Boiler installation in London

A combi heats water on demand and needs no cylinder, which is why it suits flats and smaller houses where space is tight and one bathroom is the norm. It is limited by flow rate: run two showers and both suffer.

A system boiler with an unvented cylinder handles simultaneous demand far better, which is what makes it the sensible answer in a three or four bedroom house with more than one bathroom. It needs cylinder space and it needs someone qualified to work on unvented systems. A regular boiler with a tank in the loft still has a place in older properties with poor mains pressure, where a mains-fed unvented cylinder would never perform.

A new boiler on a dirty system is a warranty claim waiting to happen. Iron oxide from years of corrosion ends up in the new heat exchanger, and manufacturers know it, which is why they write cleaning requirements into their terms.

BS 7593:2019 sets out what that means in practice: clean the system, dose it with inhibitor, fit a filter, and check the inhibitor annually afterwards. The standard recommends a filter on every new boiler installation. If the existing system is badly sludged we will say whether it needs a full power flush before the new boiler goes on, rather than discovering it later.

A boiler replacement is notifiable work under Building Regulations Part L. A Gas Safe registered installer notifies Building Control on your behalf, and you get the certificate confirming it. Without that notification the installation is not compliant, which matters when you come to sell.

The Benchmark commissioning checklist is filled in and signed at handover. That document plus your annual service records is what a manufacturer will ask for if you ever make a warranty claim, so it is worth keeping somewhere you can find it.