combi boiler losing pressure

[nic999]

My Worcester 28KW combi boiler is losing pressure. If I fill it up to 1.5bar it will have fallen to zero over 10-12hours.
Called out heating engineer who found expansion tank full of water. He re-pressurised it and cleaned PRV.
Everything worked perfectly for 1 week and now it has started losing pressure again at same rate as before. I am having to top up morning and evening.
There is no water dripping from pipe outside and we’ve had all floorboards up and no sign of a leak inside.
Engineer says it impossible to tell what problem is. Only thing he suggests is to start changing parts, starting with PRV to see if that works but there’s no guarantee that this will sort it.
Does this sound right to people? Surely he should be able to tell more precisely what the problem is. Any suggestions? Should I contact another plumber?
Many thanks.

 

[Burnerman]

He found the EV full of water.....?
Does that mean that he pressed the air valve core in, and water squirted out?
If so, the EV has had it, I'm afraid.....if this isn't what he found though, the water pressure needs to be zero in the system, and then the EV is repressurised with a foot pump or similar up to 10 psi.
John

 

[MANDATE]

I'm with burnerman here! There's a diaphram in the expansion vessel, so there's two ways it could be full of water.
(a)Diaphram may be ruptured allowing water to cross to the 'air' side.
Or
(b)the pressure on the 'air' side has leaked out through the valve, so the diaphram has moved and the vessel is full of water.
If it's (b) the valve may be leaking, so the water at 1.5bar going higher when hot will help push the air out as the diaphram moves.
Volume on air side goes down. Volume on water side goes up.
The effect of this air leak is that whatever pressure is normally reached will no longer be achievable until the lost air is replaced.
The fact the 'air' side has lost pressure means the diaphram does not return to exactly the same position.
When cooling the water contracts and the pressure decreases and due the volume being slightly higher, the pressure will go lower than previous.
When this has taken place a number of times it becomes noticeable and you wonder where the the extra water is going. Well its taking up space in the vessel and it's just a matter of time that the vessel can't take any more. So the PRV opens at 3bar and down goes the pressure. PRV closes, boiler pressure gets adjusted to 1.5bar again and the whole process starts all over again.
I think your problem seems to fit this situation
After pressurising the vessel it's best to fit a steel valve cap

 

[nic999]

Thank you very much for the prompt response.
I think Mandate's scenario B seems to fit. When the engineer found the EV 'full of water' he drained system and recharged it with a foot pump and as I say, everything worked for a week. Now pressure is dropping again I've pressed the valve on the EV and air comes out and I've checked the pressure inside with the footpump and it holding just under 10psi.
Does this mean changing the PRV may work or would you expect to see water dripping from the pipe outside if the PRV is faulty?
Thanks again,
Nick

 

[mikeyd]

PRV may only be leaking when you aren't looking
Hang a bucket / empty paint tin etc. under it to collect anything it throws out then you will know for sure.

 

[MANDATE]

PRV may only be leaking when you aren't looking
Hang a bucket / empty paint tin etc. under it to collect anything it throws out then you will know for sure.

or tie on a plastic bag

 

[nic999]

Sorry I should have mentioned before. I taped a plastic bag over the pipe 2 days ago. So far all it's collected is 2 drops which surely can't be enough to account for the pressure loss.

 

[dextrous]

Think the thing to do is identify whether the water loss is from the boiler or the CH pipework. To do this, pressurise the system and close the flow and return valve taps at the boiler. Leave for a couple of hours and see if the pressure has dropped. If yes, then it's the boiler.

 

[nic999]

that's a good idea. I'll try that. Thanks

 

[MANDATE]

From what I've read on this forum, it would appear that once a PRV has opened it hardly ever fully seals again.
Regarding the expansion vessel, it's essential when charging that there is somewhere for the surplus water to go and the water is not preventing the diaphragm from moving back. The whole operation depends on sufficient air capacity to accomadate the increased volume.
Capping the air valve after charging must be a benefit