Unvented cylinder discharging

[Mark]

I've noticed that my direct unvented cylinder discharges a small amount when heating overnight.

It's a redring one, with air bubble expansion - I've already replensihed the bubble, and while it has helped, hasn't stopped the discharge.

Basically, a slow trickle (a drop every few seconds) is discharged from the pressure-relief valve (not the temp/pressure valve). I've also got no record that it has been serviced since installed 3 years ago.

I'm guessing that this is a sign that the pressure-relief valve has worn out and needs replacing.

The other thing is that it's supposed to have a protective anode, but I've no idea where it is. There's relatively little corrosion visible, but I did drain the system down and the stuff that came out of the bottom of the cylinder was absolutely filthy. Additionally, when I refilled the cylinder again, there was some leaking from around the immersion heaters (though this stopped pretty quickly once the cylinder had pressurised).

As you might expect, this is starting to sound expensive - any idea how much a full service/inspection +/- valve replacement +/- new seals, anode and other bits is going to cost, but more importantly are there any hidden gotchas that are going to have plumbers rubbing their hands with glee?

[Ed Sirett]

Does the installation come with a pressure gauge? Does the dribble only happen at night? Did things improve when the air-bubble was renewed?

[Mark]

No.

Yes.

Yes, but it didn't fix the problem.

M

[Aidan]

Then more likely the pressure reducing valve is letting by, rather than a duff pressure relief valve. Mains water pressure generally increases at night & there's no draw off, which would remove any excess pressure that's crept through.

You'll probably find there's a blanked-off tapping on the pressure reducing valve/ multi-function valve where you could fit one. Maybe the gauge came with the kit and it was left in the back of a plumber's van. A gauge helps diagnosing problems like this.

Unlikely to be an air bubble problem. You should refill the cylinder SLOWLY when you replenish the air bubble. Try recharging it again (air is free, at present) and then maybe a new pressure reducing valve.

[adder1969]

If it's the type you can turn to test it, turn it quickly so that it opens then snaps shut quickly. Sometimes this will get them to seal properly again.

If you do replace it make sure it's of the correct type.

is where I got one from before.

Is the cold water downstream of the pressure reducing valve? If so, open the cold water first thing in the mornign to see if the intial pressure is greater than normal. That'll give an indication as to whether the mains pressure is leaking through over night.

[Ed Sirett]

That is all too common. It does not help any. < £10 to add one - probably a 1/4" BSP hole on the _inlet_ gizmo for this.

Looks like a weakish PRV - the heating is happening at night so even with a new bubble it letting by.

Most unventeds have two pressure relief valves. You _must_ fit the correct replacement it will be anything from 6 to 10 bar. If you are unlucky its the combined T&P releif valve (c. £60).

[Aidan]

That's the pressure relief valve, aka thesafety valve.

The problem is most likely with the pressure reducing valve.

If the pressure relief valve wasn't reseating, it would be dripping all the time. However the easing lever/knob should be operated at least once a year to make sure it still moves as intended. It might start dripping after doing that, but it's too bad if it does.

[adder1969]

When the water is heated and there's no demand for hot water (e.g. at night or early morning) the pressure in the cylinder will rise so it is possible for the relief valve to only leak under those conditions, but that usually is also an indicator that the bubble isn't doing it's job.

For what it's worth (see Ed's post) the pressure reducer on my unvented system was 1bar and the safety valves, both the PRV and TPRV, were correspondingly low rated. I'm not sure placing a 6 - 10 bar PRV when an e.g. 3 bar one is required is a good idea. Also my TPRV was more like 30 quid but I'm sure I could have paid double/triple had I gone elsewhere.

[adder1969]

All our flats originally came with a combined pressurised heating/hot water system and one was leakign for months over an entranceway. One day it was not leaking and a few weeks later the hot water tank blew up and gutted the flat and damaged neighbouring properties. So don't whatever you do just block up the valve!

[Aidan]

Get away.

No. That is the pressure relief valve working normally, relieving the excess pressure caused by the water expanding and the air bubble being inadequate to absorb the expansion. It is not 'leaking'. It is not failing to reseat, which would cause it to drip continuously. Operating the easing knob, as you have suggested would not effect this problem.

It isn't a good idea, but then that is not what Ed said. He wrote; "Most unventeds have two pressure relief valves. You _must_ fit the correct replacement it will be anything from 6 to 10 bar. "

He was referring to the Pressure Relief Valves which are factory pre-set at something "from 3 to 10 bar", according to the cylinder design pressure rating. You have to get the right one, set to the right pressure, for your cylinder. A PRV set at 6 bar would be little use on a cylinder designed for 3 bar and which fails at 5 bar.

Sure you got the right one?

[Mark]

Now added. The regulating valve is 1.5 bar nominal - and a gauge attached to the output reads 1.6 bar. So this valve seems to be working satisfactorily.

Unfortunately, this won't read the cylinder pressure because there's a non-return valve in the way, but it's better than nothing.

That was my original thought.

I've also checked the immersion stat - It was originally set to 60 C. Checking the installation manual for the cylinder, however, it suggests the maximum allowable temperature is 55 C - so I've made the appropriate adjustment.

I'll see what happens with this adjustment.

I think it's the pressure (not temp) relief valve as the discharging water is cold (admittedly, I can't be absolutely sure as both valves discharge through the same pipe and tundish).

As an aside, I tested the PRV by twisting the top - and it discharged an immense torrent - this hosed out of sides the tundish, and all over the walls, while at the same time overcoming the downpipe, which promptly overflowed with a rather impressive fountain. (The tundish and downpipe are quite capable of dealing with a wide open TPRV - but the PRV looks like it can manage a rather higher flow rate). Is a discharge pipe that can't actually handle the maximum discharge rate acceptable?

I'm beginning to think that this whole installation is a bit of a liability...

M

[Aidan]

Good move. You need to see what the pressure is when the PRV is dripping; the pressure might creep up overnight if the pressure reducing valve is iffy.

It will indicate high pressure if the pressure reducing valve is letting by overnight. It won't indicate high pressure if the excess pressure is caused inadequate expansion capacity.

You sometimes have to remove the pipework connecting the 2 PRVs to the tundish to see which is dripping. You have to remove the pipework to replace the valve anyway.

No. Part of the commissioning test is running either PRV (but not both simultaneously) at full-bore; the discharge pipe shouldn't back-up. There are design criteria for the size, length & number of elbows in the discharge pipe. You'll find it in the installation manual. You can wind up needing big (28mm) expensive copper pipes, which can wipe out the profit if Mr. Plumber didn't anticipate that one. It is copper, isn't it?

[Ed Sirett]

The usual order is NRV then PReducingV.

The pressure relief pipes only ever dribble but if the temperature relief operates this must be handled OK. However as you say the tundish is capable.

The problem is that the failure modes of unvented cylinders are quite a problem - set up right they work well. Vented have their problems too mostly with the storage cistern.

[Ed Sirett]

Aidan I didn't mean to imply that that the OP should replace the 3 bar PRV with a 6 bar unit if a 3 bar unit is the correct one. A 1.5 bar unvented is quite a bit less pressure that the typical unit.

[Mark]

Erm. No, it's plastic.

There's a short run of 15 mm copper from the relief valves to the tundish.

From there it's 40 mm plastic. About 30 cm straight down, then a rather impressive network of 6 elbows, then a long horizontal run (actually the

2nd half of it isn't quite horizontal, but it's definitely less than a 1:50 drop), until it joins the main waste pipe.

[Aidan]

It should be metal, plastic w> The usual order is NRV then PReducingV.

My book (CITB) shows IV, strainer, PReducingV, tee off for balanced cold supply, NRV, expansion vessel (not applicable here), PReliefV, cylinder.

The strainer, PReducingV & NRV usually come as one multi-function valve, so you can't alter the sequence.

[Aidan]

I know you didn't. Adder1 misunderstood what you had written.