price includes more labour because the installer may need to remove a cylinder or tanks and alter the existing pipework.
Is £3,000 expensive for a new combi boiler?
£3,000 is within InstantCost's typical range of £1,900 to £3,200 for a standard supplied-and-fitted combi replacement. Whether it represents good value depends on the boiler model, warranty and what the quote includes.
Can a combi boiler be installed in one day?
Yes. A like-for-like replacement in the same position normally takes four to eight hours. A conversion, relocation or installation requiring a gas-pipe upgrade is likely to take longer.
Do I need a power flush with a new combi boiler?
Not every installation needs a full power flush, but the heating system must be suitably clean. An older system containing substantial sludge may require one. InstantCost lists a typical separate power-flush cost of £350 to £650.
Is it cheaper to install a combi boiler in the same location?
Yes. Keeping the boiler in the same suitable location usually reduces pipework, flue work and labour. Moving it can add approximately £600 to £1,200 to the installation cost.
The Real Price of Installing a Combi Boiler
For most straightforward replacements, a realistic budget is £1,900 to £3,200. A conversion or back-boiler replacement is more likely to cost between £2,900 and £4,800.
The most useful quotation is not always the cheapest. It should specify the boiler model, output, warranty, parts, labour, system cleaning, controls, certification and VAT. Ask the installer to explain anything that is excluded before you agree to the work.