don't fit check valves fit isolation valves.
a check valve is to prevent backflow it doesn't isolate.
turn off the boiler incase someone turns on the hot tap and you have the mains water off.
hi-spec plumbing, June 2007
From what I've been told about combi boilers NO you don't have to touch the boiler. Turning off the mains should shut down the boiler anyway as they work under mains water pressure.
So just turn off the mains water and do what you have to with the toilet. If you're worried turn the boiler elecs off.
Anything you replace it's a good idea to put check valves in the feed pipes so each part can be turned off individually.
