Cold water flow problem has stumped 3 plumbers...can you help?!

[Alexwales]

Thanks, I've tried both of your suggestions John, no difference. N.B hot water has remained fine throughout.

The (almost) pattern that has formed is, flush the upstairs toilet and it seems to charge/kickstart the flow downstairs, which stays on until you shut the tap, when the flow stops again.

 

[John.g]

Extraordinary. I have a outside PG and took a few readings just now, D/stairs toilet, normal pressure 3.55bar, 2.85bar after flushing, pressure rose briefly to 4.8bar when fluidmaster filling valve closed, then reduced to 3.55bar. U/stairs toilet with conventional B/cock, no "kickstart", 3.55bar/2.8bar/3.55bar.

 

[Last Plumber]

Our understanding is yes, it does rise to the top before heading back down.

I don't know what the mains pressure is, but one of the first things we did was get welsh water in and they checked it and said the flow was good into the house.

As far as I know the pressure is OK, none of the other technicians have been concerned about it, it seems adequate enough to flow well on the top floor and fill the cylinder as normal.

HOWEVER - the plot thickened this morning - I woke up and went to the kitchen and for the first time in 5 weeks the cold taps were suddenly running at 80/90% power. I left them running for 4/5 mins, no stop in flow. I went the the bathroom next door, the taps worked. I left the tap running and flushed the toilet, the tap slowed to about 30/40% but kept going.

I had to pop out and when I came back, it was still working at this slightly reduced level. So I went around and checked everything that hadn't worked and it was working. 30 mins later, it's all stopped again.....

So this slightly dispels the blockage theory and seems much more a pressure issue, right? I suppose there could still be a faulty PRV somewhere behind a wall, but it seems unlikely it would just randomly start working again?

I would suggest you buy a pressure gauge and take the pressure.
It is possible with the height to the top floor and frictional resistances through pipework, valves, governors and NRV's etc, that you might struggle to get decent flow, especially at peak times.

e.g. If the top floor pipework was say 6 meter above ground, you've already lost 0.6 bar and things like NRV's can lose another 0.1 - 0.3 bar. My thinking is that if you had just over 1 bar incoming, it might struggle. Water authority would look at that pressure as acceptable.

I know there is a descending column of pipework that will help overcome pressure issues in some cases but it depends on the route and size of pipework as well as its component parts.

I am not saying this is the answer but with the fluctuations you have mentioned, I think it needs to be ruled out before you go any further.

 

[Alexwales]

Thank you for all you advice on this - i'm glad to report the issue is finally solved.

Our plumber, through a process of elimination, found the water stopped flowing in a short vertical section of piping from the spare bedroom, down into the main bathroom a floor below.

He cut a hatch in the floorboards and sent some smaller piping down the 'blocked' pipe until it came out the other end...with it came a flush of trapped water, but strangely no obvious foreign body (that we found) and no resistance met (rouge valves) - and that solved the issue.

So we were still left a little stumped as there was no obvious block, but the issue is now finally gone.