Boiler types explained
What each type of boiler does, what it needs in the property, and which one suits your home. Written for the questions our engineers get asked on the doorstep.
The short answer
Three gas boiler types cover almost every UK home: a combi, which heats water on demand and needs no cylinder; a system boiler, which heats a cylinder and suits homes with more than one bathroom; and a regular boiler, the traditional setup with tanks in the loft. Electric boilers cover properties with no gas supply, and back boilers are the obsolete units still found behind fireplaces. Every new gas boiler sold in the UK is a condensing boiler, so condensing is not a separate choice you make.
Which type is right depends less on the property size than on two things: how many bathrooms are used at the same time, and how strong the incoming cold water main is. Everything else follows from those two answers.
The five types you will come across
Combi boiler
Most common in the UK
Heats water straight from the mains the moment you open a tap, so nothing is stored.
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System boiler
Best for two bathrooms
Heats a cylinder of stored hot water, so several outlets can run at once.
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Regular boiler
Traditional open-vented
An open-vented system fed by gravity from tanks in the loft.
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Electric boiler
No gas, no flue
A heating element in place of a burner, wired to its own circuit.
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Back boiler
Obsolete — replace
A boiler built into the chimney breast behind a gas fire.
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Side by side
The five differences that decide the choice in practice.
Type
Hot water
Cylinder
Loft tanks
Typical property
Combi
On demand
No
No
Flat or one-bathroom house
System
Stored
Yes, unvented
No
House with two or more bathrooms
Regular
Stored
Yes, vented
Yes, two
Older house, weak mains
Electric
Either
Optional
No
Flat with no gas supply
Back boiler
Stored
Yes, vented
Yes, two
1960s–1990s house, due replacement
Which one suits your property
One bathroom, one shower, decent water pressure, and a cupboard you would rather have back.
Combi boilerTwo bathrooms, showers overlapping in the morning, and space for a cylinder.
System boilerAn older house with original pipework, tanks already in the loft, and a weak or shared incoming main.
Regular boilerNo gas supply, or no position where a flue could legally terminate.
Electric boilerWe measure the incoming mains flow and check the gas run on the survey, then set out the options with a price against each.
New boiler installationCommon questions
Not sure which you have?
Send a photo of the boiler and the cupboard and we will tell you what is fitted, what it is worth spending on it, and what a replacement would look like.