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Vaillant UNISTOR unvented, tundish dribble

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Discuss Vaillant UNISTOR unvented, tundish dribble in the Plumbing Jobs | The Job-board area at PlumbersForums.net

Ted808

Ted808

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Plumber
Gas Engineer
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359
Dribble in the tundish, sometimes hw sometimes c? What's going on?
Changed prv- and tprv, found exv poooped, changed that too. Continued to dribble- called V tec - said it had to be pressure reducing v, ( but the water is hot ) - changed that and guess what ? It's still dribbling!!
V tec said the only other thing it can be is the coil within, which could be scaled up. REALLY?
Has anyone had a scaled up coil?
What can it be? Usually solve this dribble in the tundish, by changing the prv/combination valve...
SUPER WEIRD is that this is one of 2 Vaillant UNV cylinders (not in the same house) that have the same problem... can't really move on to the next one if I can't sort this one...
Any hints greatly appreciated
 
Have you tested the governor/regulator to see what the pressure is on both sides?

Have you disconnected components to see where the water is coming from?
 
Last Plumber said:
Have you tested the governor/regulator to see what the pressure is on both sides?

Have you disconnected components to see where the water is coming from?
Click to expand...

I've replaced the pressure reducing valve, I could go back and test the pressure I guess, but surely if it's the gov it would be cold water in the tundish? If I turn the mains water supply off the dribble stops. The whole point of the pressure reducing valve is to stop the main pressure going above 6bar, isn't it?
Plus the dribble is hot water.

IMG_20190306_122914956.jpg
 
Ted808 said:
6bar, isn't it?
Click to expand...
3 bar Normally.
To enter the tundish it must only be coming through a prv or t&prv.
If they are new, then one of them is faulty or the pressure or temp is too high in the cylinder.
If the water is hot or cold as you say, then I would suspect it is coming from the cold feed backwards. Sometimes that will be warmer than others. If for example you run a lot of hot off then the base of the cylinder will be cold, so any backflow will also be cold for a time. After a while of not running the hot water, the water from the rest of the cylinder will be leaving via the cold feed pipe, therefore it will run gradually warmer.

My thinking (without having seen it of course or tested it) is that your mains pressure is higher than the cylinder regulator outlet pressure but there is something back filling the cylinder from beyond. It could be a mixer valve? If the pressure on the cold is higher than the cylinder and there is a valve passing back, then the cylinder will raise pressure via that source and along with expansion you could easily reach 6 bar.

Am I making sense?
 
Yes I did wonder if there was the possibility of a back feed from a mixer. I'll investigate. I guess I'll disconnect the hot supply off the top of the cylinder to prove that?
I also thought it was 3bar but the old one was marked 6bar- so I thought the v ones are different.

IMG_20190226_155706586.jpg


IMG_20190306_122907748.jpg
 
Ted808 said:
pressure reducing valve is to stop the main pressure going above 6bar, isn't it?
Click to expand...
The pressure reducing valve / governor / regulator is normally set at 3 - 3.5 bar.
The pressure relief valve you photographed would normally be 6 bar.
The Temperature and pressure relief are normally 7 bar + 90 degrees.
 
Last Plumber said:
The pressure reducing valve / governor / regulator is normally set at 3 - 3.5 bar.
The pressure relief valve you photographed would normally be 6 bar.
The Temperature and pressure relief are normally 7 bar + 90 degrees.
Click to expand...
Thanks for clearing that up for me- it's a case of I don't often work on these every time I have to go back to my books. I see where I was mixing it up!
 
Ted808 said:
Thanks for clearing that up for me- it's a case of I don't often work on these every time I have to go back to my books. I see where I was mixing it up!
Click to expand...
No worries. Good luck with it!
If you can isolate the mixer valves in the house that will tell you if it is water feeding back.
Check the incoming mains water pressure too.
It could be a mixer without NRV's (non return valves) and a higher pressure on cold or a mixer with faulty NRV's and a higher pressure on cold. It could be something else but I would start there if I were you.
If you need to reduce cold pressure to a mixer, there is a balanced cold take off on the valve in your picture if you need it remember?
You could also install a Pressure regulator on the incoming supply as another option but the issue with passing needs addressing as you know.
Let me know the outcome of your investigation?
 
YorkshireDave said:
Thats the understatement of 2019! I could not be any shorter. Its not unreasonable to assume other plumbing is just as poor / cheap so do not expect non return valves on taps.
Click to expand...
True and custard almost wants me to put old valves back as they might not have been faulty.
 
Ted808 said:
True and custard almost wants me to put old valves back as they might not have been faulty.
Click to expand...
I wouldn't worry, it is difficult to diagnose things following other peoples poor work as they do things that you would not do yourself, (I'm going for the second understatement of 2019).
When you work on something, you do expect it to have been installed commissioned and working correctly before you attend. It is not always obvious that some plant pot bodged it and scarpered. 'Tell tale signs' (as yorkshire pointed out) here, are things like the D2 discharge . Another is (possibly) that the balanced cold has been used at some point and then blanked off. That may well be innocent but under the circumstances I would rub my chin, frown and make an mmmmmmm sound!!!! Another is the white residue all over the tundish?
I cannot see what that is but I suspect from the photo that it has been wet for a while?
If you manage to solve this issue and leave it trouble free, no doubt you will gain a regular customer.
As an additional note which you can ignore if you like, i always install a pressure gauge in the side of those blocks where the black plug is, just so I can easily see what's going on. They are only cheap but it's another sale and it makes life easier.
 
Last Plumber said:
I wouldn't worry, it is difficult to diagnose things following other peoples poor work as they do things that you would not do yourself, (I'm going for the second understatement of 2019).
When you work on something, you do expect it to have been installed commissioned and working correctly before you attend. It is not always obvious that some plant pot bodged it and scarpered. 'Tell tale signs' (as yorkshire pointed out) here, are things like the D2 discharge . Another is (possibly) that the balanced cold has been used at some point and then blanked off. That may well be innocent but under the circumstances I would rub my chin, frown and make an mmmmmmm sound!!!! Another is the white residue all over the tundish?
I cannot see what that is but I suspect from the photo that it has been wet for a while?
If you manage to solve this issue and leave it trouble free, no doubt you will gain a regular customer.
As an additional note which you can ignore if you like, i always install a pressure gauge in the side of those blocks where the black plug is, just so I can easily see what's going on. They are only cheap but it's another sale and it makes life easier.
Click to expand...

And next best understatement of 2019 goes to? :rolleyes::D
 
Another is the white residue all over the tundish?
YES been leaking for months, it's in an out house you see, and the termination of the warning pipe is in a dark little corner, turned into the wall, wall has turned green and neighbour complained, that's how they found out it was leaking!!
There is no cold water taken off the combination valve never was, only cold main coming into outhouse and hot going out...
Would it be unreasonable to put a check valve on the hot supplying the taps, to stop the reverse flow. I haven't been back but I suspect the bathrooms all tiled up to the eyes and no isolation valves for the mixer taps. I did ask about his mixer taps on the first visit, he said no problems...... will have to ring him today...
 
UPDATE: Put a check valve on the hot supply from the cylinder and so far no news is good news. When I disconnected, there was definitely a good constant dribble coming down the hot- presumably from a mixer tap somewhere... they have a few.
 
Well, I was feeling chuffed too soon, back to dribble in the tundish, separated out the two into buckets and the prv 6Bar was the culprit.
So, back to the drawing board...
Tried isolating the whole lot, two port valves shut, F+R to boiler off, incoming main-OFF. Yet after some time the , now added pressure gauge, crept up, then all of a sudden raced up and I could hear the hiss of water being used in the house...
Disconnected check valve and lo +behold the bluddy thing was letting by- So think I'll return with a double check. Has anyone has this happen? 3 Weeks for a check valve??
IMG_20190418_173729018.jpg
Of COURSE HE DOESN'T WANT TO PAY EXTRA
 
Yea double check should sort it you could use a swing valve
 

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