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?

 

[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.

 

[PeteCUK]

Thanks for crunching those numbers, looks promising that a change in expansion vessel tomorrow should to the trick. He is going to change the T&P cylinder valve when he reinstates the D2 also - either way by close of play tomorrow the shouldn't be leaking on the garage floor!

 

[PeteCUK]

So the plumber has been in and replaced the expansion vessel which was shot by the sounds of the water swilling around both sides of it. He also made some other changes which I'd be interested to know your thoughts on.
Rather than fitting the expansion vessel to the top, he has tapped into the cold water supply line (for the whole house system) and fitted the expansion vessel for that. The logic was that it is better for the seals in the expansion vessel having it on the cold water side than the hot. I'm guessing that shouldn't make much difference as without a one-way valve the expansion vessel was provided space for the whole hot and cold water system anyway (hence why the pressure was going through the roof?).
Second one is rather than cap off the expansion vessel outlet on top (not required now it is fitted on the cold side) he has instead capped off what was previously (and is marked as such in the boiler manual) the hot water supply line and supplied the system from the expansion vessel fitting at the top. Any issue with either??

 

[John.g]

Re position of E.vessel, in either position, if it fails, 4 to 5 litres of water will discharge through the PRV each time the cylinder is heated from cold.
Hot water take off? just wonder if Ferroli have a specific reason for taking it from the side.