There must be a leak but can’t think where

[Anj]

Appreciate the advice...

I have attached a schematic of my heating system. Has a 2018 Worcester boiler with a UVC (but the UVC could be older not sure when that was installed, I do know the previous Viessmann boiler had failed). There a couple of motorised valves for the heating and hot water.

The problem is that the pressure in the system (as read from 2 gauges on the pipe work) keeps falling (overnight 1.5bar to 1.2bar even with the boiler off and isolated).

So I have tried to isolate as best as I can and closed off gate valves to the rads (see picture) downstairs but I can see that given the position of the valves I can’t isolate the upstairs rads (unless I add a gate valve). But I can’t see any water from upstairs (the ceiling would have caved by now).

With the boiler off and isolated and the downstairs rads isolated the only thing that was on last night was the immersion heater in the tank. The heating control was on but could only have moved the motorised valve the boiler was off so nothing else. Was sure the leak was in the rads downstairs and so totally expected the pressure to have stayed at 1.5 as the rads were isolated but no, the pressure still fell.

The expansion tank has air (quick press of the Schrader valve).

None of the PRV drain lines have any water (bone dry).

There are two gauges (no idea why) but they both read the same so not a faulty gauge.

Is it possible the UVC is leaking? Can that ever happen... the mains pressure seems to be OK and I would have thought that the mains pressure (3 bar?) would prevent the heating coil (1.5 bar) from leaking?

Any suggestion for next steps? Should I drain the system and add the extra gate valve to isolate upstairs (even though I can’t believe it is that)?

All the tank/boiler valves are in the garage exposed so no chance of a leak being missed.

Really puzzled. Am hoping those with experience on UVC (I am not a plumber) can advise?

 

[Anj]

That's good news really even though it doesn't say much about the longevity of the EV, I think Zilmet pre charge with nitrogen, also I think every external EV installed should have a isolation/drain valve, (I have seen them advertised somewhere) as it makes it a very simple operation to check the pre charge pressure without system drain down, I wonder how many are checked on the annual service?.

John you are an absolute star.

I didn’t realise there was such a thing as an expansion vessel service valve (I’ve seen loads of external EVs fitted but never seen that valve used). Draining a massive system and losing £30 of inhibitor each time is just painful - just seen one on the Screwfix website... definitely going to add that!!

Thanks even more.

 

[Anj]

John you are an absolute star.

I didn’t realise there was such a thing as an expansion vessel service valve (I’ve seen loads of external EVs fitted but never seen that valve used). Draining a massive system and losing £30 of inhibitor each time is just painful - just seen one on the Screwfix website... definitely going to add that!!

Thanks even more.

to report back.
System has been pressured for 2 days now (All valves Open) and no loss of pressure so it would seem the problem was the expansion vessel. I suppose in hindsight it was obvious if I have isolated everything but the EV and the pressure still dropped. [Still a bit mystified why a PRV hadn’t popped], but misunderstanding of how to properly test an EV threw me for a loop.

Added the service valve and must agree with John.G why this is not fitted as standard is a shame.

I’ve learnt a lot:
1. Definitely only way to check an EV is with no pressure on the water side... pressing the Schrader is only one test and not enough.
2. A 2 year old EV can still fail
3. Had a recap on Boyles law and how to calc the pressures on an EV
4. That there is such a thing as an EV service valve and should always be fitted with an EV
5. Gate valves are rubbish (although mine seem to be OK for now) but I will change them to 1/4 turn ball valves.

Huge thanks to all those responded, particularly John.G

 

[Donametchi]

What boiler do you have?
Any AAVs?
If you leave it for a few days does it continue to drop?

This is the simplest and more accurate remark.
Remember it,s a pressurised system,So once the pressure has been se to 1.5 bar of pressure it should only increase to around the 2bar mark once the heating is on.If you are finding a pressure drop and no wet ceiling marks you can only have a weep on a fitting under the floor so get a head torch and wriggle like a mole under the floor and find it.But before doing this check your radiator tails downstairs with dry paper towel as you would appear to have a very slight leak.Good luck👍