DIY Doctor

Leak in Hot Water System but Only When Cold?

Postby adriandonlan » Fri Mar 08, 2019 3:34 pm

Hi All
My hot water system keeps losing pressure overnight. It is fine when the pump and system is running but once off the pressure will drop, so at the moment it is continually on. My system has a separate hot water system so its not the same as the central heating, all of the boilers are located in an annexe and so the pipes run underground for about 30 meters. The boiler is an oil fired Grant boiler that was replaced about 6 months ago. I've had plumbers out but no one can find a leak. I tracked down the original plumber who installed the system about 10 years ago for the previous owners and he told me that they had expansion joints in the pipes. I've dug up the patio and found the trench with the pipes but cannot find the expansion joints or evidence of a leak anywhere. He also suggested changing the expansion vessel and although it seemed OK to me I changed it to a new one to eliminate it. Does anyone have any idea of things I could try or what it could be before I call out a leak detection specialist?
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Simply Build It

Postby plumbbob » Mon Mar 11, 2019 9:26 pm

When the system is cold, pressurise it to exactly one bar. Turn the rad temp up high, draw up a chair and fire the heating up. Watch the pressure constantly as the system warms up. It might rise slightly to 1.5 bar possibly, but my guess it will increase to 2,3cor even more.

When the system is up to temp, check the pressure relief valve outlet and ensure it is dry.

Has anyone checked the expansion vessel size, and what pressure is it primed to?
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