Vaillant UNISTOR unvented, tundish dribble

[Ted808]

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

 

[Last Plumber]

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?

 

[Ted808]

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?

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.

 

[Last Plumber]

6bar, isn't it?

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?

 

[Ted808]

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.

 

[Last Plumber]

pressure reducing valve is to stop the main pressure going above 6bar, isn't it?

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.

 

[Ted808]

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.

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!

 

[Last Plumber]

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!

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?

 

[Last Plumber]

I just noticed on the photo's, the distance below the tundish to the first elbow is too short. If you open the PRV's it will overflow?

 

[Welder]

I just noticed on the photo's, the distance below the tundish to the first elbow is too short. If you open the PRV's it will overflow?

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.