I have the above boiler and it's been OK for the last 7 years or so...but yesterday it developed a central heating fault it's losing pressure and the water is discharging out of the overflow, so more or less as soon as I bring the pressure up to 1 bar within a few minutes it's dripping out of the overflow even when the boiler is cold or off, the DHW is OK no water from the overflow when running hot water, so I am thinking that the central heating safety valve is faulty, or could it be other faults?

If this is the problem, is it expensive to replace the valve, approximately what should I be charged by a gas fitter and/or could a plumber do the job, is it an easy job to do, by the way the boiler is upstairs so just above the highest radiator so would the whole system need to be drained?

I had the boiler serviced last year at the same time as the living room gas fire, but the fitter I used as since moved out of the area...and I would like some idea of the fault before I call anyone I don't know out. I don't use the CH much because I have a skin condition that is made worse by heat and more so by central heating, but because we had 12 inches of snow that froze I have used the CH more, the funny thing is that this only happened after the thaw not when the CH was on most of the day.

Could be a reason why your safety PRV is opening - it could be that it has failed... but you could have a blockage - frozen pipes, or CH supply solenoid valve stuck in the closed position... plus quite a few other possibilities. You really need to get an engineer to check it out
A plumber will be able to change out the PRV - if it is still leaking when you fill-up cold, then it is a good place to start... better put a "but" here... overpressure more than likely caused the PRV to open... so, as above, get an engineer.f

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