Two quotes for “a new boiler” can be £2,000 apart and both be honest, because they are rarely quoting the same job. One is a like-for-like swap; the other is taking out a cylinder, capping loft tanks and running a new flue. So this calculator prices the boiler and the work around it as separate lines, and gives you a band rather than a single number - because anyone offering one number for a job they have not seen is guessing.
New boiler cost
A boiler is quoted supplied and fitted, by type. The type is the real fork in the British market - a combi, a system boiler and a regular boiler are three different installations, not three trim levels of one.
Real quote data is a useful reality check on that table. Across 2,000 quotes issued between January and June 2026, Heatable put the median for a standard combi replacement at £2,300, with most straightforward jobs landing between £2,150 and £3,000. In other words: most people sit in the bottom half of the combi band, and the top half is where the conversions live.
There is no £/kW figure here, and that is deliberate. Nobody publishes one for a boiler - the trade quotes the model and the cost guides tabulate the type, so the by-type band is the source. Sizing a boiler is a room-by-room heat loss calculation your engineer does, and it is a different question from pricing one. A calculator that multiplied a kW figure by a made-up rate would look more precise and be less true.
Combi boiler cost and what a conversion adds
The like-for-like-versus-conversion split is the whole price story, so each conversion is priced as its own line rather than as a percentage on the boiler.
A back boiler to combi conversion typically totals £3,000 to £5,000 all in, averaging around £4,000 - the dearest common job in British heating, because the fireplace has to be opened up and made good afterwards.
Moving the boiler is the smaller of the two decisions, and it goes up in steps: about £500 to £800 to shift it within the same room, £700 to £1,200 to another room on the same floor, £800 to £1,400 between floors, and £900 to £1,500 into the loft, where it also needs frost protection and lagging.
And the four extras that get left off a headline quote:
A complete horizontal flue run is around £200; a vertical flue is £300 to £700, and an extension kit runs £30 to £80 per metre. If the flue comes out high up, add scaffolding at £50 to £100 a day - that one is not in the band above.
The magnetic filter is the cheapest line on the page and the one worth arguing about: most manufacturers now require one for the warranty to stand.
Boiler service cost
A service is a fixed-fee annual visit averaging about £100, with most falling in the £75 to £125 range. By fuel: a gas boiler is £85 to £120, an electric one £75 to £110, LPG £90 to £120 and oil £90 to £125.
Where you live moves it as much as what you burn:
There is no service mode in the calculator because there is nothing to compute - a fixed fee with a box drawn around it is not a calculation. It belongs here as a number, and it belongs in your diary as an annual job: most manufacturer warranties lapse without a service on record, which quietly turns a £100 visit into the difference between a repair and a replacement.
Air source heat pump cost
A heat pump is quoted per kW of output installed, and then the grant comes off the top. Published UK installs land like this:
Divide those through and the rate runs roughly £1,000 to £2,000 per kW - and the reason the band is that wide is that the per-kW price falls as the unit grows. The survey, the cylinder, the pipework and the days on site are much the same work whatever is bolted to the wall outside, so a small install sits near the top of the band and a big one near the bottom.
The changeover, not the unit, is what catches people out:
A heat pump runs at a much lower flow temperature than a boiler, so the radiators that kept the house warm at 70°C may not do it at 45°C. That is why the survey matters more here than on a boiler swap - and why “my radiators are fine” is the assumption most likely to cost you.
The Boiler Upgrade Scheme
It covers England and Wales only - Scotland and Northern Ireland run their own schemes - and it is one grant per property. Your installer applies and takes it off the quote, so the money never passes through your hands, and the net band in the calculator is the number that actually leaves your account. It cannot be used for a hybrid system or to replace an existing low-carbon system, and the installer must be MCS certified. Check gov.uk before you sign anything - grant levels and closing dates move.
How this calculator works
Every figure is a real quantity times a real rate, plus the once-off lines. Nothing else:
Boiler = the boiler supplied and fitted (by type)
+ conversion (only if it is not a like-for-like swap)
+ Σ extras (flush, filter, cylinder, flue)
Heat pump = output (kW) × rate (£/kW)
+ Σ extras (cylinder, radiators, underfloor)
- the grant (a fixed sum off every end of the band)
Then every one of them becomes a band:
low = quantity × your low rate
high = quantity × your high rate
typical = the midpoint of the two
The typical is the midpoint of your own band, not a fourth estimate pulled from somewhere else, so the answer is only ever as precise as the rate you gave it. The grant is subtracted as cash, not a percentage - it shifts the whole band left by £7,500 rather than scaling it, and each end floors at zero, which is why a small install’s low end can land at nothing.
Rates are indicative 2026 UK ranges from published cost guides: Boiler Central for boiler prices by type, the conversion, the flush and the flue; Heatable for the median quoted combi replacement; MyJobQuote for the service and relocation figures; GreenMatch for heat pump prices by property size; and gov.uk for the grant levels. They move with your postcode, your property and how busy your installer is, and they are not a quote. The calculator seeds its fields from exactly these numbers so you can overwrite them the moment a real quote is in front of you.
Worked example
A 1970s semi swapping a regular boiler for a combi:
- The boiler: a combi supplied and fitted, £1,800 - £4,000, typical about £2,900.
- The conversion is its own line, not a percentage: pipework, and the cylinder and loft tanks out, at £1,500 - £2,500.
- Two extras the installer will quote anyway - a power flush at £300 - £750 and a magnetic filter at £100 - £200.
- Add them up: £3,700 to £7,450, typical about £5,575 (the midpoint, not a separate guess). Choose a like-for-like swap instead and the conversion line disappears - that is the whole £1,500 - £2,500 gap between the two quotes on your kitchen table.
The same house pricing an 8 kW air source heat pump instead:
- 8 kW × £1,000 - £2,000 per kW = £8,000 - £16,000. Per-kW falls as the unit grows, so an 8 kW job sits mid-band rather than at either end.
- Plus a hot water cylinder at £500 - £1,500 - an air-to-water heat pump needs one, where the combi above did not. Gross: £8,500 - £17,500.
- Then the Boiler Upgrade Scheme grant of £7,500 comes off the installer's quote up front: £1,000 to £10,000, typical about £5,500.
- Which is the honest comparison the two tabs exist to let you make: after the grant, the typical heat pump here lands within a few hundred pounds of the typical combi conversion - and the running costs, not the install, are what separate them after that.
Before you spend anything
- Check the Gas Safe card, not the van. Work on gas appliances in a home must be done by a Gas Safe registered engineer qualified for that work, and it is illegal for anyone else to do it. The back of the card lists what they are signed off for and whether it is current.
- Make every quote name the same scope. Ask each installer to write down the boiler model, whether the flue route changes, and whether a power flush, a filter and a gas pipe upgrade are in or out. Half the spread between quotes is scope, not price.
- Ask about the gas supply pipe. Older properties often have a 15 mm run where a combi needs 22 mm. It is a common extra, it is rarely on the headline figure, and it is not optional.
- Get the warranty terms in writing. Manufacturer warranties of 7 to 12 years are common on new boilers, but they usually depend on an annual service and often on a magnetic filter being fitted. That £100 line protects a £2,500 one.
- Price the heat pump net, then price the running cost separately. After the £7,500 grant the install gap between a heat pump and a boiler is much smaller than the headline figures suggest. What separates them afterwards is the tariff and the insulation, not the box.
