Hot water pipe to boiler was cracked and fixed but the radiators weren't bled. The boiler and house radiators worked fine apart from the upstairs ones only getting hot at the bottom. I bled the upstairs radiators as normal and re-pressured the boiler and turned the system on. The pressure was fine until the heating was turned off and then as the radiators got cooler the pressure dropped and finally got to 0. Re-pressured the boiler again and left the heating this time and the pressure stayed the same, turned the heating on the everything worked fine until I turned the heating off and the same thing happened with the pressure (as the radiators got cooler the pressure dropped slowly down to 0). I have noticed now as well that the overflow pipe outside for the boiler has a very slow drip.
Your pressure relief valve is weeping and that is one place to lose pressure. Generally when they leak the only cure is replacement.The pressure will drop as the temperature drops, its the system contracting.
The pressure relief valve is failing, and needs replacing. It is the valve that the overflow pipe is connected to. Priced at anything between ?45/65 depending on the type of boiler.
