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?

 

[SJB060685]

Its worth pointing out an absolute pressure gauge used in sealed systems with have different readings depending on where its installed

 

[Anj]

Its worth pointing out an absolute pressure gauge used in sealed systems with have different readings depending on where its installed

The pressure gauge is installed right next to the EV (in fact there are two gauges both next to the EV) I suppose that takes away the height issue.

 

[John.g]

Thanks John
the EV is a red Zilmet unit which states 35l on it built in 2018. Am not sure how it loses all the charge unless the valve is failing(?).
No PRV has let by so maybe the small amount of air has saved me. I’ve left it charged at 1.2bar and will leave it overnight. If it holds then I’ll lower it to one bar and charge the system to 1.2bar so should see the air side go up to 1.2bar?

I think I understand the first scenario...just to check - the 3.0 bar you refer to is assuming the incoming mains pressure is 3.0 bar?

as to the second how did you calculate the filling volume is 7.2l (what is the equation/formula?) Does the size of the EV make a difference? So you are saying 2.4l charge results in 0.5 bar pressure increase? Is the 2.7bar you refer to the proxy for mains pressure.

thanks for your patience with a non plumber!

OK, since we now know the EV has a capacity of 35 litres we will start from scratch and just to state that the mains pressure has nothing to do with the pressure rise, you only need sufficient mains pressure to give you the required filling pressure.


(Rem below that absolute pressure = gauge pressure +1)
Assume that the EV has a pre charge pressure of 1 bar and a filling pressure of 1.5 bar.
Now because with a gas/air, vol is inversely proportional to (absolute) pressure then the vol of air after filling to that 1.5 bar will be (35* 2/2.5) or 28 litres so the EV will now contain (35-28) 7 litres of water, now just assume that your heating system volume is 100 litres which will expand by 1.83% when heated to a average of 65C (flow+return)/2) which equals to a expanded vol of (100*1.83%) 1.83 litres, this expanded vol will now enter the EV so the EV will now contain (7+1.83) 8.83 litres (water) and the air end, (35-8.83) 26.17 litres, from this then the pressure now in the EV will be (2.5*28/26.17) 2.67 bar.abs or 1.67 bar(G) the pressure you see on your pressure gauge.
So the EV pressure will vary between cold and hot from 1.5 bar to 1.67 bar. (This might seem a very small pressure rise but you have a huge EV) Obviously if your system vol is greater that 100 litres then the final pressure will be greater and will be less with a smaller vol.

Now if the EV develops a leak at the air end probably through the Schrader valve or its seal, then the air volume will gradually start decreasing and eventually the EV will be almost full of water and the pressure will increase gradually due to expansion and reduced air end vol until eventually the PRV lifts at 3 bar. what it means in effect that you would have topped up by ~ 20 litres, not because of any system leak but because of a EV air leak.
I won't go any further at the moment, see what your test shows, but if no drop in pressure then suggest 1.0 pre charge pressure and 1.5 bar filling pressure which will give a reserve of 7 litres which will mean no top ups required for "years" (if no leaks of course).
You can do your calculations in the attached excel sheet.

 

[SJB060685]

John I was just pointing out that different types of pressure gauges are used depending on the situation. An absolute pressure gauge used in a sealed system takes absolute zero as a reference point, whereas a vented gauge takes surrounding atmospheric pressure as its reference point, obviously if atmospheric pressure changes then so will gauge reading.

 

[John.g]

Right I waited a couple of days with all the valves closed (boiler isolated), downstairs isolated. The pressure didn’t fall below 1.2bar.

Then I opened a joint in the condensate line recharged to 1.5 to see if anything came out. Nothing. System fell a little but not much.

So I decided to bleed the system but I wanted to check the gate valves so I opened the drain cock on the downstairs loop with the Gate valves for the downstairs closed (and all the rads closed as well as suggested by someone on here). Hardly any water came out despite me full opening the drain cock.
A little dribble started and continued for 1/2 hour Which I assume came from the downstairs loop as the drain cock is actually about level with the loop in the downstairs screed. (I figured if the gate valves were letting by I would see a lot water and pressure drop as the drain cock is right next to Gate valve).

I think the gate valves are doing their job so after that I opened the Gate valve in the return and drained the system.

The EV pressure (now i have put a tyre gauge on it) fell to zero! I feel like a muppet for not checking the obvious. I think the puff of air I was seeing when I pushed the valve (for a split second) was perhaps the last few puffs of air the EV had..

But I still don’t understand - none of the PRV have opened so can’t understand why the pressure was falling.
Also John G you say “if the precharge pressure fell to 0 Bar and with a filling pressure of 1.5 bar then the final pressure would be ~ 2.7 bar”.
Would you mind explaining how so? I don’t understand that.. I thought if I see 1.5 bar on the gauge the whole system is at 1.5 bar ?
I used be to an electrical engineer (sorry) so I am OK with maths but am not so familiar with hydraulics.

Anyway I am checking the EV, pressurised it to 18psi/1.25 bar to see if it holds. I will then lower this to 1 bar as suggested by John.G and then fill the system back up, monitoring both the system and EV pressure.

Thanks for the help so far.

Just to clear up a few other items...
“if the precharge pressure fell to 0 Bar and with a filling pressure of 1.5 bar then the final pressure would be ~ 2.7 bar”.
Would you mind explaining how so? I don’t understand that.. I thought if I see 1.5 bar on the gauge the whole system is at 1.5 bar ?"
The "final pressue" refers to the pressure after the system heats up again and would have been 2.7 bar with a 12 litre EV but only 1.67 bar with that 35 litre EV.

Re: now no air pressure at the air end....I calculate that the final pressure would be approaching 3 bar when there is still 6 litres of air in the EV so how it reached 0 pressure without lifting the PRV is beyond me except that the air leakage is/was so bad that it was leaking faster than the water expansion while heating up, very strange, but the test will tell alot.

 

[Anj]

John.G
Thank you thank you thank you.
I now understand the calcs (Boyles Law) I missed the Gauge v Absolute pressure part.

I have 21 rads but 9 are very small/towel rads so I think a system volume of about 150 litres. When the system was running fine it would go from 1.5bar(G) to 1.75bar(G) which according to the calcs is spot on.

Anyway the EV didn’t lose any volume overnight but stupidly I left the tyre gauge connected which might have stopped the leak if it was from the needle area, so I will repeat the test. I put some soapy water on the valve but no bubbles showed.

As for the PRV not releasing I now realise there is only one PRV for the CH system the other seems to be a pressure equalisation valve on the incoming water main and the other is on the HW cylinder. I think the latter two have nothing to do with the CH system. It might be that the PRV on the CH system (right under the EV) is jammed/doesn’t work - I haven’t tested it.
Maybe I should test it when I next pressure the system? - I have heard sometimes they don’t reseat and then leak but on balance better to know it is working. I think it must be installed with the tank and boiler in 2018 so it would be surprising if it has failed.

One question though as the EV had lost all its charge then wont this just happen again(?) hard to believe the leak will seal itself?
I guess if I see another pressure drop then the first thing to do is drain the system and check the EV and then replace it if it’s lost charge again.

I’ll redo the dry EV test and the pressure the system and see what happens.

 

[John.g]

That's all you can do, you will have to rule out the EV one way or the other before harbouring any suspicions that it still might be a system leak. The whole thing is a bit mysterious to say the least, as you say, if there is no leak at the air end then how did the pressure fall to zero because in doing so, with your constant top ups to 1.5 bar, it should have reached 3.0 bar on heat up when the air end vol fell to ~ 9 litres. The only other (unlikely) scenario that fits is that the EV pre charge pressure had lost its pressure before install (with bladder at water end), if that was the case then the final pressure would have been > 2 bar on heat up and to fit your observed pressure of 1.75 bar would mean a system contents of ~ only 75 Litres.

I presume your boiler is a regular boiler with external pump?, if its a system boiler then it will also have its own EV & PRV venting outside the building but that external EV may have been fitted if the system vol is large, even if not a system boiler it may still have a PRV, any GSR on here can tell you that if you post the make/model.
Edit: I see its a Worcester 40CDi Classic Regular so doesn't seem to have a PRV.

 

[Anj]

That's all you can do, you will have to rule out the EV one way or the other before harbouring any suspicions that it still might be a system leak. The whole thing is a bit mysterious to say the least, as you say, if there is no leak at the air end then how did the pressure fall to zero because in doing so, with your constant top ups to 1.5 bar, it should have reached 3.0 bar on heat up when the air end vol fell to ~ 9 litres. The only other (unlikely) scenario that fits is that the EV pre charge pressure had lost its pressure before install (with bladder at water end), if that was the case then the final pressure would have been > 2 bar on heat up and to fit your observed pressure of 1.75 bar would mean a system contents of ~ only 75 Litres.

I presume your boiler is a regular boiler with external pump?, if its a system boiler then it will also have its own EV & PRV venting outside the building but that external EV may have been fitted if the system vol is large, even if not a system boiler it may still have a PRV, any 'Registered Professional Gas Engineer' on here can tell you that if you post the make/model.

Thanks John
The boiler is a Worcester Greenstar 40CDi Regular

It has an external pump and am pretty sure no PRV (I couldn’t see one on the manual diagram) however it can dump water direct to the condensate line through a manual valve so maybe there is another safety valve I can’t see.

 

[Anj]

Thanks John
The boiler is a Worcester Greenstar 40CDi Regular

It has an external pump and am pretty sure no PRV (I couldn’t see one on the manual diagram) however it can dump water direct to the condensate line through a manual valve so maybe there is another safety valve I can’t see.

John.G
So the EV lost 2psi (18psi to 16psi) in a day (psi better scale on the tyre gauge) even without the system being pressured (the tyre gauge was disconnected this time).
I have ordered a new EV - will report back in a couple of days once it is installed.

thanks once again for the help.

 

[John.g]

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?.

 

[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👍