Combi boiler
Heats water on demand. No cylinder, no loft tank, small footprint. Right for one-bathroom homes with mains pressure above about one bar.
When we'd choose it
One bathroom, decent mains pressure, and the airing cupboard is worth keeping.
Boilers
The boiler question is really a comparison question.
Almost no one buys a boiler in isolation any more. The honest decision is between the boiler in front of you and the well-designed heat pump you could have instead, with the fabric and emitters priced in.
The default
A new gas boiler.
- Up-front cost
- £2,500 – £4,500installed, existing pipework
- Annual running cost
- £1,300 – £1,900gas at 2026 cap
- Efficiency
- 88 – 94%condensing, when correctly commissioned
- Lifespan
- 12 – 15 years
- Fuel exposure
- Wholesale gas
Cheap to fit, familiar, easy to service; running cost tied to gas prices for the next decade.
The alternative
A well-designed heat pump.
- Up-front cost
- £6,500 – £12,000post £7,500 BUS grant
- Annual running cost
- £900 – £1,400heat-pump tariff, fabric ready
- Efficiency
- SCOP 3.5 – 4.5flow temperature below 45°C
- Lifespan
- 15 – 20 years
- Fuel exposure
- Electricity, decarbonising
Higher up-front, lower running cost, longer life; only if the fabric and emitters are done properly.
The honest question
Which boiler should you fit in 2026?
The short verdict
A right-sized combi for most; a heat pump if you can.
Sense check
Could something simpler solve this?
The existing boiler is failing and a decision has to be made this month.
- 01
Confirm the existing boiler is actually beyond repair
A single failed component on an otherwise sound boiler is usually cheaper to repair than to replace. Worth an honest second opinion before committing.
- 02
Consider whether the decision window is a heat-pump moment
A dying boiler is the natural moment to look seriously at a heat pump. If the fabric is reasonable and there is space for a cylinder, the grant plus the running-cost gap changes the maths.
If you've considered these and a new boiler is still the right answer for your home, here's how to choose well.
Where a heat pump isn't the honest answer today, a well-specified condensing boiler on a low flow temperature is a sensible replacement. The details matter more than the brand.
The decision, in six questions
Show, don't tell.
Six questions that decide combi, system, or something better than either.
Strong candidate
Worth considering
Probably not for your home
- Q01
Is your current boiler dead or actively failing?
YesEmergency mode. Replace like-for-like on the same fuel; plan properly next time.
Worth considering
NoYou have time; use it. The best boiler decision is a planned one.
Strong candidate
- Q02
Do you have one bathroom and decent mains water pressure (>1 bar)?
YesA right-sized combi is almost certainly the answer.
Strong candidate
NoTwo-plus bathrooms or weak mains points to a system boiler with a cylinder.
Worth considering
- Q03
Are you on the gas grid?
YesGas remains the default; the question is only which type and when.
Strong candidate
NoOil or LPG boilers are increasingly hard to justify vs a heat pump.
Probably not for your home
- Q04
Are you being quoted 30kW+ for a 3-bedroom home?
YesAlmost certainly oversized. Ask for the heat loss calculation in writing.
Probably not for your home
NoSensible sizing (24–28kW for most 3-beds) is the standard to insist on.
Strong candidate
- Q05
Could your radiators be upsized in the same visit?
YesLarger emitters make either a modern boiler or a future heat pump work better.
Strong candidate
NoNot fatal, but a missed opportunity if the walls are open anyway.
Worth considering
- Q06
Have you looked at what a heat pump would actually cost with grants?
YesYou are making an informed decision either way.
Strong candidate
NoGet one quote before signing a new gas boiler contract; the £7,500 BUS changes the maths.
Worth considering
What usually changes the answer
What usually changes the answer
Change one of these and our recommendation often changes with it.
Whether the current boiler has failed.
What changes
Whether you are on the gas grid.
What changes
How well the fabric performs.
What changes
Whether the £7,500 Boiler Upgrade Scheme grant is available in your region and property type.
What changes
How long you plan to stay.
What changes
If this were our house
For a mid-terrace with a 15-year-old combi still limping through winter
We would not wait until the boiler fails. We would get one Boiler Upgrade Scheme-eligible heat pump quote and one right-sized combi quote, and compare them side by side with the £7,500 grant applied. That single hour of legwork is what separates a good decision from a Sunday-night panic decision.
If the heat pump numbers stack up and the house is reasonably insulated, we would take that route. If not, we would fit a modest 24kW combi from a mid-market brand (Vaillant, Worcester), pay the extra £150 for weather compensation controls, and put the difference into a loft top-up. In neither case would we accept a 35kW default.
When we'd say no
We probably wouldn't recommend this if...
We would rather lose the sale than take you somewhere the numbers don't stand up.
You are being sold a 30kW+ combi for a 3-bed with one bathroom. That is the lazy default; the running cost of an oversized boiler compounds for fifteen years.
You are on oil or LPG and being sold a like-for-like replacement without seeing a heat pump quote. The BUS grant changes the maths entirely.
You are being quoted a regular (heat-only) boiler in 2026 without a specific reason. Almost nobody needs one; system or combi covers 95% of homes.
The quote does not include a magnetic filter, a properly-specified expansion vessel, and power flush. Skipping those wrecks the warranty case in year three.
You are being upsold a hydrogen-ready boiler as if it is a real climate hedge. It is a marketing badge; there is no hydrogen network coming to domestic heating.
What most people get wrong
Assumptions that quietly break the answer.
The trade at a glance
What you're actually trading.
A five-step editorial scale, not a rating. Green weight is a gain; grey weight is a cost you carry.
Familiarity & install ease
Very highEvery gas engineer in Britain can fit one; parts are everywhere.
Upfront cost
Low£2,500–£3,800 fitted for a standard combi swap. Cheapest heat source to install.
Running cost (2026)
MediumGas remains cheaper per kWh than electricity, but the gap is narrowing every year.
Efficiency ceiling
MediumA modern condensing boiler runs at ~92% AFUE, well tuned; heat pumps hit 300–400%.
Regulatory risk
HighNew gas boilers face phase-out from 2035 in new builds; policy direction is one-way.
Comfort delta vs modern alternatives
MediumRadiators plus boiler is fine; underfloor plus heat pump is measurably better.
What it costs
Illustrative UK ranges, 2026.
Combi replacement (mid-market, fitted)
£2,500 – £3,800
Vaillant, Worcester, Ideal. Includes standard flue, filter, chemicals.
System boiler + unvented cylinder
£3,500 – £5,500
For two-plus bathrooms or weak mains pressure. Cylinder replacement adds cost.
Weather compensation controls (add-on)
£150 – £400
Cuts gas use by 5–15% via lower flow temperatures. Underrated.
Emergency (out-of-hours) replacement
+20–30%
The reason to plan replacement before the boiler dies.
Ranges drawn from Which?, MCS, Boxt and independent installer quotes 2025–2026. Regional variance is significant; London prices sit near the top of each band.
You've got the answer
A right-sized combi is still the pragmatic answer for most gas-connected UK homes. Get a heat-pump quote too, and never buy on a Sunday-night panic.
You can stop here if you're still deciding. The rest of this page is for readers who have already committed and want to spend the money well.
Already going ahead? Everything worth knowing follows.
If you're going ahead
What to know before you spend the money.
This half of the page is written for the reader who has decided. It reads in the order the decisions come at you.
Which version
Which version should I choose?
System boiler + unvented cylinder
Boiler feeds a pressurised hot-water cylinder. Handles multiple simultaneous showers, works with weak mains, and is the pairing for solar thermal.
When we'd choose it
Two or more bathrooms in use at once, or weak mains pressure that would leave a combi struggling.
Heat pump (as an alternative)
Not a boiler at all, but the fair comparison in 2026. Grants and running costs shift the picture materially in the right house.
When we'd choose it
Reasonable insulation, space for the outdoor unit, and appetite to right-size emitters at the same time.
Your house
How will it affect my house?
Where the boiler lives.
Combi boilers usually keep the same wall bracket; system boilers plus cylinder need an airing-cupboard-sized space forever. Flue routing matters more than most homeowners realise; a long horizontal flue through a room you use is a lifetime of low-level noise.
The pipework you already have.
Old microbore pipework limits what a new boiler can deliver, and often gets quietly ignored in the quote. A power flush cleans the existing system; a magnetic filter added on installation protects the new boiler from the debris the flush didn't get.
In use
How well will it actually work?
Weather compensation is the £200 upgrade nobody sells you.
A weather compensation control varies the boiler's flow temperature with outdoor conditions, so the boiler condenses more of the time and cycles less. Real-world gas reductions in the range of five to fifteen per cent are well-documented, and the payback is measured in a couple of heating seasons.
The 2035 question, honestly.
The Future Homes Standard bans new gas boilers in new builds from 2035. It does not, yet, ban replacements in existing homes. But the policy direction is one-way; gas gets more expensive to run and eventually harder to service. A boiler fitted in 2026 is a commitment through roughly 2040, which is fine provided you make it with your eyes open rather than because the old one died on a Sunday.
Installation
How is it installed?
Day 1 (morning)
Old boiler and controls removed; system drained; existing pipework assessed and adjusted.
Day 1 (afternoon)
New boiler mounted; gas and flue routed; magnetic filter and chemicals added; system pressurised.
Day 2
Power flush if the old system was sludgy; new controls fitted and paired; commissioning to manufacturer spec.
Handover
Benchmark logbook completed; warranty registered; controls demonstrated; gas safety certificate issued.
Common mistakes
What usually goes wrong?
- 01
A boiler oversized 'to be safe' cycles constantly and condenses rarely; it uses more gas than a smaller one would.
- 02
No magnetic filter fitted, so old debris circulates into the new heat exchanger and shortens its life.
- 03
No weather compensation control, so the boiler runs at a single flow temperature all year.
- 04
A flue routed through an occupied room produces a persistent whine you cannot unhear.
- 05
Warranty registration missed at handover; the manufacturer voids cover in year six when the pump fails.
The installer
How do I choose a good installer?
The gap between the best and worst installer on this list of trades is larger than the gap between products. Judge the person, not the brochure.
Questions worth asking
- Q01Can I see the heat-loss calculation you're sizing the boiler from?
- Q02What kilowatt rating are you specifying, and why that number?
- Q03Are you fitting a magnetic filter as standard?
- Q04Is the system being power-flushed before the new boiler goes on?
- Q05What controls are included; weather compensation, load compensation, or a smart thermostat?
- Q06How long is the manufacturer's warranty when you install it?
- Q07Are you Gas Safe registered, and can I see the ID?
- Q08What's the schedule from deposit to commissioning?
- Q09Who do I call for a callback in the first two years?
Red flags
- Boiler size chosen from a rule of thumb per bedroom, not a heat-loss calc.
- Default 30kW+ specification for a typical 3-bed.
- Weather compensation dismissed as unnecessary.
Maintenance
How do I look after it?
Annual gas-safe service, filter clean each service, condensate trap check every winter. A boiler serviced on schedule runs its full design life; one that isn't rarely reaches year ten.
Real questions
Things people actually ask.
- What size boiler for a 3-bed house?
- For most UK 3-bed homes with one bathroom, 24–28kW is correct. Anything over 30kW is almost certainly oversized.
- Should I fit a boiler or a heat pump?
- A right-sized boiler is the pragmatic answer for most gas-grid homes today. Get one heat pump quote with the grant applied before you sign.
- Will gas boilers be banned?
- New gas boilers in new builds face a 2035 ban. Replacements in existing homes remain legal for the foreseeable future.
- Is weather compensation worth it?
- Yes. The upgrade is a couple of hundred pounds and cuts gas use meaningfully; payback is well inside the boiler's life.
- Do I need a power flush?
- If the old system was sludgy, yes; refusal often voids the new boiler's warranty. On a clean system, a magnetic filter alone may suffice.
House Summary
A right-sized combi remains the pragmatic answer for most gas-connected UK homes; a system boiler for two-plus bathrooms; a heat pump if the fabric and budget support it. What we care most about is that the decision was made on numbers rather than on Sunday-night panic.
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