Unvented HW cylinder dumps HW

[Lobster]

Two days ago I found all the towels in the airing cupboard were very damp, and came to the conclusion that the only place this water could have come from is splashing from the tundish on the pressure relief valve of my unvented HW cyclinder. Judging by the amount of 'splashed' water, the amount that must have gone down the overflow pipe must have been substantial.

If I'm right, and something caused the cylinder to over-pressurise, this is obviously a serioud concern and I know I need to get it looked at ASAP.

I've been watching out for a repeat performance, to confirm the source of the water and to find the conditions under which it happened, in order to help with diagnostics, as I don't want to get a plumber in who will just condemn the whole system due to inability to localise the problem, However, it hasn't happened again (yet at least).

Can anyone suggest what might cause this? and whether in fact there's likely to be something obvious that a plumber could home in on?

The system has been in for about 10 years with no problems up to now.

Thanks David

[Matty F]

In NZ, HW cylinders are probably different However my pressure relief valve started leaking, and this was caused by the spring inside having rusted and collapsed after 20 years of sitting in a pool of water. The instructions for mounting the valve do not say to mount the valve vertical with the drain underneath so that the spring is not sitting in water. I suppose I should complain to the manufacturer.

Of course there is always some hot water overflowing whenever cold water introduced to the HW cylinder expands. Apparently this is acceptable to the experts. But not to me.

[TheOldFellow]

How is the pressure in the system managed? There seem to be two systems (that I've seen), in one the unvented cylinder has an external diaphragm-type expansion box as used in sealed HW systems, in others, like my OSO, the shape of the cylinder top has an air space who allows for expansion without a diaphragm. If it's like my OSO, there is a documented procedure for re-installing the air gap (which involves some draining down). A diaphragm type might have a split diaphragm - 10 years?

It might be that the air gap has reformed naturally from dissolved gas in the water feed, of course. When the pressure valve releases, they often go with a bang, especially if they have not opened recently.

R.

[Onetap]

There should be two valves discharging into the tundish, a PRV and a T&PRV.

Either one of them is working as intended, indicating a fault, OR one of them is defective.

The T&PRV will start to discharge at a reduced temperature if it becomes defective; I'd first suspect that. You need to stand and watch as the cylinder heats up.

It could otherwise be a defective pressure reducing valve, a deflated/ broken expansion vessel (or airgap depleted, depending on the model), defective PRV, or something I haven't thought of.

The T&PRV will discharge hot/warm water, the PRV will discharge cold.

[Onetap]

The UK Water Regulations prohibit such waste of water, so UK unvented water heaters have an expansion vessel or an airgap inside the cylinder to accomodate the expansion of the heated water. If the expansion vessel/air gap fails, you get water discharged as you describe. The rest of the world manages without expansion vessels.

[Mathew Newton]

...there could also be a third - the PRV for the central heating system (in which case, again, it could be a defective PRV or expansion vessel).

Mathew

[Onetap]

That one should terminate outside. The tundish supplied with the UVHW kit of parts has connections only for the two UVWH PRV discharges.

I thought it might be the T&PRV because I fixed one on which the owner 'found all the towels in the airing cupboard were very damp, and came to the conclusion etc......). It was an unoccupied buy-to-let property & the T&PRV discharged when the water temperature reached about 45 to 55 degC. There was no splashing on it discharging, all the damp was due to steam condensing on the uninsulated cold water pipes and so it appeared that the 'leak' was nowhere near the tundish.

The T&PRV manufacturers said they'd never heard of such a thing happening before; lying bastards.

[Ron Lowe]

The expansion vessel has lost it's air volume.

What make / model of cylinder is it?

On some models, ( eg Heatrae Sadia Megaflo )this is an internal air volume that can be re-generated by following the instructions on the tank.

Otherwise, the expansion vessel may be external. This has either lost pressure and needs to be pumped up, or has ruptured internally and needs replaced.

[Lobster]

It's a Vaillant Vantage - have found the instructions here: (or

It does have an expansion vessel sitting above the tank - so do we reckon that just needs replacing, then (looks very straightforward, ie it just unscrews - is it actually a diy job or are there big safety implications and I shouldn't touch?). On p14 of the manual it says it should be 6.0 bar - can i just check the Schraeder valve on top of the vessel with a car-tyre jobbie, and if it's less than 6 bar, try pumping it up?

Thanks David

[Ron Lowe]

Poke a poking impliment into the valve, as if to deflate a car tyre. If water comes out, it's goosed and needs replacing. The diaphram has burst.

If air, ( or nothing! ) then yes, you can pump it up. If it fails to remain pumped up, it's goosed and needs replacing.

Replacing should be a simple job with no issues.

There are no safety implications: if the expansion vessel fails or is missing, then the cylnder will continue to vent over-pressure through the relief valve as it has already done.

[Ron Lowe]

Additional note:

P5 says the pre-charge pressure on the expansion vessel should be 4 bar.

THe 6 bar reference on P14 is the cracking pressure of the Expansion Relief Valve.

Pump the thing up to 4 bar cold.

[Roger Mills]

. . . making sure that the 'wet' side isn't under pressure while you do so.

[Mathew Newton]

Are you sure it has to? Mine doesn't and, sticking my neck out here, being a new-build one would expect it to comply with the regs.

Mathew

[Lobster]

Thanks for the responses. Just checked, and upon depressing the Schraeder valve I'm rewarded with a barely perceptible little fart - that's it - just enough to know that the valve was actually depressed. I'll give repressurising a go first but presumably it'll need replacing.

Isn't it a bit odd that my system works though? Have had CH and HW on all week with no problems at all except apparently for the one 'overflow' issue.

It's going to be fun trying to repressurise - the vessel is about 5 feet off the ground in the airing cupboard, with nowhere nearby to site a footpump. Can you get extension hoses for them or something? Google's no help. Have in the past tried to use my 12V electric inflator in the house on a mains adaptor, but it wouldn't provide enough current, and I don't really want to yank the car battery...

David

[John Williamson]

Maybe get one of the fairly cheap jumpstart/ compressor units from Maplin or Halfrauds?

[Ronald Raygun]

Just use the footpump. Brace the pump against the wall or the doorframe and operate it with your hands.

If you can clip the end of the pump's hose onto the valve, as you can with a car tyre valve, great. Otherwise it'll just be a trifle more challenging because one hand will be needed to hold it on. Perhaps you could hang the pump on the wall/frame from a small nail, or use an assistant to hold the hose in position. Then you can operate the pump with both hands (one hand pretending to be your foot, the other pretending to be the wall or ground).

Alternatively it might be possible to go at it with a stepladder and stand the foot pump on one of its rungs.

It may not be very comfortable, but if you only need to do it every

10 years, then up with it you can surely put.

[John Rumm]

Did you need an excuse for a compressor and airline?

[John Rumm]

which probably means shut off the mains inlet, and open a hot tap...

[Lobster]

I assumed I'd need to open a drain c*ck on the CH to reduce the pressure to zero, thereby losing all that precious inhibitor - is that not the case?

David

[Lobster]

Will certainly give it a go, sure; just wondering what sort of volume of air we're talking about shifting: am thinking it could be a fair bit? Have since come across a stirrup pump affair on ebay with a 2m hose which would probably cut the mustard if need be.

David