How much does a combi boiler cost fitted?
A combi (combination) boiler heats your radiators and your hot water from one compact unit, with no cylinder or loft tanks. That is why it is the cheapest type of gas boiler to fit and by far the most common choice for a swap. The fitted price depends mainly on the brand tier and the output in kW.
The ranges below are for a like-for-like swap by a Gas Safe registered installer, including the boiler, a standard horizontal flue, labour, disposal of the old boiler, commissioning and the Building Regulations notification. They are the same baseline figures used throughout the new boiler cost guide that this article sits under.
| Combi boiler tier | Typical output | Supply only | Fitted (like-for-like swap) | Guarantee commonly offered |
|---|---|---|---|---|
| Budget (e.g. Ideal Logic, Baxi 400/600, Glow-worm Energy, Vokera) | 24-30kW | £600-£1,100 | £1,800-£2,500 | 5-10 years |
| Mid-range (e.g. Worcester Greenstar 2000/4000, Vaillant ecoFIT pure, Ideal Vogue Max, Baxi 800) | 25-35kW | £900-£1,600 | £2,200-£3,200 | 7-12 years |
| Premium (e.g. Worcester Greenstar 8000, Vaillant ecoTEC plus, Viessmann Vitodens 200, ATAG) | 30-40kW | £1,300-£2,200+ | £2,800-£4,500 | 10-12+ years |
The combi boiler fitting cost, meaning labour alone, is £500 to £1,200 for a straightforward swap and does not vary much with the brand. What moves the fitted total from £1,800 to £4,500 is the boiler you choose, the extras added, and how much work your home needs beyond the swap itself.
Model names above are given for orientation only; outputs, guarantee terms and which ranges are current change from year to year, so check the manufacturer's site for the model you are quoted. Brand-by-brand fitted and supply-only prices are compared in boiler prices by brand.
How much is a combi boiler supply only, and should you buy one that way?
Supply-only prices from merchants and online retailers look tempting next to a fitted quote. A 30kW mid-range combi at £1,100 against a fitted price of £2,600 seems to leave £1,500 on the table.
In practice, buying the boiler yourself rarely saves money, for four reasons:
- Guarantees. Manufacturers' extended guarantees (7, 10 or 12 years) almost always require installation and registration by a Gas Safe registered engineer, and often by one accredited with that brand. A boiler you supply may only get the standard two to five years, or nothing if it is from a grey import.
- Installer margin. Installers buy at trade prices and add a margin, but they also take responsibility for the boiler arriving correct and undamaged. Many simply charge a higher labour rate when you supply the appliance.
- Wrong model risk. A combi that is too big, has the wrong flue option or needs a different gas pipe size can cost more to install than the saving.
- No single point of responsibility. If the boiler fails early, the installer blames the appliance and the retailer blames the installation.
Supply-only buying makes sense in one situation: you already have a trusted installer who has sized and specified the boiler and is happy to fit a customer-supplied unit at an agreed labour rate. Even then, ask them to confirm the guarantee you will receive in writing.
How do system boiler prices compare?
A system boiler heats radiators directly and sends hot water to a sealed, unvented cylinder, which stores it under mains pressure. The boiler costs a little more than a combi, and you have the cylinder to consider.
| Item | System boiler | Combi boiler |
|---|---|---|
| Boiler, supply only | £800-£2,000 | £600-£2,200 |
| Fitted, like for like (existing cylinder kept) | £2,000-£4,000 | £1,800-£3,500 |
| New unvented cylinder, if replaced | £800-£1,500 extra | Not needed |
| Typical labour time for a swap | 1-1.5 days | 1 day |
| Best suited to | 2+ bathrooms used together, 4+ bedrooms | 1-2 bathrooms, flats, smaller homes |
If your existing cylinder is sound, a system boiler swap is only a few hundred pounds dearer than a combi swap. If the cylinder is 15 to 20 years old, poorly insulated or vented from a loft tank, many installers will recommend changing it at the same time, which is where the £4,000 to £5,500 totals come from.
A regular (heat-only) boiler, which works with a vented cylinder and cold water tanks in the loft, is priced much like a system boiler: £700 to £1,800 supply only and £2,000 to £4,000 fitted. It is almost only ever fitted as a like-for-like swap in older homes. For the differences between the three layouts, see the explanation of combi versus system boilers and which to choose.
Which factors push a combi boiler price up or down?
Beyond brand tier, these are the variables that make one home's combi boiler cost £1,900 and the neighbour's £3,400 for a similar boiler.
- Output. A 24-25kW combi suits a flat or small house with one bathroom; a 28-30kW model suits most three-bedroom homes; a 35kW or larger is for homes with higher hot water demand. Each step up adds £100 to £300 to the boiler price. The sizing guide on which size of boiler you need helps you avoid paying for output you will never use.
- Flow rate for showers. Combis deliver 10 to 16 litres a minute of hot water depending on the model. Two showers at once pushes you to a higher output or a system boiler, which is a cost decision as much as a comfort one.
- Flue position. A standard horizontal flue is included. A vertical flue through the roof, a long run or a plume kit adds £150 to £500.
- Gas supply. Most combis need a 22mm gas pipe for the full run from the meter. Upgrading an old 15mm pipe is £150 to £400.
- Condensate drain. If there is no internal drain nearby, a condensate pump or an insulated external run adds £100 to £300.
- Controls. Boiler Plus rules in England require a time and temperature control, and for a combi one additional energy-saving measure such as a smart thermostat, load or weather compensation, or flue gas heat recovery. Budget £120 to £250 for a smart thermostat if you do not already have suitable controls.
- System condition. Sludge in the radiators means a powerflush (£300 to £800) before the new boiler goes in, or the guarantee may be at risk.
- Location. London and the South East carry higher labour rates, often £200 to £500 more than the national average for the same job.
Which hidden extras are not in the headline price?
A low headline figure often excludes items that later appear on the invoice. Ask for every one of these to be listed as included or excluded before you accept a quote.
| Extra | Typical cost | Why it comes up |
|---|---|---|
| Magnetic filter | £100-£200 fitted | Protects the heat exchanger; a condition of many long guarantees |
| Powerflush | £300-£800 | Old or sludged systems; chemical cleanse (£100-£200) may suffice on newer systems |
| Smart thermostat or programmer | £120-£250 fitted | Boiler Plus compliance and efficiency |
| Vertical or extended flue, plume kit | £150-£500 | Boiler not on a suitable outside wall |
| Gas pipe upgrade to 22mm | £150-£400 | Older 15mm supply cannot feed a modern combi |
| Condensate pump or external pipe | £100-£300 | No gravity drain nearby |
| Thermostatic radiator valves | £15-£40 each | Required on most radiators without a room thermostat |
| New radiators | £150-£400 each fitted | Corroded, undersized or mismatched to a lower flow temperature |
| Cupboard or boxing-in work | £150-£600 | New boiler is a different size or needs service clearance |
| Removing cylinder and loft tanks | £150-£400 | Conversion from a cylinder system to a combi |
| Scaffolding | £200-£600 | Roof or high-level flue terminal |
To put the radiator line in context, the cost of new combi boiler and 7 radiators together is typically £3,300 to £6,300: £2,200 to £3,500 for a mid-range combi fitted, plus £1,050 to £2,800 for seven radiators with TRVs. Doing both jobs in one visit is almost always cheaper than two separate bookings.
Every one of these extras is explained line by line, with what installers charge for labour on each, in the guide to boiler installation cost, labour, parts and extras.
Worked examples: how much is a new combi boiler for three typical homes?
The figures below use the ranges above and assume a reputable installer and a boiler from one of the major manufacturers. Your quotes may sit slightly outside them, but if they are far outside, ask why.
Example 1: one-bedroom flat, existing combi, 1 bathroom
| Item | Typical cost |
|---|---|
| 24-25kW budget or mid-range combi | £700-£1,200 |
| Labour, half to one day | £500-£800 |
| Flue, valves, cleanse, disposal | £150-£300 |
| Magnetic filter | £100-£200 |
| Total fitted | £1,450-£2,500 |
A flat is the cheapest combi swap there is. The boiler is usually on an outside wall in the kitchen, the pipe runs are short and the system has only four to six radiators.
Example 2: three-bedroom semi, old combi being swapped, 1 bathroom
| Item | Typical cost |
|---|---|
| 28-30kW mid-range combi | £1,000-£1,500 |
| Labour, one day | £600-£1,000 |
| Flue, valves, cleanse, disposal | £150-£300 |
| Magnetic filter and smart thermostat | £220-£450 |
| Powerflush of 7-9 radiators, if needed | £300-£600 |
| Total fitted | £2,270-£3,850 |
This is the average cost of a combi boiler most people have in mind: a 30kW model swapped in a day for £2,000 to £3,200, or up to £3,850 with the system cleaned and modern controls fitted.
Example 3: four-bedroom detached, 2 bathrooms, system boiler kept with cylinder
| Item | Typical cost |
|---|---|
| 24-30kW mid to premium system boiler | £1,200-£1,900 |
| Labour, 1-1.5 days | £700-£1,200 |
| Flue, valves, cleanse, disposal, controls | £300-£600 |
| Magnetic filter | £100-£200 |
| New unvented cylinder, only if the old one is replaced | £0 or £800-£1,500 |
| Total fitted | £2,300-£5,400 |
Keeping a sound cylinder makes a system boiler swap cost much the same as a large combi. Replacing the cylinder pushes it well past a combi, but gives a home with two bathrooms hot water that a single combi often cannot match. The guide to how boilers and hot water cylinders work together explains when a cylinder is worth keeping.
How much does converting to a combi add?
Many people searching for a combi boiler replacement cost are not swapping like for like. They have a regular or system boiler with a cylinder and want to switch to a combi to free up the airing cupboard and loft. This is a conversion, and it costs more than a swap.
| Conversion | Typical fitted cost | What the extra pays for |
|---|---|---|
| Regular or system boiler to combi | £2,500-£5,000 | Removing cylinder and tanks, new hot and cold pipework, gas pipe upgrade, often a second day of labour |
| Back boiler unit (behind a fire) to combi | £3,000-£5,500 | Removing the unit and fire, capping gas, making good the fireplace, siting the new boiler elsewhere with a new flue |
The replace conventional boiler with combi cost is driven by pipework more than by the boiler. The old cylinder fed hot water by gravity from the loft; a combi delivers it at mains pressure from wherever the boiler sits, so new 15mm hot water pipework often has to be run to the bathroom and kitchen. Mains water pressure and flow also need checking, because a combi cannot give you more hot water than the incoming cold main supplies.
A back boiler to combi conversion cost is higher still because the old unit sits in a fireplace that must be stripped, capped and made good, and the new boiler needs a new location and flue. If this applies to you, the step-by-step guide to replacing a back boiler covers the job in full, and the explainer on regular and heat-only boilers describes what you are converting from.
Are combi boilers cheaper to run than system or regular boilers?
The combi vs conventional boiler running costs question comes up in almost every conversion decision, and the honest answer is that the gap is smaller than most people expect.
- Standing losses. A cylinder loses heat to the room around it. A modern, factory-insulated unvented cylinder loses only a modest amount each day, so the saving from removing it is typically tens of pounds a year, not hundreds. An old cylinder with a thin jacket in a cold loft loses considerably more.
- Boiler efficiency. All new gas boilers are condensing and rated at roughly 90 to 94 per cent. There is no meaningful efficiency difference between a combi and a system boiler of the same tier.
- Flow temperature. Running the heating at 55-60°C rather than 75-80°C saves more than the choice of layout. A system boiler's cylinder needs a periodic 60°C-plus store for hygiene, which limits how low you can set its hot water circuit.
- Behaviour. A combi only heats the water you draw, so households that use little hot water save more; a large family running baths and showers back to back will see almost no difference.
So for system boiler vs combi boiler running costs, expect a combi to be a little cheaper in a small household and roughly equal in a large one. Choose the layout on hot water performance and space, then focus on efficiency through controls and flow temperature.
How can you keep the combi boiler cost down?
Finally, look after the boiler once it is in. An annual service, typically £70 to £130, is a condition of nearly every manufacturer's guarantee and is far cheaper than an avoidable repair. The guide to the cheapest way to get a new boiler brings together all the timing, brand and installer tactics in one place, and how long a boiler lasts explains what a well-kept combi should give you back for the money.
Frequently asked questions
How much is a combi boiler on its own, without fitting?
Supply-only prices run from about £600 for an entry-level 24kW model from a major brand to over £2,200 for a premium 35kW or 40kW boiler. Most mid-range 28-30kW combis cost £900 to £1,600. Bear in mind that manufacturer guarantees normally depend on a Gas Safe registered installer fitting and commissioning the boiler, so the supply-only price is rarely the price you actually pay.
How much does a new combi boiler cost fitted in a 3 bed house?
For a straightforward swap of an existing combi, most three-bedroom homes pay £2,000 to £3,200 for a 28-30kW mid-range model fitted. That covers the boiler, flue, labour, disposal and commissioning. Adding a magnetic filter, smart thermostat and a powerflush of an older system takes a fully refreshed installation to £2,500 to £4,000.
What is the combi boiler replacement cost if I am switching from a system or regular boiler?
Converting from a cylinder-based system to a combi typically costs £2,500 to £5,000. The extra over a like-for-like swap pays for removing the cylinder and any loft tanks, running new hot and cold pipework, often upgrading the gas pipe to 22mm, and a second day of labour. A back boiler conversion is a bigger job again, usually £3,000 to £5,500.
Is a system boiler more expensive than a combi?
Usually, yes. System boilers themselves cost a little more, typically £800 to £2,000 supply only, and fitted prices run £2,000 to £4,000 compared with £1,800 to £3,500 for a combi. If the hot water cylinder also needs replacing, add £800 to £1,500 for a new unvented cylinder. The pay-off is strong hot water to several outlets at once.
What does a new combi boiler and 7 radiators cost?
A mid-range combi fitted at £2,200 to £3,500 plus seven new radiators with thermostatic valves at £150 to £400 each comes to roughly £3,300 to £6,300. The spread depends on radiator size and style, whether pipework needs moving and whether the installer does both jobs in one visit, which is normally cheaper than two separate ones.
Do combi boilers cost less to run than system boilers?
Slightly, in most homes. A combi heats water only when a tap runs, so there is no stored water losing heat overnight. A modern well-insulated cylinder loses only a small amount, so the difference is typically tens of pounds a year rather than hundreds. Boiler efficiency, flow temperature and controls matter far more to the gas bill than the choice between the two layouts.
Sources and further reading
- Energy Saving Trust - Boilers
- Gas Safe Register
- Worcester Bosch - Combi and system boiler ranges
- Vaillant UK - ecoFIT and ecoTEC boilers
- Ideal Heating - Logic and Vogue boilers
- Baxi - Combi and system boilers
- GOV.UK - Boiler Plus standards for heating in England
Prices and scheme details were checked on 22 October 2026 and are shown as typical ranges. Always get written quotes from Gas Safe registered installers.