if you have checked all rads,your ch drain (for leaks), and youve changed the press relief valve it sounds like your expanssion vessel has lost its charge or is faulty.to check if its the exp vessell.
1.set ch press to 1 bar
2.turn on ch
3.watch pressure needle.is it continually rising? if it steadily rises up to 2 then if it gets to 3 bar the pressure relief valve activates(which it should its a safety valve)your pressure will drop to zero or at least nearly,your exp vessel is at fault.
to recharge the expanssion vessel(check manufacturers instructions to locate they are the single largest part in the boiler .they are usually at the side or back and more often than not are bright red.
on the vessell is an air valve like a bike/car tyre.
1.blow off pressure to zero you can use press relief to do this.
2.attach footpump or similar device to valve on expansion vessel.
3.pump up pressure to approx 1-1.2 bar.(on the foot pump gauge NOT ch press gauge
4.remove pump and replace dust cap.top up pressure and bleed system of air.
5.when free of air set press to 1 bar.
turn on ch and keep eye on pressure.
as it heats up it will still rise but only to about 2 bar itshouldnt go near 3 and lose pressure out prv again .good luck i,m a heating engineer obviously, and 9 times out of 10 this will do the trick.if not i,m afraid you need new expansion vessel
cp, October 2008
