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Gas boilers

Back Boilers Explained

Back boilers sit behind a fireplace, often with a gas fire in front. Many still work, but they're becoming harder to maintain.

3 min read · Updated October 2026

Fireplace with a gas fire and the outline of a back boiler hidden behind it

What a back boiler is

A back boiler is installed in the fireplace opening behind a gas fire, using the chimney as a flue. It typically heats a hot water cylinder and radiators, with tanks in the loft. They were common in UK homes for decades but are no longer fitted new.

Keeping one going

A back boiler that's serviced and passes safety checks can be used, but older models are less efficient than modern boilers and parts can be hard to find. Annual servicing by a Gas Safe registered engineer remains important.

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Replacing a back boiler

A replacement usually involves:

  • Decommissioning the back boiler and fire
  • Fitting a new boiler elsewhere, often in the kitchen or a cupboard
  • Running new pipework and a new flue
  • Often converting to a combi and removing tanks
  • Making good the fireplace

Common questions

How long does a back boiler replacement take?

Often two to three days.

Can I keep my fireplace?

Often yes, with a new fire or simply as a feature of the room.

Is a back boiler replacement expensive?

It's more work than a like-for-like swap, so it costs more.

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