G3 Registered, replacement control valve blowing off

[Jones82]

I fitted a 15mm prv on the incoming mains today. The mains runs about 1m in 15mm before teeing off and rising in 22mm. Not sure if I mentioned that the unvented cylinder is in the loft.

Well the cold supply is 22mm to the loft however the rest of the house is fed from the same supply as the unvented. Now the customer is complaining of pressure / flow loss on the cold taps, outside tap and the hot flow is significantly reduced when the downstairs taps are running.

I've tried to explain that this is a design flaw of the original system but they are adamant that fitting a 22mm prv instead of 15mm will improve things.... Thoughts?

I'm thinking 22mm dcv on the hot outlet from the cylinder and remove the prv. I've also mentioned installing a new dedicated 22mm main to the loft then connecting the balanced cold up there to the cold circuit.... this may also cause issues.

The standing pressure on the prv is only 2.8bar but we think the mains creeps up over 3bar during the night which would result in different pressures on the hot and cold

 

[EvilDrPorkChop]

I fitted a 15mm prv on the incoming mains today. The mains runs about 1m in 15mm before teeing off and rising in 22mm. Not sure if I mentioned that the unvented cylinder is in the loft.

Well the cold supply is 22mm to the loft however the rest of the house is fed from the same supply as the unvented. Now the customer is complaining of pressure / flow loss on the cold taps, outside tap and the hot flow is significantly reduced when the downstairs taps are running.

I've tried to explain that this is a design flaw of the original system but they are adamant that fitting a 22mm prv instead of 15mm will improve things.. Thoughts?

I'm thinking 22mm dcv on the hot outlet from the cylinder and remove the prv. I've also mentioned installing a new dedicated 22mm main to the loft then connecting the balanced cold up there to the cold circuit.. this may also cause issues.

The standing pressure on the prv is only 2.8bar but we think the mains creeps up over 3bar during the night which would result in different pressures on the hot and cold

Main question is firstly, has it stopped your pressure relief from discharging?

 

[Jones82]

Main question is firstly, has it stopped your pressure relief from discharging?

Literally left a couple of hours a go but I think so yes

 

[SJB060685]

As above. First question is did it solve original problem?

 

[Chalked]

So you have found the problem.
its getting a back feed from somewhere. I would be looking at shower mixers.
fitting a prv, is just sorting the symptoms. Not the cure.

 

[EvilDrPorkChop]

So you have found the problem.
its getting a back feed from somewhere. I would be looking at shower mixers.
fitting a prv, is just sorting the symptoms. Not the cure.

Whilst you aren't incorrect I don't completely agree here, all supplies really should be balanced to showers/baths, and IMO most taps on a unvented system. So finding the back feeding culprit isn't really solving the issue completely as the pressures would still be unbalanced.

I know when retro fitting cylinders to existing houses you can't always do the balanced cold easily but then really a PRV should be fitted to the incoming main as the OP has done.

OP - The pressure decrease the customer is noticing - could any dirt have got in the PRV filter when you've fitted it or joint compound? 22mm might help with flow but then you really need to be making it a 22mm main too.

Really the main wants upgrading to 22mm to the unvented and then the balanced cold from the combination valve connecting into the rest of the cold supply. Know this probably isn't the easiest option.

 

[SJB060685]

As Chop says unvented cylinders require good incoming flow and pressure from supply outside of dwelling, quite often an unvented cylinder is installed, albeit by a G3 certified person but not always do they fully understand the requirements that need to be met in order to achieve the outcome that is expected. All outlets should be balanced, so the installation of a PRV as you did is not necessarily wrong, especially when 3 bar pressure is more than enough for most situations. I suspect the reduction in flow is down to inadequately sized supply piping, you may have good pressure but flow rates is largely down to size of pipes.