Pressure problem on Potterton Ultra 2 boiler

2 Apr 2004
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United Kingdom
I have a potterton Ultra 2 boiler which seems to have a mind of it's own.

I used to get a slight leak inside the boiler - close to the pressure relief valve. It only happened when the outside temperature fell below zero. The leak was only about a saucerful. Unfortunately most days there was nothing to see by the way of a leak so I couldn't show anything to the plumber. One day last summer, when the heating had been off for a few days, I found a very slight leak outside the boiler at the isolation valve on the boiler flow pipe. I sealed this with a little silastic.

Since then the leak inside the boiler disappeared. The boiler pressure increased. It now rises to 3 bar when the boiler is on and drops to 2 bar when the boiler is off.

A few weeks ago I noticed there were a few drips outside coming from the safety valve discharge pipe. This only happened when the boiler came on and the pressure rose above 3 bar. Once the boiler pressure dropped to 2.8 bar the leak stopped,

I thought I would reduce the pressure by bleeding a radiator when the heating was off. When I bled the radiator I could see the pressure drop by about 0.2 bar. Then lo and behold after a couple of minutes the pressure rose back to what it had been before - without me repressurising the system.

I then thought I'd try another approach - I turned the pressure relief valve just a touch anti clockwise. I could hear the air escape. The pressure dropped by 0.5 bar.

Fine I thought only to discover 5 minutes later the pressure was back up to what it was - this was with the boiler OFF.

undeterred I reduced the pressure again by about 1 bar. I felt happier until I went outside and found that there was now a constant drip from the safety valve discharge pipe.

The boiler is 6 years old I assume the pressure relief valve is faulty. The overflow pipe has been dripping constantly for the past 2 days with no drop in pressure. It now sits at 1.6 when off and rises to 2.1 when on.

My question is how come the pressure stays up without me repressurising or topping up the system.

The sealed watertank is in a cupboard directly above the boiler. Several on the estate where I live have been known to explode

Any thoughts or comments greatly appreciated.
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