I have a worcester bosch combi boiler BG C1 4731153 [NG] the pressure keeps rising even if the heating isnt on causing it to leak water from the outside overflow The only way to reduce the pressure is to keep venting it from the pressure relief valve or turning the mains water off if I leave it for any length of time. Just wondered if any one else heard of this problem . Also not sure what the WB model No is .

Mr S, March 2009

OK, I've found it. A silvery connection between a cold pipe and a warmer pipe going into the boiler. (This is with the boiler on and functioning hence it being warm).
Am I right in thinking I have to unscrew the large 'wing nut' type taps at both ends of the flexible piping to allow the water to flow through in order to top up the boiler?
Also, do I have to switch the boiler completely off to do this or just the heating (making sure we don't turn on the hot tap as well).
I'd appreciate any tips!

karina, March 2009

Not an answer I'm afraid but I've the opposite problem. My pressure is low. I suspect the 'filling loop' - but should that be something pretty obvious? Are there any pictures online as we inherited our boiler and the plumbing in the house is a little 'idiosyncratic' to say the least.

karina, March 2009

Thanks for that guys been told also that it could be the heat exchanger seal which allows CW pressure to feed into central heating side causing it to over pressurise

Mr S, March 2009

If its OK when the water is turned off it is almost certainly the filling loop valve passing, if external remove the braided hose to check, if you have the inbuilt filling loop remove the white key from underneath boiler to check.

Mick A, March 2009

The expansion tank is a likely cause.

Ben, March 2009