Water pressure too high even after PRV

[Stealthdude]

Honeywell Home Pressure Reducing Valve 1.5-6bar 15mm x 15mm - Screwfix

Order online at Screwfix.com. Lightweight, simple construction. Designed to protect household appliances from excessive supply pressures regardless of fluctuating inlet pressure. Can reduce water consumption and damage from over pressurisation. 1.5-6bar adjustable outlet pressure. Supplied with...

www.screwfix.com

That's the prv which is installed straight after the stopcock, the relief valve is installed after the prv outside about 1 meter after in the car port just above a drain.
Just had a mate here who is a plumber for the council, he thinks it's strange the relief valve isn't working and can't believe how high the pressure is. He thinks it's because we live half way down a steep bank with a village at the top. However he does suggest the EV may work so will try that next.

 

[Johntheo5]

You have a Honeywell D04FS, its OK for 16bar inlet pressure but don't know if its drop tight or not, the Honeywell D06FM is drop tight but so too was the Reliance Predator, good for 16bar (and drop tight).
If whatever PRV is installed doesn't keep the pressure to 5bar with no DHW demand and it rises to 8bar instantly after closing a Hot tap then a EV is useless as its ony designed to accomodate the expansion.

You said in your first post that "when checking the pressure on the PRV in the morning it often says it is over 8 bar (off the scale).", this would have been with the Reliance Predator installed? so did you notice if the pressure stayed at 5bar after drawing off HW?, if so, then if your boiler has DHW preheat and without a EV the pressure could certainly rise to 8bar overnight.
Suggest you reinstall that Reliance PRV or buy a new one, or a Honeywell D06FM

Is 8bar written on the end of the pressure relief valve?

What is the make/model of your boiler?.

 

[Madrab]

However, when checking the pressure on the PRV in the morning

I presume this is after the PRV was set the day before and the cold mains has been sitting in the pipework overnight and the CH has been on? You may or may not know if the water pipework runs alongside the CH pipework? If so then it could quite possibly be expansion.

One way to test, set the mains pressure to 3 bar the night before and then shut off the mains, then check the pressure in the morning, if it has risen above 3 bar then it can only be expansion (unless the mains tap is compromised of course)

 

[Stealthdude]

I don't think this is a CH problem as we never have the heating on overnight, we switch it on manually so pipes are not warm etc in the morning. I think it's solely back pressure from the boiler when the hot tap is turned off quickly and I just assumed it happened overnight. The pressure drops to about 3 bar when running hot or cold water it only 'bounces' up to 8 bar when when the hot tap is turned off quickly, cold tap is fine.
The exact same issue was happening with the predator valve, that's why I changed to the honeywell.
I'm going to get a 3 bar relief valve and see if the at least relieves the pressure.

 

[Johntheo5]

So, if you open a cold tap then the pressure will fall to ~ 3.0bar and will remain there, or certainly will not rise higher than say 5bar for a considerable time but bounces back up to 8bar when a hot tap is opened and then closed?.

 

[Madrab]

I'm going to get a 3 bar relief valve and see if the at least relieves the pressure.

I wouldn't look at that as an option, it's really not designed for that. It will release the pressure of course, if all set to say 3bar but not a usual approach.

Could hydraulic shock create that jump in pressure? I guess so, very rare though, not something I've ever encountered and I have worked on a good number of 'high' pressure systems. Usually check valves (non slam) would be the first thing I'd look at.

Not something that would be expected just on the hot system either, it would normally be both unless there is something quite specific to the hot.

 

[Stealthdude]

The 3 bar relief valve has sorted it but as you say not the best approach, but at least the system is within the limits now. Can't understand why it is just the hot tap that causes it, to me suggests the boiler is causing it somehow.

We have a boiling water tap (Qettle) which states a max of 5 bar and we have had a hose blow off that. Fortunately we were in and damage was minimal but I've been concerned if it happened again while we are out that would be a different story.

Maybe the hillside location of our street is a factor aswell, pressure during flow is fine, flow rate is adequate but not huge as you'd expect with such high pressure.

 

[Johntheo5]

Water hammer can happen in mysterious ways, it certainly seems that shutting off a hot tap is causing this, for whatever reason, so the installation of a shock arrestor (EV) can only help, maybe install it close to the boiler?.

 

[Mr.B]

Did it not, or is his issue expansion? All the predator valves are drop tight, that being said, could he also have got a bad one, always possible.

I was recently going to fit a 15mm Predator (model PRED800015) ... the one they sell in Screwfix with the green top for about £35.

It says in the Screwfix description that it's drop tight. But the tech dept say not. And the installer did apparently agree with them.

Long story short - I rang Reliance tech. dept. and was definitely assured that that model (and the 22mm equivalent) are NOT drop tight.

To add to that I recently had an installer round and the conversation got round to PRVs and I said that I had, just a few days earlier, ordered a Caleffi 535 series 15mm PRV (with no gauge included) to replace the old Caleffi 533 model which also was not drop tight and which I'd had for 17 or 18 years .... (I rang Caleffi technical and they also confirmed all this).

So the upshot is -
Reliance 312 series IS drop tight. The one in Screwfix IS NOT, despite the description saying it is.
Caleffi ... the 535 series IS drop tight and the Caleffii 533 series IS NOT
Honeywell ... the DO4 is not, the D05 and DO6 is.

As I say I paid £62 for a 535 valve and it had no gauge (you can't buy that model with a gauge) so I swapped the one from the old 533 over to it.
I now have a drop tight gauge after 17 years with one that did creep up .

The installer said that my info had been useful as he didn't know about drop tight valves and had been messing about replacing valves due to upwards creep without realising not all pressure reducing valves are the same.

 

[Madrab]

But the Predator details page doesn't say it is

Interesting and as @Johntheo5 posted >>>> straight from their website, in sections - overview & features and benefits

https://www.reliancevalves.com/sites/rv/files/2022-06/Reliance%20Valves%20Predator%20Pressure%20Reducing%20Valves%20Data%20Sheet.pdf

Whilst I wouldn't argue with their tech dept. - (edit) though it wouldn't be the first time a techies got it wrong (edit) - someone needs to tell them to update their data sheets then as that would normally be my go to reference for a valves specs and I'm sure SF won't be happy that they are mis-representing.

 

[Mr.B]

It had me confused, but that's not difficult. As mentioned I chose to go with a Caleffi 535 and it's holding nicely.