Why is my mains water PRV not maintaining 3 bar?

[PeteCUK]

Hi all,

I had a problem recently with the pressure to the house exceeding the 3 bar that the caleffi pressure regulator was set to. The gauge on it indicated that static pressure would sometimes increase to 6 bar; it's not just a gauging issue because the HW cylinder next to it starts leaking from the 6 bar relief valve (which leaks onto the garage floor as the previous owners have cut through the copper pipe, by the looks of it because it was blocked up with limescale!).

I took the valve out and cleaned it as suspect it probably hadn't been done by the previous owners in 10 years and it was a bit gunged up but once refitted I had the same issue. Replaced it with a brand new like for like and while most of the time it sits at 3 bar nicely, every few days I'll see the garage floor wet, normally in the morning presumably when the static pressure has had a chance to build up. Operating any tap drops the pressure to 3 bar immediately from where it holds pressure pretty well for a while.

Is this normal static pressure behaviour? The HW cylinder is protected by the relief valve, but is it likely to damage other parts of the system at all, eg the water softner?

 

[Stigster]

Expansion vessel (or internal expansion bubble depending on the design of the cylinder) fault most likely although there are other things that could be wrong to cause those symptoms. You need a G3 registered plumber (in the UK) to work on these. You shouldn't have worked on that yourself really.

It's going over pressure as the water heats, the pressure is building and the safety device is opening to do what it is supposed to do and preventing the cylinder bursting or exploding. Get someone in ASAP to service and repair it properly. Again, make sure they have an up to date G3 card.

 

[PeteCUK]

Expansion vessel (or internal expansion bubble depending on the design of the cylinder) fault most likely although there are other things that could be wrong to cause those symptoms. You need a G3 registered plumber (in the UK) to work on these. You shouldn't have worked on that yourself really.

It's going over pressure as the water heats, the pressure is building and the safety device is opening to do what it is supposed to do and preventing the cylinder bursting or exploding. Get someone in ASAP to service and repair it properly. Again, make sure they have an up to date G3 card.

Just to confirm, I didn't go any work to the HW cylinder, I replaced the Caleffi 536250 pressure reducing valve on the main water line into the property i.e. straight after the stop-cock; is that still something I shouldn't have touched? I had originally intended to just replace the valve for a new one (the filter screen had broken on the old one) as that is suggested user maintenance in the instructions, but by far the cheapest way to get a new valve in the UK was to buy a whole new unit and when it arrived, I couldn't for the life of me get the new valve out, so resorted to replacing the whole unit.
But either way, the issue is with the HW cylinder and not the pressure regulator and I'm best to get a G3 plumber in?

 

[Peterdevon]

As said above it is either the pressure vessel or the bubble you must get a G3 plumber to do this work

 

[Stigster]

You are fine to work on any of your cold stuff and also the hot distribution side of the cylinder, just not the cylinder itself.

It was this bit I've copied and italicised below that led me to think you had taken off, cleaned and then replaced one of the safety devices on the cylinder itself, rather than your pressure reducing valve on the cold main.

"the HW cylinder next to it starts leaking from the 6 bar relief valve (which leaks onto the garage floor as the previous owners have cut through the copper pipe, by the looks of it because it was blocked up with limescale!).

I took the valve out and cleaned it as suspect it probably hadn't been done by the previous owners in 10 years and it was a bit gunged up but once refitted I had the same issue"

Sorry I got that wrong but I hope you understand why I read it that way.

Now, back to the problem. Your hot water cylinder will (or should) have a 3 bar pressure reducing valve built in to the combination valve, where the cold feed first arrives at the cylinder so any increase in pressure on the cold mains should not increase the pressure in the cylinder beyond 3 bar. Obviously it is going over pressure and discharging on to your garage floor. This is a separate issue to the PRV on your mains.

Either the pressure reducing valve on the cylinder has failed (stuck open or just disintegrated within the valve body) allowing the cylinder to come up to full mains pressure, or the most likely cause is a problem with the expansion vessel (or internal air bubble within the cylinder itself on some models) and this needs looking at.

It is also possible that a thermostat has failed and the cylinder is trying to boil and the Temperature and Pressure relief is opening because of that. That would usually be a bit more dramatic at the discharge pipe when it blows off so I don't think it's that.

You need a G3 plumber to reinstate the D2 Discharge pipe that you say was scaled up and cut off and to find the cause of the cylinder going over pressure and discharging.

 

[ShaunCorbs]

Unless they’ve put the main unvented pressure reducing valve on after the mains stop tap

 

[PeteCUK]

Unless they’ve put the main unvented pressure reducing valve on after the mains stop tap

The Caleffi pressure reducing valve is fitted immediately after the mains stop tap enters the house. From what I can tell the only water that bypasses this is one external cold water tap.

 

[John.g]

Don't know if you have (or require) a drop tight PRV....
The Caleffi 533841H doesn't specifically state that it is drop tight other than saying that it conforms to EN1567
The Honeywell D04 is not stated in their brochure as being Drop Tight whereas their D05 is stated as being Drop Tight,

 

[Stigster]

Drop tight? Not a term I've heard before. What does that mean?

 

[John.g]

A drop tight or static PRV will maintain the set point pressure even under no flow conditions, if not drop tight, a dynamic PRV, it will not and the pressure can creep up to the upstream pressure.
For example, in the above case, if one assumed a 24 litre E.vessel then the pressure would rise from 3 bar to 6 bar (if available) when ~ 10 litres have passed through the PRV, if no demand for water (say at night) then the PRV would only need to flow ~ 0.03 LPM (30cc/min) over a 6 hour period.

 

[Arfur Fuxaik]

Drop tight? Not a term I've heard before. What does that mean?

A simple explanation here,
All About Pressure Reducing Valves – Hidden Treasures | Reliance Worldwide Corporation (UK) Ltd. - https://www.rwc.co.uk/pressure-reducing-valves-hidden-treasures/

 

[Stigster]

I understand. Thanks for the explanation.

 

[John.g]

A simple explanation here,
All About Pressure Reducing Valves – Hidden Treasures | Reliance Worldwide Corporation (UK) Ltd. - https://www.rwc.co.uk/pressure-reducing-valves-hidden-treasures/

Caleffi also give a very good explanation but don't seem to state whether the OP's 536250 is/is not drop tight. One might expect that any PRV used in a "house" should be drop tight because of potential long periods of no usage.

 

[John.g]

Below, I am pretty sure is a link to the Caleffi drop tight version, if not happy with this or its description, then the Honeywell D05 is the alternative.

PresCal™, Pressure reducing valve (sweat) | Caleffi S.p.a.

www.caleffi.com

 

[PeteCUK]

Below, I am pretty sure is a link to the Caleffi drop tight version, if not happy with this or its description, then the Honeywell D05 is the alternative.

PresCal™, Pressure reducing valve (sweat) | Caleffi S.p.a.

www.caleffi.com

Thanks John, have also emailed Caleffi to ask them about the one I have in there already and see how drop tight they think that is. Otherwise I might try one of those linked.
Out of interest, is there any reason why someone wouldn't want a drop tight PRV for a domestic water system? Seems that if you want a specific pressure in your house it would be good to have that all the time, not just when the water is in regular use!

 

[John.g]

Yes, one might think that the static version would be the default but If only supplying taps etc then the dynamic version is probably fine as you mightn't even notice the difference as once you open the tap etc the pressure should drop instantly to 3 bar.
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I emailed Caleffi (UK) this am and got a very prompt reply. The 535 series is a static (drop tight) PRV as is also the 536, I also asked them could the downstream pressure creep under no flow conditions if the upstream pressure increased and they answered that there would have to be a very dramatic rise for this to occur.

Bearing the above in mind, I would ensure that there is no HW cylinder heating either from gas/oil coil or electric immersion (if fitted) and observe the pressure rise under no flow conditions for a hour or so, if the pressure does not rise then carry out the following to rule out any chance of a defective E.vessel, If you have a separate pressure gauge on the cylinder this would be a big help as you could then run off say 100 litres of "hot" water, then shut the cold feed at the cylinder, heat up the water to normal temperature and observe the pressure, it should not increase to > 4/4.5 bar assuming 3 bar initially and PRV pressure should remain at 3 bar. If no cylinder pressure gauge (only the one on the PRV) then after running off that 100 litres of hot water shut the stopcock, heat up the cylinder and observe the pressure at the PRV, even if you cannot shut the stopcock it will still tell something.

What type of HW cylinder do you have and has it got a external E.vessel, if so you might post capacities of both?.

 

[fezster]

Yes, if it's not drop tight, the static pressure could rise under no flow conditions.

Does the pressure only rise when the unvented cylinder is being heated? If so the EV needs charging (as said above).

You should also have a NRV between the cylinder and the balanced cold, to stop hot water expanding to the cold side. This might be part of your unvented combination valve.

 

[Stigster]

I had to ask what "drop tight" meant here and thankfully I got a good explanation but they could have saved me asking if they'd only called them "Raise tight"!

 

[John.g]

Drip tight maybe?. Anyway attached are some nice diagrams courtesy of Caleffi describing PRV operation.

 

[PeteCUK]

Thanks for the continued help folks. Reassuring to know the PRV I have on is drop tight so can stay on there. I've now got the HW cylinder off and am keeping an eye on the water pressure. The rise appeared intermittent so it will probably be a few days before I can pinpoint the heated cylinder as the culprit.

 

[PeteCUK]

Well that would seem pretty unequivocal; hot water turned off all weekend pressure rocksolid at 3 bar. Turn hot water on go back in to garage a bit later and floor is wet and pressure up at 4 bar (though we've been using water, so who knows what it has been up to. Seems expanding water in HW cylinder would seem to be the culprit.

 

[PeteCUK]

Finally got a plumber in - he pointed out (confirmed by me with Ferroli) that the expansion tank is only 12L vs the min 18L that it should be so he's going to change that and reinstate the D2 pipe tomorrow. Hopefully it will be sorted then!

 

[John.g]

12 litre is certainly too mean even though theoretically it would accomodate the expansion from (nearly) a 300 litre HW cylinder heated to 60C and the pressure should still be less than 6 bar. Ideally IMO the pressure rise should be limited to 1 to 1.5 bar, say 4 to 4.5 bar in your case.
As a matter of interest, what is the capacity of your cylinder and has the pressure always risen to 6 bar since installation?.

 

[PeteCUK]

12 litre is certainly too mean even though theoretically it would accomodate the expansion from (nearly) a 300 litre HW cylinder heated to 60C and the pressure should still be less than 6 bar. Ideally IMO the pressure rise should be limited to 1 to 1.5 bar, say 4 to 4.5 bar in your case.
As a matter of interest, what is the capacity of your cylinder and has the pressure always risen to 6 bar since installation?.

Cylinder is 250 L Ferroli Aquacyl. We've only been in the property a year so don't know how long the pressure has been increasing, but judging by the fact they had cut through the discharge pipe which is almost blocked off due to limescale I'd say it has probably been reaching 6 bar for much of its 10 years since installation.

 

[John.g]

The 250/18 litre combination should result in a final pressure of 4.25 bar @ 60c & 4.87 bar @ 70C so should be fine. The 12 litre E.vessel results in final pressures of 5.2 bar @ 60C & 6.7 bar @ 70C (T&P valve discharge). Maybe consider changing the T&P cylinder valve as well but if its not dripping now at 3 bar probably OK.