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My boilers optimum working pressure is 1.1 bar but yesterday it was at 0.25. This,so i was told, was due to lack of water in the system which tallied up as one radiator was stone cold for no reason whatsoever. I opened the filler loop and put water in till pressure reached 1.1, everything working great. Today however i noticed a pipe that runs through to the outside was constantly dripping (outside), boiler now reading 3 bar! I bled all the rads but very little air came out so emptied about 1.5l of water from one of the rad valves, boiler back to normal pressure. But 3 hours later its back up to 3bar without the heating being on in that period. Is it possible the lever on the filler loop isnt closing properly and its slowly filling the system and the pipe outside is the overflow? The lever on the filler loop is proper stiff and not smooth at all.
Could be the pressure relief valve is faulty, or could be the expansion tank bladder is ruptured
pressure vessel is bust.. replace with new or fit auxilary.. good practice to change prv as well.. typical combi problem.. no short cuts i m afraid.
As above, pressure vessel. It's a steel tank with an air filled bladder in it, you pump up the bladder with a Schrader type bike pump. As the water heats up, it expands and squashes the bladder. If it bursts, there's nowhere for the water to expand and it gets released by the pressure release valve. When mine failed, I bought a new one from boilerparts.co.uk, it was a relatively simple plumbing job and I didn't need to go near any of the gas (which I wouldn't have). Don't know what yours is like.
BTW you should leave the filling loop disconnected when not in use, then it doesn't matter if there's a small leak in the valve. Otherwise, when your pressure vessel fails, the water can backflow and contaminate your mains rather than out through the PRV.
You were right in thinking that it's the valve on filling loop which is passing, to prove this just disconnect and see what happens to the boiler pressure. If the pressure remains stable and the filling loop is dripping you have solved and just require a new filling valve.