Fluctuating cold water pressure up from PRV

[OGRI]

Hi, we're experiencing fluctuation readings in a PRV here's the story:

Cold water into premises is 7 bar. This runs unregulated to four outlets:
Kitchen sink
Outside tap
Washing machine
Dishwasher

The cold water to the first floor runs through a PRV set to 3 bar. This then supplies:
Central heating boiler Worcester Bosch 37CDi
WC cistern
Basin mixer tap
Bath )
Shower head ) These three are controlled by a Crosswater manifold
Bath handset ) with a single common thermostat mixer.

Recently we experienced hammering in the cold water pipe when flushing the WC.
We then observed that the PRV needle was swinging between 0 - 10+ bar, often coming to rest on the maximum stop - indicating a pressure in excess of that coming into the premises.

We had the PRV exchanged by a qualified plumber. The PRV we replaced had a clean filter and appeared to be in good working order. It was less than a year old.

However the new PRV behaved exactly the same - swinging between 0 - 10+ bar.

Turning a coldwater tap on downstairs, has no effect on the PRV reading, as it doesn’t pass through the PRV.
Neither does flushing the WC upstairs though this is ‘after’ the PRV. (the PRV gauge drops to 2.5 whilst the cistern is filling and returns to 3.0 bar when filled.)
Nor does the PRV misbehave when running the cold water in either the basin, bath or shower.

However when running hot water or a mixture of hot and cold water to either the basin, bath or shower the PRV remains at 3.0 bar but when the hot water is shut off the gauge in the PRV swings to 10+ bar (takes a few seconds). The pressure can then only be released by flushing the WC.

We can’t get the PRV to ‘misbehave’ other than by calling for hot water from the CH boiler and whilst the water is running the PRV shows 3 bar. Once we shut the tap the PRV climbs to 10+ bar. We can’t get the PRV to replicate this behaviour by running any cold water tap - it remains at 3 bar.
Worcester Bosch technical department rule out any issues with the boiler.
Any ideas what may be causing this?

 

[Last Plumber]

The water passes through a NRV (non-return valve) inside the PRV and the gauge is on the outlet (guessing as I haven't seen it). If water flowing through stops suddenly, you get a build up of pressure which is trapped on the outlet side.

If the WC valve closes too quickly that would be my suspicion.

 

[OGRI]

Hi, Thanks for the response but the issue isn't with the WC, that simply highlighted a different problem to us ie the high pressure in the cold water pipe. Maybe my original post was a bit long winded. In summary on investigation and a bit of trial and error we have ascertained that the PRV only reaches an indicated 10+ bar when we run hot water and then shut it off, and it always goes to 10+ bar. We have a combi boiler and no storage tanks/cisterns.

Turning a coldwater tap on downstairs, doesn’t move the PRV gauge, it remains at an indicated 3 bar, - it’s downstream of the PRV so I assume this would be expected.

When opening a cold water outlet upstairs - upstream of the PRV - the PRV gauge drops to 2.5 whilst the water is running and returns to 3.0 bar when the outlet is closed.

However when running hot water (or a mixture of hot and cold water) through the boiler, the PRV remains at 3.0 bar but when the hot water is shut off the gauge in the PRV swings to 10+ bar. We then discovered that the pressure can then only be released by opening a cold water outlet either downstream or upstream of the PRV.
Hope that's a bit clearer.

 

[John.g]

Just a thought.
when you shut off the hot water then you will have the boiler Hx plus pipework full of hot water "trapped" by the PRV (non return valve) at one end and by the shut tap(s) at the other end, the boiler hot water will possibly increase in temperature in the Hx very rapidly (residual Hx heat) and cause the pressure rise due to expansion with no where to go?.
Can you run "cold" hot water through the boiler with it switched off and see does the same thing happen?.

 

[townfanjon]

What make / manufacturer and rough cost of the is prv please

 

[centralheatking]

Just a thought.
when you shut off the hot water then you will have the boiler Hx plus pipework full of hot water "trapped" by the PRV (non return valve) at one end and by the shut tap(s) at the other end, the boiler hot water will possibly increase in temperature in the Hx very rapidly (residual Hx heat) and cause the pressure rise due to expansion with no where to go?.
Can you run "cold" hot water through the boiler with it switched off and see does the same thing happen?.

If thats the case and I like your theory maybe the non return is in the wrong location especially as boilers over run centralheatking

 

[John.g]

The PRV will probably still act as a NRV even if none fitted, maybe worth installing a 5 litre E.vessel pre pressurised to 3 bar in the combi mains supply.

 

[OGRI]

Just a thought.
when you shut off the hot water then you will have the boiler Hx plus pipework full of hot water "trapped" by the PRV (non return valve) at one end and by the shut tap(s) at the other end, the boiler hot water will possibly increase in temperature in the Hx very rapidly (residual Hx heat) and cause the pressure rise due to expansion with no where to go?.
Can you run "cold" hot water through the boiler with it switched off and see does the same thing happen?.

Hi John,
Yes, just tried it with boiler switched off and when the "cold" hot water is run and shut off the PRV immediately jumped to an indicated 7.5 bar. I flushed the WC to release pressure but once the valve shut the PRV again went to 4 bar and carried on rising by 1 bar every 5 minutes, up to 8 bar when I flushed WC again.
Does this give a hope of a diagnosis?

 

[John.g]

The above would indicate that the PRV isn't "drop tight" and is allowing the pressure to creep up; to the upstream pressure of 7.5/8 bar, even though you stated previously that this rise in pressure only happened when you ran the combi hot water, so a bit strange. Does it still continue to rise to 7.5/8 bar after every flush of the WC, if it does then the PRV is certainly creeping. If you can crack open a cold tap to maintain say 4 bar and measure this flow rate then that will give the leak past flow rate, another suggestion is that the WC "ballcock" will close relatively slowly after a flush and will give the PRV more time to close.
As post #5 asked, can you post the make/model etc of your existing PRV (or/and the old one) and that will tell alot.

Just to be clear, right now, does this still happen??
"When opening a cold water outlet upstairs - upstream of the PRV - the PRV gauge drops to 2.5 whilst the water is running and returns to 3.0 bar when the outlet is closed."
[automerge]1577475478[/automerge]
One other thing, if the pressure rise is only initiated by the combi boiler and you have established that you do in fact have a drop tight PRV fitted then you might consider the installation of the 5 litre E.vessel near the combi boiler, can't think of anything else jyst now.

 

[OGRI]

The above would indicate that the PRV isn't "drop tight" and is allowing the pressure to creep up; to the upstream pressure of 7.5/8 bar, even though you stated previously that this rise in pressure only happened when you ran the combi hot water, so a bit strange. Does it still continue to rise to 7.5/8 bar after every flush of the WC, if it does then the PRV is certainly creeping. If you can crack open a cold tap to maintain say 4 bar and measure this flow rate then that will give the leak past flow rate, another suggestion is that the WC "ballcock" will close relatively slowly after a flush and will give the PRV more time to close.
As post #5 asked, can you post the make/model etc of your existing PRV (or/and the old one) and that will tell alot.

Just to be clear, right now, does this still happen??
"When opening a cold water outlet upstairs - upstream of the PRV - the PRV gauge drops to 2.5 whilst the water is running and returns to 3.0 bar when the outlet is closed."
[automerge]1577475478[/automerge]
One other thing, if the pressure rise is only initiated by the combi boiler and you have established that you do in fact have a drop tight PRV fitted then you might consider the installation of the 5 litre E.vessel near the combi boiler, can't think of anything else jyst now.

Hi John,
The PRV we replaced was a Honeywell D04FM (and appeared OK when taken off), the replacement is a Caleffi 533841H.
Just to be clear, right now, does this still happen??
"When opening a cold water outlet upstairs - upstream of the PRV - the PRV gauge drops to 2.5 whilst the water is running and returns to 3.0 bar when the outlet is closed."
Yes, it does. Cold or hot water run upstream of the PRV drops the pressure to 2 to 2.5 bar.

NB, the way we get the system to 'behave' itself is to:
Turn the boiler off and let it go cold
Turn the cold feed water off.
Release pressure by running cold water upstream of the PRV (usually flushing the WC)
Open a hot tap upstream (or downstream) of the PRV
Turn boiler back on.
The PRV will remain at 3 bar when either or both hot/cold is run upstream of the PRV.
BUT - as soon as DHW is run in the kitchen (down stream of the PRV) it all goes wrong.

Think I mentioned earlier that the Boiler is in the bathroom upstream of the PRV

 

[centralheatking]

Hi John,
The PRV we replaced was a Honeywell D04FM (and appeared OK when taken off), the replacement is a Caleffi 533841H.
Just to be clear, right now, does this still happen??
"When opening a cold water outlet upstairs - upstream of the PRV - the PRV gauge drops to 2.5 whilst the water is running and returns to 3.0 bar when the outlet is closed."
Yes, it does. Cold or hot water run upstream of the PRV drops the pressure to 2 to 2.5 bar.

NB, the way we get the system to 'behave' itself is to:
Turn the boiler off and let it go cold
Turn the cold feed water off.
Release pressure by running cold water upstream of the PRV (usually flushing the WC)
Open a hot tap upstream (or downstream) of the PRV
Turn boiler back on.
The PRV will remain at 3 bar when either or both hot/cold is run upstream of the PRV.
BUT - as soon as DHW is run in the kitchen (down stream of the PRV) it all goes wrong.

Think I mentioned earlier that the Boiler is in the bathroom upstream of the PRV

Honeywell Do4 PRV is the best on the European market and the most expensive we use literally 1000's per year never a comeback in the past 3 years, I know as all our units registered and numbered.
I think in a domestic situation they must not be horizantal . You have put an inferior prv on anyway
replaced a rolls royce with a fiat ...centralheatking

 

[John.g]

OK, just to be clear again...Upstream of the PRV means Unregulated pressure ie 7.5/8 bar or whatever.
Downstream of the PRV = REgulated pressure ie 2.5/3 bar.
Havn't read back through the posts but the pressure displayed on the boiler front should never read higher than 1.0/2.3 bar, otherwise the boiler "safety" valve (also called a PRV) will lift. The HW supply to/through the boiler may or may not be regulated as combi boilers I think can accept pressure up to 10 bar so IF the combi HW (mains) supply is unregulated then this can be reflected elsewhere.

The Honeywell D04 is not stated in their brochure as being Drop Tight whereas their D05 IS stated as being Drop Tight, the Caleffi 533841H doesn't specifically state IMO that it is drop tight other than saying that it conforms to EN1567.