Replacing radiators on a sealed system

I had a new bolier fitted ove the summer (a Valiant combination one) and had removed some radiators for redecoration. I'm sure the heating engineer said there was a special procedure for replacing these on sealed system amd wondered if amyone could explain what this is. I assume is something to do with keeping the pressure up ?
TIA
Stan
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It is easier with two people. Then you can have one at the radiator, to turn the bleed key off when it is full and one at the filling point, keeping up the pressure.
For the lazy or friendless, you could always install a pressure reducing valve set to 1 -1.5 bar in the filling loop and just leave it on whilst going around the radiators to bleed. Must remember to turn it off and disconnect the hose afterwards, though. Obviously, this is an option that you don't have, unless you fancy some plumbing practice.
Christian.
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