Oil fired central heating - pressure problems

[suealan1]

I have an oil fired system which loses pressure but only when it is OFF for a while, when we are away for a few days. Various bits (RPV, pressure tank, valves) have been replaced but the problem persists. We have turned off the underfloor heating section which seems to make no difference, so it seems the problem is not there. The rest of the heating system can't be isolated (or so I am told) so we haven't tried that.

Any thoughts as to how I might address this problem?

 

[twgas]

You "turned off" the underfloor, switched it off or isolated the flow and return(s)?
I dont believe the rest of the system cant be isolated! Although it might take some effort and maybe some building damage but if you have a hidden leak the damage will, in the long term, be much worse.

 

[suealan1]

Thanks for your reply. You're right - isolated, but only the flow. I know it's difficult to believe the rest of the system can't be isolated but the whole thing was clearly a diy job by my predecessor, whose expertise was in flooring.

I was hoping for a suggestion as to where to start looking given that we lose pressure only when the system is COLD.

 

[oilhead]

If you isolate only the flow, then you are isolating nothing as pressure will reach all points through the return. Examine every radiator valve, every joint and pay particular attention to any auto air vents. If you do this whilst everything is cold, you will stand more chance of seeing it.

 

[oilboffin]

It may pay to check the boiler drain off cocks and air vents also check the prv I've just found one leaking on a boiler and replaced it and got no problems any more

 

[suealan1]

Thanks to both. I understand the logic of pressure being maintained, even without the flow. We have checked every radiator (only 3) and every visible joint - but not when cold. We'll do that.

Yes, we have checked and indeed changed the pressure vessel and the prv and this has had no effect.