Low Cold Water Pressure Inside on Unvented Cylinder

[techsearchuk]

Hi,

I've been struggling with low water pressure inside my house for past 6 months...details as follows:

Flowrate measured in street = 30 liters per minute
Flowrate at outside tap = 24 liters per minute
Flowrate at kitchen tap = 8 liters per minute
Hot water pressure is decent and much greater than 8 liters.

I have an unvented cylinder that is probably 8 years old. The expansion vessel was replaced a couple of years ago.
I've had the stop-cock replaced as that was not fully opening or closing correctly but that has made no difference.

I'm trying to work out what might be causing the drop-off in flow rate/pressure of the cold water. Could it be the PRV on the unvented cylinder?

I'm content to get an engineer out but I'd really like to know first what kind of problem I am facing... if it is something to do with the internal water pipes, the unvented cylinder or something else?

Any thoughts or troubleshooting would be appreciated!

 

[Johntheo5]

Quite possibly the (normally) 3bar pressure reducing valve (PRV) on the cold water supply to the UV cylinder, could be a blocked strainer in this or a faulty PRV, there may be a tapping on the PRV to attach a pressure gauge.

 

[techsearchuk]

Thanks for the reply would this be the one?

 

[Madrab]

I think we need to understand a few thing first -

L/Min is an indication of amount of flow from an outlet or pipe
Bar/PSI is an indication of the pressure that pushes that water out of the outlet/pipe.

You are quoting flow figures so the fact that your kitchen tap is only flowing 8 L/Min does not indicate low pressure, all it indicates is that the flow from that outlet is low. That could be down to the design of the tap, whether it uses an aerator/flow saver, does it have flexi pipes and is there an issue there are there any restrictions at their ISO valves etc.

From the sounds of it, the district mains is fine, the outside tap (which usually is a full flow tap) is fine, your unvented HW (UV) is fine, so that indicates that the mains into the house and up to the UV is fine. What are the other cold outlets like, in the bathroom etc? Not sure if all the cold outlets will be fed from the balanced supply from the UV combination valve, sometimes that isn't the case.

With all of that though I would be looking at the kitchen tap and it's supply first.

 

[Rodders53]

@Johntheo Why would the Pressure reducing on Unvented hot water cylinder affect the cold water to anything other than the balanced shower outlet --- balanced tapping has been unused in both homes I have had unvented in (from original new build install)? (It's the blank black cap on that photo above).

Note to OP
Pressure and Flow are two separate things (although one affects the other) --- in electricity Voltage and Current are the equivalents.
You are only complaining of the flow.
(Beaten to that by @Madrab who is far more expert than I !!)

Does HW flow rate remain good for many minutes or drop off to the same slower flow rate as cold after a while?

 

[techsearchuk]

Hi, you're right my terminology is not quite right. I can only easily measure flow rate but in doing so I know that the hot water flow rate is as good as it has always been and I have no issues with that, even after running the hot tap for a while.

The flow rate from the outside tap is very good. The flow rate from the showers and taps inside is a lot less that the outside tap and a lot less than it used to be.

 

[Johntheo5]

Thanks for the reply would this be the one? View attachment 390550View attachment 390551

Yes, thats the one, the pipe going off to the right is the balanced cold, (after the PRV), the black plastic plug underneath it is where the expansion vessel is often teed off from. If the kitchen cold tap is supplied from this balanced cold and has a reduced flow from normal but the hot water supply is normal then yes, very unlikely to be the PRV (pressure reducing valve) although 8LPM from a kitchen tap is IMO pretty good.
Also, of course there is a relationship between flow and pressure but its not directly proportional, flow is proportional to the sq.root of pressure, if you assume that the pressure after the PRV is normally 3.0bar but for whatever reason has now fallen to 1.5bar, if it is restored to 3.0 bar then the flow through that cold tap, if supplied from the balanced cold, will not increase to 16LPM but will increase to 8*sqroot(3/1.5), 11.3LPM.

 

[techsearchuk]

That makes sense and I'd say the drop off in flow rate HAS probably been in the region of 25% so working from your figures that would be 12LPM down to 8 LPM. The PRV only replaced in December 2023.

Edited to add..

At the top of the grey PRV there is a small round back cap, I was expecting to see a calibration screw that could he adjusted? Does the black cap come off or is there a different way to adjust the pressure on this one?

 

[Madrab]

At the top of the grey PRV there is a small round back cap, I was expecting to see a calibration screw that could he adjusted

Not usually - the control group's (combination/multibloc valve's) PRV for the UV will be factory set to the UV's max inlet pressure, that should not be adjustable.

The point I would make is if everything is equal but there has been a noticeable change to the cold outlets (you say the hot outlets haven't changed?) starting 6 months ago then something must have changed. If everything else is the same but it's just the cold then I would look at that those outlets locally and work back from there. If the UV hasn't changed then anything fed from the balanced cold should be the same, equally unaffected, as there is nothing after the PRV that should then restrict the balanced cold supply.

 

[techsearchuk]

I decided to try and get used to the drop in flow/pressure but unfortunately it seems to have reduced slightly again. I think I'll get the unvented cylinder serviced to see if that identifies anything. Any other thoughts or advice in the meantime are welcome!!

 

[Johntheo5]

I would get a isolating valve and pressure gauge installed where that black plug is and leave it permanently installed, as a first step.

 

[Madrab]

If the HW pressure and flow is fine then it's nothing to do with the unvented. As the unvented is controlled by the combination valve (CV) and if that was at fault then the HW would also be effected.

I would probably be looking more towards the kitchen tap and the pipework @ that location rather than anything to do with the BS. Though the BS could easily be tested though by isolating the CV, disconnect the BS pipe and then connect a test piece of 22mm with an elbow @ the end and check the flow into a bucket @ that point.

If all is good there then I'd be looking at the tap (may have an flow limiting aerator fitted) and also check its supply.