Unvented cylinder leak. Experts only - is anything wrong? (Ed.)

[alwaysproblems]

Long story short I've had issues with the hot water tank leaking either from the tundish, from the fittings or now from the element. It's 8 years old, both the prv and t&p have been replaced, expansion vessel tested and repressurised but the problems won't go away. After both hot and cold water leaking through the tundish I had both valves replaced. I noticed a leak from the controls unit and my only guess is steam is passing at the element. Have I just been really unlucky or is there something wrong with the install.

Apologies for this but I live in the Highlands and its almost impossible to get trades up here.

Am I better off getting this ripped out and starting again?

Appreciate any help / guidance you can offer.

 

[Deleted member 294929]

The temp & Pressure relief look a bit suspect.
But pictures are not clear enough.

 

[alwaysproblems]

Here you go Bod, rated at 7bar and 90c

 

[Deleted member 294929]

Lower

This set up

 

[alwaysproblems]

There's a valve on the drain which is odd but it's open so no issues.
Could the problems be caused by an unbalanced supply? Mains pressure is 7-7.5bar, no other prv fitted on incoming mains. If that is an issur how can the system be balanced? Is this done via the inlet valve?

 

[Johntheo5]

There's a valve on the drain which is odd but it's open so no issues.
Could the problems be caused by an unbalanced supply? Mains pressure is 7-7.5bar, no other prv fitted on incoming mains. If that is an issur how can the system be balanced? Is this done via the inlet valve?

The mains pressure of 7/7.5bar should not be a problem if a balanced cold is taken off at the cylinder valve set at the PRV (pressure reducing valve), what setting is written on the expansion relief valve?, you might post a clearer photo of the valve set showing all the pipework.
If you havn't a balanced cold or a PRV somewhere on the cold mains then its possible that cold water could be pressurizing the cylinder back through the hot and lifting the expansion relief valve.

 

[alwaysproblems]

Thanks John, ive attached more pics. Whats reqlly strabge is the thermostat on the hw was turned down. I turned it up and put the hot water heater on but nothing leaking. Its almost random.the incoming water to the property is at the kitchen with little to no.space to fit a prv. What are my options other than dig a pit between the mains shut off valve and my house and put a prv in there?

 

[Johntheo5]

I would put the non PRV mains fairly well down the list just now. The pressure might be building up say overnight with no HW draw off if the valve set pressure reducing valve isn't "drop tight", what is the valve set make/model and the expansion valve setting??.

Can you identify the items marked on the photo, is one of them connected to the white expansion vessel?

 

[Madrab]

Looking at the pipework downstream of the combination group (right of the group) the cold feed that tees downwards is the cold feed into the cylinder and the pipe that heads right and upwards looks to be connected to the Expansion Vessel?

If that's the case and there is no other balanced cold supply downstream of the Combi group feeding the rest of the house, then if there are any 'mix in the body' monobloc/mixer taps there could be back pressure from the higher 'unbalanced' cold mains back feeding through the hot supply into the cylinder causing one of the RV's to lift.

 

[alwaysproblems]

Thanks John, hopefully all the below makes sense and shows things a little clearer

 

[alwaysproblems]

It looks like the t&p valve is rated at 7 bar. All the taps in the house are mixers.

 

[Madrab]

If your mains is >7bar then it's a distinct possibility that there is a back feed from the unbalanced cold mains through one of the mixers into the cylinder or a passing PRV as suggested.

One test would be to isolate the cold mains, release the pressure from the unvented until the tundish no longer runs, then isolate the HW feeds to the taps and turn the cold back on and leave the system for some time, if the tundish starts to run again then it could be a PRV issue. If it stays dry then heat the cylinder as normal, if the tundish then starts running again then it's back to looking at an expansion issue.

This should all be done by a qualified engineer though as these cylinders shouldn't be DIY'ed and TBH they should have already performed all these tests as part of their fault finding procedure

 

[Johntheo5]

Try and get some details off the Caleffi valve set, this info might then tell whether drop tight or not.
Also what is meant by this, "I noticed a leak from the controls unit"

Edit: Caleffi ... I believe the 535&536 series IS drop tight and the Caleffii 533 series IS NOT.

What is the HW cylinder volume and the EV volume?

 

[Johntheo5]

I see the EV volume is 24L.
A 200L cylinder with pre&filling pressures of 3.0bar will have a final pressure of 3.72bar when heated to 65C and 6.64L of PRV leak past would be required to lift the expansion relief valve at 6.0bar.
A 250L cylinder with pre&filling pressures of 3.0bar will have a final pressure of 3.94bar when heated to 65C and 5.71L of PRV leak past would be required to lift the expansion relief valve at 6.0bar.
A 300L cylinder with pre&filling pressures of 3.0bar will have a final pressure of 4.19bar when heated to 65C and 4.8L of PRV leak past would be required to lift the expansion relief valve at 6.0bar.

 

[alwaysproblems]

I'm almost certain the back pressure is the problem. After doing a bit more research I have a hissing tap and no non return valves fitted on the mixers. Is the hissing tap the faulty mixer or could that just be a symptom?

After a plumber flooded my kitchen from.the main 3 times I'm reluctant to fit a prv under the kitchen sink. I'm guessing that's my only option as there's no room between the mains isolator outside and my house.

Could I fit a non return valve on the hot water discharge at the cylinder?

Thank you for all your help, I really feel like you've gotten to the bottom of this issue which has cost me too much money already.