Combi vs system boiler: which suits your home?
The short answer
- Combi: smaller household, one bathroom, good water pressure, no cylinder
- System: bigger household, hot water to several taps at once, heat pump ready
- £1,800 to £3,500 for a like-for-like swap of either; £3,000 to £5,700 to convert to a combi
A combi suits a smaller household with one bathroom and good water pressure; a system boiler suits a bigger household that uses hot water in several places at once, and keeps the cylinder a heat pump would need. A like-for-like swap of either typically costs £1,800 to £3,500 installed.
Which suits which home
A combi for one bathroom and good pressure; a system boiler for bigger households; a regular boiler for low pressure with loft tanks.
| Your home | Usually suits | Why |
|---|---|---|
| Flat or small home, one bathroom | Combi | No cylinder or tanks to find room for; hot water on demand |
| Family home, more than one bathroom | System | Stored hot water can run to several taps at once |
| Low mains water pressure | Regular (heat-only) or system | A combi depends on mains pressure; a regular boiler suits low pressure |
| Planning a heat pump later | System | Most heat pumps work with a hot water cylinder |
| Solar water heating, or solar panels heating water | System | A cylinder can store the sun's energy as hot water |
| Existing regular boiler with loft tanks, working well | Regular (heat-only) | A like-for-like swap avoids the cost of a conversion |
Read the detail
A combi suits a smaller household with one bathroom and good water pressure; a system boiler suits a bigger household using hot water in several places at once; a regular (heat-only) boiler suits a home with low water pressure that already has loft tanks. That is the Energy Saving Trust's advice, summed up below.
From the Energy Saving Trust's guide to boiler types. Every home is different, so ask the installer to check your water pressure and hot water use before choosing.
Combi boilers
Hot water on demand, no cylinder or tanks, often the least expensive kind.
Read the detail
A combi heats water straight from the mains when you turn on a tap, so there is no hot water cylinder and no tanks. That saves space, and the Energy Saving Trust says combis are often the least expensive kind of boiler. Heat and hot water come on demand, which makes them an efficient choice for smaller households.
The Trust says combis are less suitable for houses with several bathrooms or high hot water use, because on-demand hot water works less well when it is wanted in several places at once, and for houses with low water pressure.
System boilers
A cylinder stores hot water for several taps at once, and suits a heat pump later.
Read the detail
A system boiler heats water into a separate hot water cylinder. The Trust says that makes it more efficient than a combi at producing hot water, though some heat is lost from the cylinder over time. Being able to store hot water makes it a good choice for larger families, who can draw it to several taps at the same time.
A cylinder can also store energy from solar panels, and it is what most heat pumps use, so a system boiler keeps the door open to a heat pump later.
Regular (heat-only) boilers
A cylinder plus two loft tanks: suits low water pressure, not flats.
Read the detail
A regular boiler, also called a heat-only or conventional boiler, works like a system boiler but also has two header tanks for cold water, usually in the loft. It tends to suit larger homes or homes with low water pressure.
The drawbacks, according to the Trust: you need room for the tanks, your hot water pressure may be much lower than your cold, and it is unlikely to suit a flat, because the tanks have to sit higher than the highest hot tap or radiator.
Hot water demand decides it
How much hot water you use at once. Space and heat pump plans come next.
Read the detail
How much hot water you use at once is the deciding question. One shower at a time suits a combi; two showers running together, or a bath filling while someone washes up, favours stored hot water from a cylinder.
Space comes next: a combi needs no cylinder cupboard, which matters in a flat. Then the future: if a heat pump is likely when this boiler wears out, keeping the cylinder now saves putting one back later.
The type also sets the size: a combi is rated higher than a system boiler for the same home, because it heats water on demand. See what size boiler you need.
What each costs
£1,800 to £3,500 for a like-for-like combi or system swap, £3,000 to £5,700 to convert to a combi.
| Job | Typical installed cost |
|---|---|
| Combi, like-for-like swap | £1,800 to £3,500 |
| System, like-for-like swap | £1,800 to £3,500 |
| Regular (heat-only), like-for-like swap | £1,800 to £3,000 |
| Regular or system converted to a combi | £3,000 to £5,700 |
Read the detail
A like-for-like swap costs about the same for each type: £1,800 to £3,500 for a combi or a system boiler and £1,800 to £3,000 for a regular boiler, in the quotes of the online installer Heatable from January to June 2026. Converting a regular or system boiler to a combi costs £3,000 to £5,700, for the new pipework and taking out the tanks or cylinder.
Typical installed prices from 2,000 Heatable quotes, January to June 2026: one firm's prices, so a local installer may quote more or less. See our new boiler cost guide for the national averages.
Unless the household's hot water needs have changed, a like-for-like swap is the cheaper choice. Our new boiler cost calculator prices a swap or a conversion for your home.
Frequently asked questions
Is a combi or system boiler better?
A combi for a smaller household with one bathroom and good water pressure; a system boiler for a bigger household that uses hot water in several places at once, according to the Energy Saving Trust.
Is a system boiler more efficient than a combi?
At producing hot water, yes, the Energy Saving Trust says, but some heat is lost from the cylinder over time. A combi heats only the water you use.
How many bathrooms can a combi boiler handle?
Combis work best with one bathroom. The Energy Saving Trust says they are less suitable for houses with several bathrooms or high hot water use, because hot water on demand struggles when it is wanted in several places at once.
How much does it cost to change from a system to a combi boiler?
About £3,000 to £5,700 to convert a regular or system boiler to a combi, against £1,800 to £3,500 for a like-for-like combi swap.
Do I need a system boiler for a heat pump?
You do not need the boiler, but most heat pumps work with a hot water cylinder, so keeping the cylinder that a system boiler uses saves putting one back later.
Data sources
The Energy Saving Trust for the advice and Heatable quotes for prices.
Show every source
- Energy Saving Trust, Boiler types explained (last updated 20 May 2026): yearly savings for a gas heated home fitting an A-rated condensing boiler with a programmer, room thermostat and thermostatic radiator valves in place of an older boiler, by the old boiler's rating, at July 2026 fuel prices; £3,500 typical cost of replacing a G-rated gas boiler. Checked 5 October 2026.
- Heatable, New boiler installation costs 2026 (updated 25 September 2026): typical installed prices from 2,000 of its own quotes, January to June 2026. Heatable is an online boiler installer, so these are one firm's prices. Checked 5 October 2026.