Overflow discharge from Unvented cylinder

26 Aug 2007
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United Kingdom

I have a water discharge issue via the overflow from my unvented cylinder. The cylinder is overflowing only when the cylinder is being heated.

It appears the muppet who installed it did not do a proper job and will not return to fix.

I have been told that the pressure relief valve requires replacement and the overflow MUST discharge via tundish and copper pipework.

I've looked on NHBC who produce a technical guidance document 8.1/26 that states that indeed the overflow should include a tundish, which I dont have, but does state that plastic overflow pipe work is an acceptable alternative to copper.

My questions are this.

1. I believe the tundish gives a visual sign of water, but its an open fitting. Is there no chance of a tundish spilling water? Are there enclosed ones available with a glass or similar window?

2. Surely to unitise the existing PP pipe for overflow is the most convenient?

3. What would be the ball park cost to supply and fit the valve, tundish and utilise existing pipes? One quote I have appears excessive - north of £600.

Help and advice appreciated.

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Pressure vessel needs recharging or internal air bubble needs reinstating. (Same thing basically, there is no pocket of compressed air to allow for the expansion). 15 minute job.
which I dont have, but does state that plastic overflow pipe work is an acceptable alternative to copper.
Has to be high temp PE pipe not just normal over flow, a tundish must be installed, when installed properly you will not get any water spilling out, post some pics of what you hae
Thanks so far.

here are some pics of what I have.

image1 (5).jpeg

White pipe is overflow.
Red hand wheel is cold water supply

image2 (2).jpeg

This shows overflow coming off top of cylinder

image3 (2).jpeg

Overall view. expansion vessel off to right - not shown.


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Oh dear, that's just plain wrong.

Has the benchmark in the MI been completed by the chap that installed it? That install isn't right. I would hazard a guess that if it isn't filled in then the chap may not be qualified to fit UV cylinders. Do you have a BC (building control) certificate covering the install?

Can I please ask you to turn off the hot water until you can get a qualified engineer in to check it over and sort it out. That's an accident waiting to happen, unvented cylinders can be very dangerous.
As @Madrab says so many things wrong with that install, turn off the supply and open a hot tap and dont use until it is installed correctly
The 2 key safety devices need to be re-plumbed properly a tundish introduced and disharge pipe upgraded and run properly. The stop tap ideally needs changed, a gate valve isn't really designed for mains water. Your qualified chap will know what to fix and what to sort out to stop it releasing pressure.

What plan is the heating, I presume S plan?
Last edited:

thanks again.

don’t know what you mean my s plan. Sorry.

I’ve been quoted as follows
Parts required:
Inlet Control Valve £60.38 plus VAT
9m 22mm pipe £45.00 plus VAT
3m 15mm pipe £7.50 plus VAT
15/22mm tundish £6.89 plus VAT

No of Engineers: 1 = 1 apprentice
Time Required: 6 Hours £342.00 plus VAT

What do you think??

Stop using the cylinder..it's unsafe. Who installed it...they can be prosecuted for such illegal work.

Get a G3 licenced installer and at least the following needs checking/doing...

Who's stolen the immersion heater...needs installing/wiring for backup.
Expansion vessel plumbed into the wrong connection.
Check the expansion vessel pre-charge pressure.
No balanced cold take-off by the look of it.
Needs a proper full bore ball valve for inlet isolation.
Raise the pressure reducing valve/expansion valve inlet set above the cylinder for easy maintainence.
Check there's a drain-off on the cylinder.
Re-do the safety valve pipework and fit a tundish and check where the termination is.
Re-do the cabling...twin & earth is not acceptable.
Check all the wiring and controls...highly likely to be non compliant.
Insulate all the pipework.
Check the structural support...chipboard is not really acceptable.
Fill in the cylinder label and notify to building control and receive the control notice from them.
etc etc....
Shocker. Very frustrating to see knowing that 99% chance of no comeback to installer.
Looks like expansion vessel may be off the balanced cold connection prior to check valve. Doesn’t matter what pressure is in that vessel, it ain’t gonna change anything!
Looks like the kind of install you'd see in a training centre, with an assessor breathing down your neck asking you to list all the faults on it.
Shocker. Very frustrating to see knowing that 99% chance of no comeback to installer.

If the cylinder has a serial number then, given the dangerous installation, the cylinder manufacturer may be able and willing to help trace who bought it
In your dreams Bernard...manufacturers don't give a stuff.
Several boiler manufacturers actually rebrand their models (and reduce the warranty to the legal min) and sell them through the builders merchants
knowing there's every chance they will be installed by illegal non registered workers.

It's all about shifting stock...

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