Unvented hot water cylinder problem

[splash gti]

The pressure relief valve on our unvented hot water cylinder is only lasting approx 18 months to two years before it starts dripping.

In approx 8 years we have already had 3 replacement valves fitted (by different plumbers) plus the original. On all occasions we've had the full part fitted (made by Caleffi) not just the red-capped valve. The valve part is marked PN10 and it is rated at 6 bar.

The latest - fitted about 18 months ago - has just started dripping into the tundish so looks like another big plumbing bill coming my way!

Anyone else have problems with the pressure relief valves not lasting or is it just us?

A friend suggested our water pressure could be too high.

Is there anything that can be done? Can I make any suggestion to the next plumber?

Thanks in advance.

[gareth h]

Presumably you have a pressure reducing valve as well, perhaps this isn’t working and pressure is too high, and the relief valve is just doing its job?

[g7jtk]

Do you get your cylinder serviced?

[springfan62]

Check your expansion vessel isn't full of water, they don't last long.

[RammyMP]

springfan62 said:

Check your expansion vessel isn't full of water, they don't last long.

I have to replace mine every 3 or 4 years

[Busterbulldog]

Definitely check the charge pressure of the expansion vessel. This causes most pressure relief valve fails.

[Jaguar steve]

RammyMP said:

springfan62 said:

Check your expansion vessel isn't full of water, they don't last long.

I have to replace mine every 3 or 4 years

Mine was and that's what caused the PR valve to fail.

No need for a plumber 'tho. Just turn the cold supply off, open the lowest HW tap in the house to depressurize and partially drain the system and replace the vessel and valve. Inflate the expansion vessel to about 3.5 bar and turn the supply back on. Open all the HW taps to bleed the air out and it's sorted

ETA. Easy enough to check the vessel before you pull it all apart - there's a Schrader vale just like a car tyre one on the top of the cylinder, usually under a plastic cover. Just push the valve in just the same way as you'd do to let air out of a tyre and if water comes out then the vessel is knackered.

Edited by Jaguar steve on Wednesday 28th August 21:59

[Gingerbread Man]

It would be odd for an unvented cylinder to not come with a combination valve (isolation, non return, pressure reducing valve), so I'd guess that you would have one fitted. Depending on how the cold pipework is plumbed, it might not be balanced with the hot. It could be back tracking into the hot line through mixer valves (typically) and over pressurising the hot.

A nice installation idea has a pressure gauge before and after the reducing valve. An easy check to see how it's behaving and what's happening.

[Somebody]

Busterbulldog said:

Definitely check the charge pressure of the expansion vessel. This causes most pressure relief valve fails.

This. A handheld compressor like this https://www.amazon.co.uk/VonHaus-Cordless-Digital-... is handy for pumping the PV. The first fail on mine saw an engineer use a manual foot pump with his hands and he was soaked through with sweat when he had finished!

Edited by Somebody on Thursday 29th August 12:10

[dmc26]

most likely your vessel is flat or your pressure reducing valve is faulty causing the re-occurring fault

id get these checked first of all. remember that the vessel is a serviceable item and doesn't generally need replacing.

water coming out a vessel doesn't necessarily mean its burst. and 3.5bar can also be too much pressure for the vessel depending on your pressure reducing valve so that can also cause a fault.

[splash gti]

Thanks to everyone for the advice and recommendations.

In answer to some of your questions, yes, there is also a 3 bar pressure reducing valve before the 6 bar pressure relief valve. - it's all part of the same piece. I have had the complete part replaced each time.



I've tried pressing the valve on the top of the expansion vessel but it seems like there is no pressure in there at all - no air or water coming out.

I have noticed that the hot water tap temperature is very high, however, I turned the tank thermostat down but this had made no difference. Could this be at fault - my theory being that hot water increases pressure?

Thanks for the air pump suggestion, although I am a little scared to touch anything due to the pressure risk, and I am well aware that those working on it should have a G3 qualification.

I have called a plumber but have no fixed date for arrival as yet. I'll let you all know the outcome.

Many thanks again.

[dhutch]

splash gti said:

I've tried pressing the valve on the top of the expansion vessel but it seems like there is no pressure in there at all - no air or water coming out.

That is your problem then.

Every time you heat water it expands, and it should expand into the expansion vessel compressing the air, and then be drawn off next time you open the tap. Only if something goes wrong should the prv or thermal prv open.

If the expansion vessel has no air in it, there is no air bubble, and it must open the prv every time, which leads to seal erosion and a short life. (you also should only need to replace the red prv part, not the whole pressure reg)

You may have some joy blowing it up with a bike pump to around 2.5bar (blow it up, crack a hot tap, check pressure) but if it is flat again in a week or so the diaphragm is holed and you need to replace it. £30-50 ish.



Daniel

[g7jtk]

splash gti said:

Thanks to everyone for the advice and recommendations.

In answer to some of your questions, yes, there is also a 3 bar pressure reducing valve before the 6 bar pressure relief valve. - it's all part of the same piece. I have had the complete part replaced each time.



I've tried pressing the valve on the top of the expansion vessel but it seems like there is no pressure in there at all - no air or water coming out.

I have noticed that the hot water tap temperature is very high, however, I turned the tank thermostat down but this had made no difference. Could this be at fault - my theory being that hot water increases pressure?

Thanks for the air pump suggestion, although I am a little scared to touch anything due to the pressure risk, and I am well aware that those working on it should have a G3 qualification.

I have called a plumber but have no fixed date for arrival as yet. I'll let you all know the outcome.

Many thanks again.

Turn the water off, open the lowest hot tap and use a bike pump or better still a compressor to put some air in there.

[Busterbulldog]

It will most likely be cured once you recharge the expansion vessel. No need to buy a complete valve if needed just buy the prv insert and save ££££s.

[splash gti]

Thanks to everyone for taking the time to help me. I'm going to give it a go.

I presume the order is: switch off cold water feed, open hot tap downstairs to drain any pressure, close hot tap downstairs, pump up expansion vessel to 2.5 bar, turn water back on, open hot tap downstairs to bleed any air out, job done? Can someone confirm order is correct please?

[IJWS15]

Some strange ideas here - water is incompressible, it is why water in your engine cylinders will bend the con rods

The expansion vessel sits at the same pressure as the water in the tank. The pressure in both rises and falls at the same time.

[Bullet-Proof_Biscuit]

splash gti said:

Thanks to everyone for taking the time to help me. I'm going to give it a go.

I presume the order is: switch off cold water feed, open hot tap downstairs to drain any pressure, close hot tap downstairs, pump up expansion vessel to 2.5 bar, turn water back on, open hot tap downstairs to bleed any air out, job done? Can someone confirm order is correct please?

• Open hot tap upstairs to bleed air

Also, same method to drain the system and replace the pressure release valve yourself, it has compression fittings so once drained just loosen the nust off the pipes and replace, using new compression olives, no need for plumber, spend saved money on beer.

[Somebody]

splash gti said:

Thanks to everyone for taking the time to help me. I'm going to give it a go.

I presume the order is: switch off cold water feed, open hot tap downstairs to drain any pressure, close hot tap downstairs, pump up expansion vessel to 2.5 bar, turn water back on, open hot tap downstairs to bleed any air out, job done? Can someone confirm order is correct please?

Yes that is the sequence. Have you tried tapping the pressure vessel? Does it sound full or empty? Full means there's nowhere else for the heated water to expand into. Also, the pressure should be 3 bar and not 2.5 - at least it is on mine.

Good luck

[Busterbulldog]

Leave a hot tap open when you recharge the cylinder , if you don't the water that's in the tank will have nowhere to go.

[dhutch]

Yes:
- Turn off cold feed
- Open hot tap and leave
- Pump up to pressure
- Return tap to normal operation

If you cant turn off the cold feed, you can also just leave a hot tap running a bit while you do it.


Daniel