About six months ago a pressure valve at the back of my rayburn cooker started to leak. The rayburn runs on oil and heats the hot water in the house. The plumber told me there was dirt in the valve and replaced it. It continued to leak, he replaced it again, and the leak stopped. My tenants were away over Christmas, the well water system was drained and the heating turned off for approx one month since Dec 31st, to avoid freezing pipes. The water pump was turned on about a week ago, but to my knowledge, the first time the heating system was turned on was today. When I checked, I found that the same pressure valve is leaking again - enough to fill a large jamjar in the space of two hours. Can anyone tell me why this is happening again? I also noticed a small leak in my airing cupboard upstairs. The plumber told me that pressure valves can drip periodically over years but surely this shouldn't be the case, as water leaking anywhere is damaging in a house? Any help appreciated.

Is there an expansion tank on your heater? When cold water fills a heater and is heated it will expand and possibly raise the pressure to the point that the safety valve opens. An expansion tank will absorb this excess psi

Not being familiar with your system my guess is that you have air in the system.

If you drain the heating system pipes, you let air in. After refilling you must bleed off the air. How you do that will depend on just how the system is set up. Air expands more than water when heated, therefore the excessive pressure is causing the T&P valve to release.

Don't know what plumber means about pressure valve in closet leaking. Don't know why you would have any kind of pressure valve on the system. It could be that what he is referring to is bleeder valve. Bleeder valves are installed at high points in the line for the purpose of letting air out of the system. Air will rise to the highest point.

What does this valve look like.

What's a Rayburn cooker?
What's an airing cupboard?
Where are you located?