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Combi boiler

Checked against its sources: Updated

A combi boiler is a single appliance that runs the central heating and heats hot water as it is drawn, with no storage cylinder or loft tanks.1 Its heating side is a sealed circuit, filled through a filling loop.1

Covers
Combination boilers on gas, LPG or oil, UK. Building rules quoted are for England and are labelled.
You
Read the gauge and display, set temperatures and timers, top up pressure as your own user manual allows.
A professional
Anything inside the case, the gas supply, the flue. A Gas Safe registered engineer for gas and LPG.

1 How a combi makes hot water

A gas burner fires when the heating or a hot tap calls for it. Vaillant describes the heat passing through a main heat exchanger into the primary circuit, then either straight to the radiators or across a second, plate heat exchanger to the taps3. The water at the tap comes straight from the mains, which is why flow is usually good while one outlet is open1.

Many modern combis also keep a small amount of water warm inside the boiler, a pre-heat function, so a tap runs hot sooner3. That small store does not make a combi a storage boiler.

2 Where a combi runs short

Heating water as it flows sets a ceiling on how much it can deliver at once. Vaillant says that on the average combi only one tap or outlet can be used effectively at a time, and that homes with several bathrooms may suit a system boiler or a regular boiler better3. There is no store to run out, but there is also no reserve to draw on when two showers open together.

3 What goes with it

  • A pressure gauge and a filling device, because the heating side is sealed. The right reading differs by model, so see boiler pressure.
  • An expansion vessel, which Vaillant says every combi has4. See expansion vessel.
  • A condensate pipe, because modern combis are condensing boilers5. Outdoor runs can freeze, covered under frozen condensate pipe.

4 Rules that touch combis in England

Fitting a gas combi in an existing home in England brings one extra requirement, a choice of four: flue gas recovery, weather compensation, load compensation or a smart control that automates and optimises6. In hard water areas, Approved Document L also says the hot water circuit of a combi should have treatment to reduce limescale, defining hard water as over 200ppm6. It says treatment, and does not name one product. More under hard water treatment.

Work on a gas combi is only for those allowed by the Gas Safety (Installation and Use) Regulations 1998, which cover England, Wales and Scotland; Northern Ireland has its own version7. The detail is under who can work on a boiler.

Sources

  1. Baxi's glossary read 3 November 2026.
  2. Combi boilers range page, Worcester Bosch and oil boilers page, both read 3 November 2026.
  3. What is a combi boiler, Vaillant read 3 November 2026.
  4. Vaillant on expansion vessels read 3 November 2026.
  5. Is a condensing boiler a combi boiler, Baxi FAQ read 3 November 2026.
  6. Approved Document L Vol 1, 2021 with 2023 amendments, paras 6.2 and 8.8, England read 3 November 2026.
  7. Regulation 3, Gas Safety (Installation and Use) Regulations 1998 and the Northern Ireland 2004 equivalent, read 3 November 2026.