How long does it take to fit a new boiler?

The honest answer depends on how much of the job is boiler and how much is plumbing.

Straight swap: one day

Old combi off the wall, new combi on, same position, same pipework. A competent engineer starts at eight and is testing by mid-afternoon. You will be without heating and hot water for most of the day, and you should have it back the same evening.

Conventional to combi: two to three days

This is a bigger job than people expect. The hot water cylinder comes out, the header tanks in the loft come out, the pipework is reworked, and you often end up with holes to make good in an airing cupboard.

Three days is normal. Anyone quoting you one day for this either has not understood the job or is not planning to do it properly.

Relocating the boiler: two days plus

Moving from the kitchen to the loft or garage means new gas runs, new flue, new condensate drainage. The boiler is the easy part.

What should happen on the day

The engineer should show you their Gas Safe ID card without you asking. They should protect your floors. They should fit a magnetic filter - most manufacturer warranties now require one. They should flush the system, not just drain it.

At the end you should get a Building Regulations compliance certificate (the installer notifies Gas Safe, and you receive it within a couple of weeks), the manufacturer warranty registered in your name, and the old boiler taken away.

The bit people forget

You will have no heating and no hot water on the day. If it is January and you have a baby or an elderly relative in the house, say so when you book, and ask the installer to bring temporary heaters. A good one will.