Heating Pressure Loss

I’m losing pressure in my heating system but, confusingly, it seems to mostly happen overnight. I can’t figure out what’s going on. The system is currently pretty butchered because of house work but I can’t find a leak anywhere and I drop from 2 bar to 0.8 bar overnight.

I can only think of two things:
1. It’s a boiler issue that’s internal and the leak occurs when it’s off duty.
2. It’s in the hot water circuit and the pressure drops only when hot water heating occurs (this one doesn’t really make sense as the systems are common on the return)

Any ideas?

I'll ask Stepson Senior (plumber/heating engineer)...

Thanks, Jim! I’m a bit baffled by the symptoms

The response was that it could be the expansion vessel. He says check any vents (probably to exterior) as it could be the pressure relief valve.

Is the system fully bled - you could be seeing a pressure change as any trapped air expands or contracts as it heats/cools?

Cool - will look. Thank him, please (and you too, Jim)

I don’t know if totally bled though I’ve topped it up loads of times thinking that this may be what it is - but, if anything, the problem is getting worse - the reverse to what you’d expect if it was air

Watching. Mine does EXACTLY the same. Have consulted Dr Google and pressure relief valve seems to be high on the list.

Thing I have, is pressure is 1.5 bar when the boiler is operating, then when the system is off and cold it sits at 0.5 bar in the red. Switch it on and it rises to 1.5 when going.

I should only be concerned what the pressure is when it's on, is my line of thought?

This strangely feels like an oil pressure conversation!!

All this does sound like the expansion vessel is broken. It's like the lung of the system keeping everything at a regular pressure if it fails the pressure will rise when hot and the relief valve will blow then when cold it will be under pressure.

Except the pressure never gets very high and hardly fluctuates - 2.4 bar max - which is below the 3 bar gauge “red line” (the boiler’s gauge goes up to 6 bar). I also expect to see steam rising somewhere in this cold weather if the hot water was venting.

I’ll ask the builder’s heating guy to take a look at it when he’s doing the underfloor heating connections.

I chased a similar problem for weeks and didn't find a hint until I chanced to wonder why there were water marks on the coupe... the manifold in the room above the garage had a pinhole leak and the gap below the floorboards had two inches of water sitting in it.

Loss of pressure normally points to a leak somewhere. And shows more when the system is heating and pumps are running.

A small leak will show a lot of pressure loss over 24hours.

Another vote for the expansion ‘bladder’. A friend had the same issue.

Have you checked the pressure relief valve in case its draining into a hidden drain
And normal domestic combi boilers only need to be about .8 bar if you are topping up to 2.4 im not supprised its dropping pressure when it cools down as it will certainly vent as it expands then be less when cold

It’s a big, f-off, Boulter oil-fired boiler that basically cuts out at 1 bar or less. The mid-point of the “green” zone on the pressure gauge is 2 bar.

Anyhow, I think I’ve found the problem - or, more accurately, Mrs MRS did. Visiting the “building site” that serves as my home, she asked Charlie why his bedroom carpet was wet. He said he’d been wondering about that...! Clearly, a first from Cambridge doesn’t mean you have a brain.

I’ve tightened the offending joint and hopefully that’s it solved. The first floor level at Charlie’s part of the house is 8” concrete slab hence no sign below and it was under his desk.

Thanks for the suggestions - as usual, it’s the simple answer - compounded by the chaos that my home currently is. Waiting on windows that, in true Grand Designs style, are months late.

Sounds like a nightmare, but also very exciting!

Kinda got here

I remember those days all too well