PRV Banging

[chirpychippy]

Good morning,

A bit of background first then ill try and explain the problem. At the inlaws house they dont have the conventional combi boiler or even a tank, what they do have is the main cold water supply coming in in the bathroom, this supply feeds the toilet next to it which is t'd off the main, the main then feeds an electric shower and then onto the bathroom under sink unvented water heater and then into the kitchen where it feeds another unvented water heater for the kitchen sink.

The problem they are experiencing is the PRV on the water heater in the bathroom, its a 6 bar prv as specified by the manufacturer, its a Crown CPU10 heater. What happens now and again is that prv opens and closes with extreme speed making the heater jump around and the pipes sound like a jack hammer. The only way to stop this carrying on is to turn the main supply virtually to nothing.

I nipped round there yesterday and had a look at the PRV being that it was only changed 2 months ago for the same reason, i took it off and opened it up to check it to see if its seating properly or was full of carp but it is in perfect condition. Put it back on turned the water back to moderate pressure and then manually opened the prv at this point it got stuck again, the spring jumping up and down like a lunatic pipes banging - back to square one.

Nowhere on this 1 pipe system is there an expansion vessel but tbh im not sure why im mentioning it.

I am trying to get a plumber round again but the good ones are always really busy.

I have some plumbing knowledge but this is beyond me could you help please??, Thank you.

 

[jeff the gasman]

Thermostat may have faulted to boiling point or insufficient expansion. Does it have an expansion vessel?
Do not modify it or you have a potential bomb.

 

[Madrab]

If you can stop it by shutting down the mains feed then it may be the incoming pressure is too high, it's too close to a draw off point (3m is the usual min distance) or there's a check valve installed, in each case a mini expansion vessel should be installed. As @jeff the gasman says though, don't play around with the heater's only safety device, it really needs someone experienced with unvented systems to test and advise.`

 

[Newboy]

It sounds like potentially a poor installation and/or high incoming mains pressure. If the Safety Pressure relief valve is chattering then it suggests that the pressure in the system is above 6 bar

Usually on an unvented heater there would be:

incoming supply > isolator > cold draw off to other appliances > check-valve > expansion vessel > Pressure relief valve (6 bar) > water heater

If the mains is high pressure (>3 bar) then a Pressure Reducing Valve is fitted after the isolator

incoming supply > isolator > PRV (3 bar) > cold draw off to other apps > check-valve > expansion vessel > Safety Pressure valve (6 bar) > water heater

 

[chirpychippy]

Thank you for your replies. For the record i have no intention of modifying anything " i dont have the knowledge".

The main is about 2 metres away from the heater. So we have incoming supply "bar pressure unknown" and "no" isolator which then becomes the cold draw off to shower and kitchen sink then "no" check valve "no" expansion vessel but there is a 6 bar prv before the cold supply enters the water heater it is this prv that chatters.

Tell me im wrong, i normally am....i dont understand how the prv is supposed to work the way it has been plumbed in. The prv is "in line" with the flow of the cold supply to the heater. Ill explain we have a vertical pipe then we have a horizontal pipe to join the two without bending the pipe to form a 90 degree one would use an elbow...in this instance the prv is the elbow, i thought the prv needed to discharge excessive water pressure but because its plumbed in with flow it cannot discharge...

I am trying to get a plumber around there as its beyond me....

Thanks again.

 

[jeff the gasman]

The prv is merely a means of disposing of excess pressure which can cause ruptured pipework or the heater
If your incoming Main is already at 5 bar, and plenty are, with insufficient expansion provision you’ll get this happening.
A decent plumber will sort it mate.
Ps it’s fitted on the cold because otherwise it may be spitting boiling water out.

 

[dilalio]

This is one of those instances where the acronym PRV can become confusing... "Pressure Relief Valve" or "Pressure Reducing Valve". It sounds like you need both. Some photos of the install would help. How long has this been in and did it always have this issue?

 

[DP]

The unit is undersink heater. There will be no expansion vessel as expansion will be to mains incoming line due to miniscule size of the heater.

If pressure reducing valve fitted to fix the problem of high incoming pressure ( suspect hammering action will be happening when water demand is low in the area), then EV will be needed.

 

[chirpychippy]

Thank you for all the replies, it is a pressure relief valve not reducing valve as i said it was .
The heater has been in about 3 years and this problem has been in the last 3 months. I spoke to a plumber this evening and he agrees with you all that it is probably a pressure issue and to install a prv reducing valve in an appropriate place.
If it had been a simple fix i would have done it, but tbh, im not a great lover of plumbing best left to those who know best "and have the insurance"..
Thanks again.

 

[AdvanceHelpline]

If I read one of your posts correctly it appears that the PRV is connected both ends? Is this right? If so it is wrongly installed as one of the connections should be open to a tundish.

 

[jeff the gasman]

If I read one of your posts correctly it appears that the PRV is connected both ends? Is this right? If so it is wrongly installed as one of the connections should be open to a tundish.

How would it bang if pressure equalised on both sides?

 

[DP]

Thank you for all the replies, it is a pressure relief valve not reducing valve as i said it was .
The heater has been in about 3 years and this problem has been in the last 3 months. I spoke to a plumber this evening and he agrees with you all that it is probably a pressure issue and to install a prv reducing valve in an appropriate place.
If it had been a simple fix i would have done it, but tbh, im not a great lover of plumbing best left to those who know best "and have the insurance"..
Thanks again.

Thereby a PROBLEM agas been created

By fitting a pressure reducing valve, where is the system pressure increase going to end up?
Into the pressure relief valve. What has been remedied is not a remedy but a botch as there is lack of understanding as to how to fix the problem.

 

[dilalio]

It needs an EV... Make sure it's one that's right for the job. I've seen disastrous results with the wrong type being used

 

[chirpychippy]

Evening all,

Straighten a few things out, the pressure relief valve is correctly fitted it does go to a tundish.

So a pressure reducing valve set at 3 bar has been installed on the cold feed that only feeds the water heater, after the pressure reducing valve and before the cold feed enters the heater is a "T" solder fitting which t's into the pressure relief valve and onwards to the tundish.

Presume this is correct it was done by a plumber. "Shrugging shoulders at the moment" .

 

[Newboy]

Quite possible that the problem may still exist - now the new valve will be stopping the heated water expanding back along the supply pipework. An expansion vessel should have been fitted.
The old PRV (safety device) may no longer chatter but it's quite likely that the expansion due to heating will cause the same PRV to open.

Also it's not clear where the new pressure reducing valve has been fitted.

The sequence should be:

Incoming supply > isolator > PRV (3 bar) > cold draw off to other apps > check-valve > expansion vessel > Safety Pressure valve (6 bar) > water heater

edit:typo corrected