Before the day
Clear access to the boiler, the gas meter and the radiators; the engineer will visit all three. If the boiler lives in a cupboard, empty it. Expect the water and heating to be off for most of the day, so plan showers around it, and if the old boiler is being moved or removed there will be some making good to arrange afterwards.
A proper installation follows a survey, not just a phone quote. If nobody has measured your radiators, asked about bathrooms or looked at the flue position before installation day, the price is guesswork, which is exactly the trap comparing quotes properly is designed to avoid.
The running order
Morning: the system is drained, the old boiler comes off the wall, and any pipework changes are made. A straight swap reuses most of what is there; a conversion from a conventional system is where tanks and cylinders come out, and that is a two-to-three-day job rather than one.
Midday onwards: the new boiler goes up, the flue is fitted and sealed, and the system is refilled with a cleaner and inhibitor. Most installers fit a magnetic filter at this point, and on a quality job the system gets flushed rather than just refilled. Late afternoon: commissioning, where the engineer checks gas rates, tests safety devices, balances radiators, sets the controls, and registers the installation.
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Compare boiler quotesWhat you should be holding when they leave
Three things. The Benchmark commissioning checklist, filled in, which lives with the boiler manual. Notification to Building Control, which the installer arranges and which produces a compliance certificate in the post; a buyer's solicitor will ask for it when you sell. And the manufacturer's warranty registration, which the installer should submit within days because an unregistered boiler often reverts to a short base warranty.
Before the van leaves, have them show you three things: how to repressurise the system, how the controls work, and where the condensate pipe runs, because that is the pipe that freezes in a cold snap. Five minutes then saves a callout in January.
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