Adjusting the PRV to go to 4 bar..?

[Mark K]

Can someone please explain to me why plumbers do not understand how flow is a consquence of pressure & bore? Keep the bore the same & increase the pressure & flow increases. Similarly, increase the bore & keep. Pressure the same & flow increases.

The point of a pressure reducing valve is to provide a maximum pressure with 'unduly' limiting flow. All PRVs impeed flow somewhat.

In terms of increasing flow in this instance 'bouncing out of the sink' is zero indication of anything. The only real test is a weir gauge or physically timed test. The real test is looking at the flow rate of a second outlet.

I hope this basic information helps people.

Agreed, a restrictive PRV will allow the pressure to build while there is no use, but not allow enough flow to "replenish" that pressure when there is a draw.

A 15mm supply and PRV could get a 6" main in a factory up to pressure given time, but obviously could not flow enough to cope with a 6" valve being opened.

Flow measurement at the tap and pressure readings before and after the PRV and just before the tap while it's running would help identify the issue. i.e. incoming pressure not maintained, pressure not maintained through the PRV or pressure loss in the system to the tap. Obviously, the rated pressure at the tap maintains the flow through it to atmosphere.

 

[64degrees]

If you want maxperformance from your combi give it a dedicated ( first tee from incoming main) 22 mm feed and distribute the hot water to the first outlet in 22mm pipe. There’s a flow restrictor in the boiler, but if you give all the water it should perform as per manufacturers specs - unless they maybe talk them up a bit? Didn’t read the whole the thread, think you mentioned a combi store I would have fitted an Atag

 

[dan_the_plumber]

Also going above 3bar can impact other things such as the boiler from a safety perspective

Hi Riley. Sorry are we talking about the heating or DHW circuit here?

 

[Riley]

As it’s a stored combi then I’m guessing it doesn’t matter. I was equally confused

 

[Old Account]

No never , you might well be creating a pressurised bomb
Combi set ups operate at 1.5 bar
And no more ever. There have been accidents ! With over over pressurised unvented systems
I know it’s my specialist area
Centralheatking

What bomb are you talking about mate? In Europe we have 5 bar mains incoming cold water and have combi boilers installed never encounter any time bomb hahaha don’t get your point to be fair

 

[Riley]

Are they not reduced before entering appliance though?

 

[dblvkrb]

Small update:

Static pressure is 4 bar, dynamic pressure (read from PRV at mains) is 2.4 bar when 1 tap is running and 1.5 bar with 2 taps running. Is this normal?

I still intend to dial up the static pressure and test the flow with a Weir gauge, but likely won't be able to test for a week or so.

 

[John.g]

Small update:

Static pressure is 4 bar, dynamic pressure (read from PRV at mains) is 2.4 bar when 1 tap is running and 1.5 bar with 2 taps running. Is this normal?

I still intend to dial up the static pressure and test the flow with a Weir gauge, but likely won't be able to test for a week or so.

You would need to take some more readings as Mark K suggested.
In the very first post you said that you renewed the old lead piping in 25 mm and that you then installed a PRV because the pressure was/is too high. Assuming the mains pressure hasn't changed then the static head (without PRV) will be exactly the same as before so no danger of over pressures in the combi as it will be rated for a maximum static pressure?, so why not remove the PRV and fit simple restrictors to your taps or just don't open them fully.
For interest only, I have 2 x 1/2 ins kitchen taps, one flows 10 LPM from a static mains pressure of ~ 2.5/2.8 bar at the tap, the other flows 5 LPM from the attic tank with a static head of ~ 0.6 bar.

 

[dblvkrb]

I know more tests need to be run and I will run them and report back.

I agree we could run the mains at full pressure and put restrictors on at each outlet, but I believe the plumber has gone the other way around by simply reducing all incoming pressure, which isn't ideal.

 

[Mike wareham]

Small update:

Static pressure is 4 bar, dynamic pressure (read from PRV at mains) is 2.4 bar when 1 tap is running and 1.5 bar with 2 taps running. Is this normal?

I still intend to dial up the static pressure and test the flow with a Weir gauge, but likely won't be able to test for a week or so.

 

[Mike wareham]

How long is the new leingth of 25mm plastic pipe? does it go to to a stoptap in the path or road? Is this stoptap turned on fully? sounds to me like there is a restriction somewhere, the new PRV in the house has a filter is this clean, you also have a stoptap in the house is this blocked with debris,as pressure is droping when there is a greater demand

 

[dblvkrb]

How long is the new leingth of 25mm plastic pipe? does it go to to a stoptap in the path or road? Is this stoptap turned on fully? sounds to me like there is a restriction somewhere, the new PRV in the house has a filter is this clean, you also have a stoptap in the house is this blocked with debris,as pressure is droping when there is a greater demand

25mm pipe is maybe 4 meters long, yes it does go to a stop tap in the road which is fully open, the stopcock in the house is fully open.

Why do you think there is debris - should I not be losing as much pressure when opening a tap/2 taps from mains pressure?

 

[Mike wareham]

25mm pipe is maybe 4 meters long, yes it does go to a stop tap in the road which is fully open, the stopcock in the house is fully open.

Why do you think there is debris - should I not be losing as much pressure when opening a tap/2 taps from mains pressure?

I would expect about 20l/min from the kitchen cold tap at that pressure,and if the mains pipe from the road is only 4M away not a great deal of flow would be lost, does your new 25mm pipe go to the main in the road or is it joined to the exhisting smaller diameter pipe somewhere, ( was it connected by the water company) ?

 

[Mike wareham]

I would expect about 20l/min from the kitchen cold tap at that pressure,and if the mains pipe from the road is only 4M away not a great deal of flow would be lost, does your new 25mm pipe go to the main in the road or is it joined to the exhisting smaller diameter pipe somewhere, ( was it connected by the water company) ?

Is it a shared supply that feeds other houses?

 

[John.g]

I would expect about 20l/min from the kitchen cold tap at that pressure,and if the mains pipe from the road is only 4M away not a great deal of flow would be lost, does your new 25mm pipe go to the main in the road or is it joined to the exhisting smaller diameter pipe somewhere, ( was it connected by the water company) ?

I seem to remember that my kitchen taps have a flow coefficient (Kvs) of something like 0.4 M3/h @ 1 bar which equates to 6.7 LPM @ 1 bar, if the OPs taps are similar then it would require 5 bar to get his expected 15 LPM, his reading of 2.4 bar (one tap) would suggest a flow rate of ~ 10.4 LPM (easy to measure this with a empty milk bottle) the 1.5 bar reading (two taps) would suggest a combined flow rate of ~ 16.4 LPM. The PRV will have its own Kvs which affects flow rate as well plus the differential pressure across it will have a big effect on its output. This Kvs is not known but the manufacturers should certainly know it.
I would suggest (as has been suggested in other posts) increasing the PRV outlet pressure to max with both taps open and then measure both tap flow rates, I would also suggest cleaning the PRV strainer if only to rule it out.
Depending on the above test, to really narrow down the problem I would replace (temporarily) the PRV with a spool piece and repeat the measurements, that will tell a lot IMO.

 

[dblvkrb]

Is it a shared supply that feeds other houses?

The 25mm pipe goes directly to the mains (someone from the water board did the job). It is not a shared supply.

 

[dblvkrb]

I seem to remember that my kitchen taps have a flow coefficient (Kvs) of something like 0.4 M3/h @ 1 bar which equates to 6.7 LPM @ 1 bar, if the OPs taps are similar then it would require 5 bar to get his expected 15 LPM, his reading of 2.4 bar (one tap) would suggest a flow rate of ~ 10.4 LPM (easy to measure this with a empty milk bottle) the 1.5 bar reading (two taps) would suggest a combined flow rate of ~ 16.4 LPM. The PRV will have its own Kvs which affects flow rate as well plus the differential pressure across it will have a big effect on its output. This Kvs is not known but the manufacturers should certainly know it.
I would suggest (as has been suggested in other posts) increasing the PRV outlet pressure to max with both taps open and then measure both tap flow rates, I would also suggest cleaning the PRV strainer if only to rule it out.
Depending on the above test, to really narrow down the problem I would replace (temporarily) the PRV with a spool piece and repeat the measurements, that will tell a lot IMO.

This is interesting, I thought the indoor tap nozzles would be restricting flow in some manner despite there not being separate flow restrictors in place and I have readings similar to yours on the l/m side. Though I haven't measured it yet, I believe the garden tap is putting out more l/m.

100% agree with you and others who have mentioned I need to test by increasing the pressure or replacing the PRV (or both), hopefully I can get the plumber in this week to talk through and test.

 

[dblvkrb]

Conclusion..

We tested the garden tap and it was putting out >20 l/m, the plumber explained that it is the 12mm flexi pipes that came with the tap sets are restricting the flow a great deal. We tested some of the shower taps with and without the nozzles attached and we measured a difference of about 3-4 l/m (going up to 16 l/m).

That being said the plumbing of the boiler wasn't completely finished, with the blending valve being installed a couple of weeks back. The results are a bit odd. Most of the taps still output at 10-12 l/m, however, the bath taps output at near 20l/m with both taps fully open, the shower output is at 16/m which is certainly good enough for what we need.

I'm not sure why there is such variance but it's worked out for the best, I can only assume it's the type of connections used for each tap combined with the fact that the boiler is now a great deal more efficient now that the blending valve is in. We're still at 4 bar of standing pressure.

 

[John.g]

Conclusion..

We tested the garden tap and it was putting out >20 l/m, the plumber explained that it is the 12mm flexi pipes that came with the tap sets are restricting the flow a great deal. We tested some of the shower taps with and without the nozzles attached and we measured a difference of about 3-4 l/m (going up to 16 l/m).

That being said the plumbing of the boiler wasn't completely finished, with the blending valve being installed a couple of weeks back. The results are a bit odd. Most of the taps still output at 10-12 l/m, however, the bath taps output at near 20l/m with both taps fully open, the shower output is at 16/m which is certainly good enough for what we need.

I'm not sure why there is such variance but it's worked out for the best, I can only assume it's the type of connections used for each tap combined with the fact that the boiler is now a great deal more efficient now that the blending valve is in. We're still at 4 bar of standing pressure.

Shower heads are designed to give a nice spray effect so there is a considerable pressure drop across them so no really big surprises there. You are getting 20 LPM from a bath tap and 10 LPM from the other taps, my bath taps are 3/4", all the remaining taps are ~ 1/2"or even less (metric), my upstairs bath tap flows 12 LPM @ gravity head of 0.25/0.3M, the upstairs bathroom basin tap flows ~ 4 LPM @ the same head.

All in All I think you should be happy enough.

Did you observe the PRV pressure with a flow rate of say 20 LPM?, even assuming a mains PRV upstream pressure of 6/8 bar one would expect a "drooping" in the PRV pressure of (depending on its type & quality) 0.25 bar to 0.5 bar giving 3.75/3.5 bar? @ 20 LPM.

 

[dblvkrb]

Shower heads are designed to give a nice spray effect so there is a considerable pressure drop across them so no really big surprises there. You are getting 20 LPM from a bath tap and 10 LPM from the other taps, my bath taps are 3/4", all the remaining taps are ~ 1/2"or even less (metric), my upstairs bath tap flows 12 LPM @ gravity head of 0.25/0.3M, the upstairs bathroom basin tap flows ~ 4 LPM @ the same head.

All in All I think you should be happy enough.

Did you observe the PRV pressure with a flow rate of say 20 LPM?, even assuming a mains PRV upstream pressure of 6/8 bar one would expect a "drooping" in the PRV pressure of (depending on its type & quality) 0.25 bar to 0.5 bar giving 3.75/3.5 bar? @ 20 LPM.

I haven't read the drop off since my last test - suspect it is going down to ~2bar with 20l/m coming out. The plumber was nervous about going up to 5 bar to test as the fittings are only rated to 3bar. I now see little value in testing an increase in pressure given the l/m had increased where needed.