I have a Glow Worm Swift Flow 80 that keeps dropping the pressure down to about 0.5 to 0.7 PSi when the heating in not on. When it's on, the pressure can go up as high as 3 Psi. It all seems to work o.k. but could I have a problem. I've bled the radiators and they seem fine so I'm just wondering what else I can do? Any help would be much appreciated.

Neil Potter, January 2006

hi,
the problem is that when the water in the radiators heats up, it expands. if there is nowhere for it to expand to, then the pressure goes up so high that it discharges outside through the safety valve, then when the system cools down the water contracts again down to a pressure much lower than where it started.
so the fault here is that either there is too much water expanding for the boiler to cope with, or that the method the boiler uses to cope with the expansion is faulty.
in the former case it means you have too many radiators and need to add an expansion vessel into the syatem, or in the latter case you need to replace the expansion vessel in the boiler. in either case you need a heating engineer...

graham, January 2006