Pressure loss only when boiler shut down

[Bradfield2010]

Hi there,
I am trying to help my sister with a problem that is baffling me. She has a boiler that heats a swimming pool and three small radiators. The pool heats via two small heat exchangers. It is a sealed system with Expansion Vessel, Air Release Valve and Emergency Pressure Relief Valve. The boiler runs nicely and when left on permanently for days it cuts in and out correctly and the system pressure stays rock solid. However when you shut down the boiler and therefore the pump, the system pressure goes from 1.5 bar to zero in a matter of hours and the system needs refilling before starting up again. I have checked the air and pressure relief valves and both are fine (no leaks). I am assuming the expansion vessel is working as once I refill the system and fire up again the pressure stays fairly constant. Have checked the boiler heat exchanger and cant see any sign of leaking. I cant find a leak anywhere. The two pool heat exchangers look simple enough, hot feed in and out and pool flow in and out. Has anyone any idea where the leak could be such that it doesn't leak under pressure and heat but shut it down and !!!!!
Any comments would be very greatly appreciated
Many Thanks
Bradfield

 

[crockett]

Hi
Fittings of differing materials will contract and expand under heat and cooling down at different rates, this is more than likely the case with your issue. As for where it is, lords knows!!