Facts last checked 17 September 2026
The short answer
A combi boiler heats water only when you turn on a tap, so it needs no cylinder or loft tanks and suits most homes with one bathroom. A system boiler stores hot water in a cylinder, which suits homes where two showers often run at once. A regular (heat-only) boiler also uses a cylinder plus cold water tanks in the loft, and mainly makes sense where those tanks already exist or mains pressure is poor. If you might want a heat pump later, think twice before removing a cylinder, because most heat pumps need one.
What is the actual difference between the three boiler types?
All three burn gas to heat water for your radiators. The difference is how they give you hot water at the taps.
| Combi | System | Regular (heat-only) | |
|---|---|---|---|
| Hot water | Heated on demand, straight from the boiler | Heated and stored in a cylinder | Heated and stored in a cylinder |
| Hot water cylinder | No | Yes | Yes |
| Cold water tanks in the loft | No | No | Yes, usually two (a storage tank and a small feed-and-expansion tank) |
| Space needed | Least: just the boiler | Boiler plus an airing-cupboard-sized cylinder | Boiler, cylinder and loft tanks |
| Several taps at once | Weaker: flow is shared | Good: draws from stored water | Good, but pressure depends on tank height |
| Works with a future heat pump cylinder | You would need to add one | Cylinder space already exists | Cylinder space already exists |
Combis are now the normal choice in England. The English Housing Survey found that 63% of homes had a condensing combi boiler in 2024, and a further 19% had a condensing system or regular boiler. A condensing boiler is one that recovers heat from its own exhaust gases, and condensing models have been required for new and replacement installs since the mid-2000s.
When is a combi boiler the right choice?
A combi suits you when hot water use rarely overlaps. Picture a two or three bedroom home with one bathroom: someone showers, the kettle boils, the dishwasher runs later. A combi handles that well, frees up the airing cupboard and loses no heat from a stored tank.
- One bathroom, or two that are rarely used at the same moment.
- Good mains water pressure and flow. The Energy Saving Trust notes combis are less suitable where pressure is low.
- You want the cupboard or loft space back.
- You are not planning a heat pump in the next few years, or you accept that a cylinder would need to go back in.
Two rules apply to every new combi in an existing home in England. Under Approved Document L, the boiler must reach at least 92% efficiency on the ErP scale (the EU-derived energy label test), and it must come with at least one extra efficiency measure: weather compensation, load compensation, flue gas heat recovery, or a smart thermostat with automation and optimisation. Any quote that leaves all four out is not compliant.
When does a system boiler make more sense?
A system boiler keeps a cylinder of hot water ready. When two people shower at once, both draw from that store instead of splitting one boiler's output. That is why the Energy Saving Trust describes system boilers as a good choice for larger families with higher hot water use.
- Two or more bathrooms that are regularly used together, such as a busy morning in a four-bed family home.
- Mains pressure is good, so the cylinder can be fed directly (an unvented cylinder) without loft tanks.
- You have, or might add, solar panels. A cylinder lets you store surplus solar electricity as hot water.
- You are keeping the door open to a heat pump, which works with a cylinder.
The trade-off is standing heat loss: a stored tank slowly cools between uses, and you give up the cupboard space. For a household that uses a lot of hot water at once, that trade is usually worth it.
Is a regular boiler ever the best option in 2026?
Sometimes, yes. A regular boiler relies on gravity from tanks in the loft, so it can suit homes with weak mains pressure, and older systems whose radiators and pipework were designed around an open-vented setup. If yours works and you like it, replacing like-for-like is the least disruptive job.
It is rarely right for a flat, because the tanks have to sit higher than the highest tap or radiator. And hot water pressure at the taps can end up noticeably weaker than your cold supply. If those two things bother you and your mains pressure is decent, a system boiler with an unvented cylinder keeps your stored hot water and loses the loft tanks.
How do you decide for your own home?
- Count how often two hot taps or showers genuinely run at the same time. Rarely points to a combi. Daily points to stored hot water.
- Check your mains pressure and flow with an engineer before choosing a combi. A combi can only deliver what the incoming main supplies.
- Decide whether you care about the space a cylinder takes up.
- Ask yourself whether a heat pump is likely within the life of this boiler. The government's Heat and Buildings Strategy puts a gas boiler's lifetime at around 15 years, so today's choice shapes what is easy next.
- Get a survey. The right size and type come from your home's heat loss and hot water use, not from bedroom count alone.
If you are weighing that last question seriously, read heat pumps in older British homes and try the BUS eligibility checker before you commit to removing a cylinder.
What does switching boiler type cost compared with like-for-like?
Conversions cost more than straight swaps because pipework, cylinders and tanks change. These are Smart Energie's published fixed amounts on top of the boiler price, which starts at £1,895 for a like-for-like combi, the same wherever you live.
| From your current boiler | To | Added to the price |
|---|---|---|
| Combi | Combi | Included |
| System | System | +£295 |
| Regular | Regular | +£345 |
| Regular | System | +£645 (loft tanks removed) |
| System | Combi | +£795 (cylinder removed, pipework reworked) |
| Regular | Combi | +£995 (cylinder and loft tanks removed) |
| Back boiler | Combi | +£1,150 |
A replacement hot water cylinder, if yours has failed, is not in the boiler price and is quoted separately. Remember that no government grant covers a gas boiler, and the 0% VAT relief on energy-saving materials does not apply to boilers. You can see a fixed price for your home in two minutes with the instant boiler quote, and how we price explains what sits behind the numbers.
Common questions
Which boiler type is most efficient?
All new gas boilers must reach at least 92% ErP efficiency under Approved Document L, so the gap between types is small. Combis avoid the heat a stored cylinder loses, while system boilers heat water more efficiently in bulk. How well the boiler is set up, including its flow temperature and controls, matters more than the type.
Can a combi boiler run two showers at once?
It can, but the flow is shared, so both showers weaken. If that happens every morning in your house, stored hot water from a system or regular boiler will feel much better.
Do I need to keep my loft tanks?
Only with a regular boiler. If your mains pressure is good, a system boiler with an unvented cylinder or a combi removes the tanks. If pressure is poor, the tanks may be doing a useful job.
Is there a grant for a new combi boiler?
Not for a gas boiler in a typical owner-occupied home. The Boiler Upgrade Scheme gives £7,500 towards an air source or ground source heat pump instead. See the Boiler Upgrade Scheme explained.
Will a new boiler be banned soon?
No. Replacing a gas boiler in an existing home is legal, and nobody has to remove a working one. The detail is in what has actually been announced about gas boilers.