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 refil 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 doesnt leak under presure and heat but shut it down and !!!!!
Any comments would be very greatly appreciated
Many Thanks
Bradfield

 

[WHPES]

Have you checked the air in the expansion vessel? If there isn't enough , once it is up to pressure it will discharge any expansion of water and when it cools down the pressure will drop very low. You fill them with a bike pump (refer to manufacturers instructions).

Otherwise (unlikely but not impossible) a leak could be somewhere where a joint contracts as it cools and lets the water leak out.

Isolate different sections and use a pressure test pump on the circuit to narrow it down.

 

[wnjswozo]

expansion vessal/recharge it,put some LS1 in system

 

[oldplumber]

can afford to heat a pool, can afford to buy a boiler/call in a plumber!

 

[newbie1]

well said

 

[Bradfield2010]

Thank you WHPES, will investigate expansion vessel (although it sits on a self above the boiler and I haven't noticed and leaks from it). Will let you know. Best Regards B

Oldplumber & Newbie1, many thanks will see if the butler can locate a plumber!

Thanks wnjswozo, it sounds like pressure vessel is where to look.

 

[Winston]

The pressure will drop when the boiler is off as the water is cooling down, and then increase when boiler is back on due to the expsansion of the water.

Oops not to zero though you have a leak

 

[oldplumber]

"Oldplumber & Newbie1, many thanks will see if the butler can locate a plumber!"


Good answer - down in flames

 

[WHPES]

If there is a problem with the air leaking out of the expansion vessel, when the water heats up, there is nowhere for it to expand so expended water will be discharged, keeping the pressure at 1.5 bar or whatever. When the boiler goes off, the water cools down and as you don't have any air in the expansion vessel the pressure will drop off compltetly to zero and the boiler won't be able to fire again until you have topped up.