Combi Boiler Loses Pressure only when Off

[ripley500]

My combi boiler works fine when running, however when it's left off for a while such as overnight, it loses all water pressure.
- I've checked and replaced the PRV and confirm that no water is lost at the PRV
- I've installed a new Expansion Vessel and confirmed this is working ok
- I've replaced the air valve
- I've isolated the CH flow and return valves overnight and the boiler still loses pressure - this indicates any leak is in the boiler but there is no evidence of water anywhere in the boiler, including the main heat exchanger
- There is no visible signs of any pipe leaks and all radiators and hot water works fine.

Water pressure is being lost somewhere but I cannot seem to fathom just where and yet the boiler is fine when in use.

Given that pressure is lost even when isolating the CH flow & return, my assumption is that one or both of these valves must not be closing fully and hence letting pressure out through the CH pipework.

Any ideas or suggestions for things I may have missed are appreciated.

 

[terryplumb]

Does water constantly drip from condensate pipework when boiler is not in use ?

 

[ripley500]

Does water constantly drip from condensate pipework when boiler is not in use ?

So the boiler is a Vokera Linea 28 - according to the manual, there should be a condensate trap/pipe installed, however, I can't see where this is or indeed if one is actually installed.

 

[terryplumb]

Look for a white plastic pipe connected to the boiler ,or show us pics of pipework entering boiler.

 

[ripley500]

Look for a white plastic pipe connected to the boiler ,or show us pics of pipework entering boiler.

The only white plastic pipe appears to be on the first pic and goes into the area of the Gas valve

 

[terryplumb]

That's a non condensing version. How old is that boiler ?
Are any radiator pipes buried in concrete flooring or in underfloor voids ?

 

[ripley500]

That's a non condensing version. How old is that boiler ?
Are any radiator pipes buried in concrete flooring or in underfloor voids ?

Thanks - it's about 20 years old - there are radiator pipes under the floor voids, however, pressure is lost even when I isolate the flow and return CH valves. This is why I'm suspecting that one or both of these valves is not sealing and hence even when the boiler is isolated, pressure is being lost through radiator pipes overnight.

That said, when the boiler is running, no pressure is lost - when boiler is off, pressure drops (from 2 bar to 0 bar within an hour), so any leak must be quite significant and I'd therefore expect to lose pressure even when the boiler is running but it doesn't.

 

[terryplumb]

Pressure would be lost when boiler is isolated,as the loss is on the system side. If it's under flooring ,is there a deep void below ?
Any access below ?
It's probably losing water when heating is on too, but the system water expansion is hiding the fact.

 

[ripley500]

Pressure would be lost when boiler is isolated,as the loss is on the system side. If it's under flooring ,is there a deep void below ?
Any access below ?
It's probably losing water when heating is on too, but the system water expansion is hiding the fact.

It's a fairly modern house, built around 20-25 years ago. About half the floors downstairs are tiled and the rest floorboards & carpets so some would be easy enough to pull up and take a look. The tiled floors will obviously be a nightmare to do.

What do you mean when you say the 'loss is on the system side'? I assume if the boiler flow & return are isolated then the pressure loss must be in the boiler and not the radiator pipes (assuming the isolation valves are doing their job correctly).

 

[terryplumb]

Sorry ,to be clear ,if the boiler isolation valves don't actually fully close ( not unheard of on a 20 year old boiler ) then the boilers pressure gauge will record any loss on the system ( pipework/ radiators/ valves etc).

 

[ripley500]

Sorry ,to be clear ,if the boiler isolation valves don't actually fully close ( not unheard of on a 20 year old boiler ) then the boilers pressure gauge will record any loss on the system ( pipework/ radiators/ valves etc).

Many thanks, makes sense.
Your help is appreciated