How long does a boiler installation take?

A like-for-like combi boiler swap usually takes a single working day (4–6 hours of actual work). A system or regular boiler swap takes 6–8 hours. A full conversion — regular boiler with a loft tank to a new combi — typically takes 1.5 to 2 days. New-build installs in a property with no existing system can take 2–3 days. We confirm the schedule on the quote so you know exactly when to expect the engineer in and out.

What happens on day one of a typical install

Most installs start between 8am and 9am. The first hour is set-up: dust sheets down, water and electric isolated, old boiler drained and disconnected.

By mid-morning the old unit is off the wall and the gas, water and flue connections are being prepped for the new boiler. Pipework changes, magnetic filter installation and flue alterations happen through the middle of the day.

The new boiler goes on the wall after lunch. Electrical connections, system fill, smart thermostat pairing and the chemical flush typically run through the afternoon. Final commissioning and a hand-over to you usually wraps up by 4–5pm.

How long for a like-for-like combi swap

If your existing combi is on the same wall as the new one, with no major flue or gas changes needed, the install is typically a single day — often finished by mid-afternoon.

Heat will be off during the install but back on the same evening. Hot water is back as soon as commissioning completes.

How long for a regular-to-combi conversion

Conversions take longer because the engineer is also draining and removing the hot water cylinder, capping or removing the loft cold tank, running new gas pipework and decommissioning the old cylinder cupboard.

Allow 1.5 to 2 days. Day one usually ends with the new boiler installed but not fully commissioned. Day two is testing, system balancing and clean-up.

Factors that can extend the timeline

Difficult flue routes (e.g. a horizontal flue passing through a bedroom wall), the need for a new gas supply from the meter, scaffolding for high-level external flues, or major pipework changes can add half a day to a day.

In leasehold flats, freeholder permissions and Building Control notifications may need lead time before the install can begin — something we sort during quoting, not on the day.

What you should do to prep

Clear access to the existing boiler and any cylinder or loft tank. Move anything fragile out of the boiler cupboard. If you're heading out, leave a key or set of access details we've agreed in advance.

Expect the heating to be off for most of the day, and hot water off until commissioning is complete. We'll always make sure the heating is back on before we leave.

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