Leak in Hot Water System but Only When Cold? All aspects of plumbing questions and answers, help, tips and information

[adriandonlan]

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?

[plumbbob]

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?

[adriandonlan]

If anyone wants to know about this problem, it ended up being a faulty expansion joint (a stainless steel bellows). I ended up having to dig up four places in the patio to find it. This whole process was very frustrating as the original builders should have installed an inspection chamber as I know have to refill lots of holes. Anyway I have purchased a replacement and have fitted it however I can't seem to get a perfect seal and there is a tiny drip between a union and the bellows (http://www.stourflex.co.uk/products/exp ... oints.html). I have used ptfe, then jointing compound and finally jointing flex (which worked the best). There is no leak in the olives on the two union joints. Anyone have any advice? I have thought of whether to completely replace the copper pipe with PEX in the outside run (30m) as this should eliminate the need for expansion joints.