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Discuss Can water flow rate be measured at the supply pipework? in the DIY Plumbing Forum area at PlumbersForums.net

Messages
87
Hello Guys

I recently posted a thread about low water pressure at taps in a new kitchen installed by a company using sub-contractors. I have 0.44 bar of hot water pressure and am getting a flow rate of 4.5 Lpm at the new hot monobloc. The Blanco people say that the taps are rated at minimum 0.3 bar and that a 0.3 bar head should deliver 7.9 Lpm. At my 0.44 bar I should be getting 11.6 Lpm, over twice as much.

The Company are saying that 0.3 bar is the lowest pressure tap they use. That doesn’t seem to me to address the issue. Does it? The overhead supply mains water supply goes on a further 3 metres to supply the Utility Room monobloc (combined hot and cold flow-rate of 25 Lpm) and the central heating boiler. Yet the combined hot and cold flow rate in the new kitchen monobloc is 14.5 Lpm. It seems that both hot and cold are throttled back.

A clogged aerator has been identified as a possible problem and 15mm service valves are installed. Would ‘full-bore’ service valves help? Also is it possible to measure the flow rate at the tails of the pipework in both kitchen and utility as Company is saying the Utility monobloc could be a lower minimum bar rating.

Any advice in any of these areas will be gratefully received as the Kitchen Company are beginning to fade away with the final comment of “We don’t do taps of less than 0.3 bar” without offering further investigation.
 
Messages
87
Hello Guys

I recently posted a thread about low water pressure at taps in a new kitchen installed by a company using sub-contractors. I have 0.44 bar of hot water pressure and am getting a flow rate of 4.5 Lpm at the new hot monobloc. The Blanco people say that the taps are rated at minimum 0.3 bar and that a 0.3 bar head should deliver 7.9 Lpm. At my 0.44 bar I should be getting 11.6 Lpm, over twice as much.

The Company are saying that 0.3 bar is the lowest pressure tap they use. That doesn’t seem to me to address the issue. Does it? The overhead supply mains water supply goes on a further 3 metres to supply the Utility Room monobloc (combined hot and cold flow-rate of 25 Lpm) and the central heating boiler. Yet the combined hot and cold flow rate in the new kitchen monobloc is 14.5 Lpm. It seems that both hot and cold are throttled back.

A clogged aerator has been identified as a possible problem and 15mm service valves are installed. Would ‘full-bore’ service valves help? Also is it possible to measure the flow rate at the tails of the pipework in both kitchen and utility as Company is saying the Utility monobloc could be a lower minimum bar rating.

Any advice in any of these areas will be gratefully received as the Kitchen Company are beginning to fade away with the final comment of “We don’t do taps of less than 0.3 bar” without offering further investigation.
Update. The Company are saying that the tap manufacturers report a combined hot and cold flow rate of 7.9 Lpm for their 0.3 min pressure taps. I am getting 14.5 Lpm combined and the implication is that I should be happy with that. But surely that figure of 7.9 Lpm combined flow-rate must be when both hot AND cold are at 0.3 bar. My pressures are hot 0.44 bar and cold (guesstimate) 1.5 bar. Is it even possible to get a flow-rate of 7.9 Lpm at those pressures at 0.3 min bar taps?
 

Ric2013

Plumber
Messages
3,133
Have you considered loss of head due to flow?

Many things can reduce the working head/pressure. Examples include limescale in connections to the cylinder, long runs of 15mm pipe, traditional stopcocks, and narrow-bore isolation valves. Gate valves and full bore isolation valves do not. That 0.3 bar Blanco is discussing will be the working pressure, not the standing pressure.

You might have 0.5bar standing head (I think you said your cistern is 5 m above the tap) but that pressure will drop when you open the tap. Probably the tap is rated at 0.3bar working head. This is the pressure you would see at a manometer installed on the pipework immediately before the tap with the tap open and therefore very different from the standing pressure.

Also is it possible to measure the flow rate at the tails of the pipework in both kitchen and utility as Company is saying the Utility monobloc could be a lower minimum bar rating.
If you have a water meter, it is very easy to measure flow. Check nothing is flowing first, and then open the relevant tap and see how fast the numbers go round. But as the flow into the tap must equal precisely the flow out of the tap, the bucket method you have presumably used up to now should suffice.

But surely that figure of 7.9 Lpm combined flow-rate must be when both hot AND cold are at 0.3 bar.
Agreed.

Is it even possible to get a flow-rate of 7.9 Lpm at those pressures at 0.3 min bar taps?
Hard to say. The manufacturer may be basing the flow on a 67% hot, 33% cold mix. As the relationship between flow and pressure is non-linear, it is hard to say what the tap will do with mixed pressures, particularly if it is a thermostatic mixer. At a guess, I'd suggest the tap flow would = half of the stated combined flow at the higher pressure + half of the stated combined flow at the lower pressure. That said, if we do not know the actual woking pressures (as above) then this is a bit hypothetical.
 
Messages
87
Have you considered loss of head due to flow?

Many things can reduce the working head/pressure. Examples include limescale in connections to the cylinder, long runs of 15mm pipe, traditional stopcocks, and narrow-bore isolation valves. Gate valves and full bore isolation valves do not. That 0.3 bar Blanco is discussing will be the working pressure, not the standing pressure.

You might have 0.5bar standing head (I think you said your cistern is 5 m above the tap) but that pressure will drop when you open the tap. Probably the tap is rated at 0.3bar working head. This is the pressure you would see at a manometer installed on the pipework immediately before the tap with the tap open and therefore very different from the standing pressure.


If you have a water meter, it is very easy to measure flow. Check nothing is flowing first, and then open the relevant tap and see how fast the numbers go round. But as the flow into the tap must equal precisely the flow out of the tap, the bucket method you have presumably used up to now should suffice.


Agreed.


Hard to say. The manufacturer may be basing the flow on a 67% hot, 33% cold mix. As the relationship between flow and pressure is non-linear, it is hard to say what the tap will do with mixed pressures, particularly if it is a thermostatic mixer. At a guess, I'd suggest the tap flow would = half of the stated combined flow at the higher pressure + half of the stated combined flow at the lower pressure. That said, if we do not know the actual woking pressures (as above) then this is a bit hypothetical.
That information has been really helpful Ric2013, particularly the confirmation that his 7.9 Lpm from Blanco must be based on a head of 0.3 bar for both hot AND cold and that the company can not therefore rest on that as an expectation as I have 0.44 and Mains pressure (estimate 1.5 to 2.0 bar).

The taps are not thermostatic but I see your point about how Blanco might configure the mix.

The (actually very good) company are going to fit full-bore service valves to replace the standard service valves.

Much obliged.
 
Messages
87
How have you measured or calculated 0.44 bar?
Hi Ric2013

I added together the following measurements:

tap outlet to ground floor ceiling - first storey floor to ceiling - loft joists to bottom ow CW tank. I then added in 10cm for first-floor ceiling void and a further 17cm for ground-floor ceiling void (inc plasterboard and floor decking. I came out with approx 4.41m

MDLJK
 
Messages
1,760
Update. The Company are saying that the tap manufacturers report a combined hot and cold flow rate of 7.9 Lpm for their 0.3 min pressure taps. I am getting 14.5 Lpm combined and the implication is that I should be happy with that. But surely that figure of 7.9 Lpm combined flow-rate must be when both hot AND cold are at 0.3 bar. My pressures are hot 0.44 bar and cold (guesstimate) 1.5 bar. Is it even possible to get a flow-rate of 7.9 Lpm at those pressures at 0.3 min bar taps?
Just bear in mind that flow is proportional to the sq.root of pressure so the increase in flow at 0.44bar from 0.3bar is a by a factor of 1.21 and not 1.47.
One would think that valves that have internal mixing (either thermostatic or manual) would tend to mix at the lower pressure, or possibly like explained in post #3 above, you might just set your mixer to your normal temperature, measure the individual flowrates with hot/cold isolated, add them and compare it with your actual combined flow and that will give a interesting comparison.
 

Ric2013

Plumber
Messages
3,133
Hi Ric2013

I added together the following measurements:

tap outlet to ground floor ceiling - first storey floor to ceiling - loft joists to bottom ow CW tank. I then added in 10cm for first-floor ceiling void and a further 17cm for ground-floor ceiling void (inc plasterboard and floor decking. I came out with approx 4.41m

MDLJK
Grand. Yes, that's your standing pressure, not your working pressure.

It's occurred to me that you could get an idea how much resistance there is in the pipework by seeing what happens if you try to run your kitchen tap at the same time as your utility room tap. From your previous comments, I suspect the loss of head (difference between standing and working pressures) may not be much, but if you run, say 5 lpm from your utility room tap and the kitchen tap almost ceases to flow, that would tend to indictate that there is a restriction in the pipework.
 
Messages
87
Grand. Yes, that's your standing pressure, not your working pressure.

It's occurred to me that you could get an idea how much resistance there is in the pipework by seeing what happens if you try to run your kitchen tap at the same time as your utility room tap. From your previous comments, I suspect the loss of head (difference between standing and working pressures) may not be much, but if you run, say 5 lpm from your utility room tap and the kitchen tap almost ceases to flow, that would tend to indictate that there is a restriction in the pipework.
Hi. I tried this and bringing on the Utility Room hot water did not seem to greatly effect the flow rate of the kitchen hot water. Kitchen is 4.0 Lpm Utility is 7.5 Lpm. The new kitchen install was done completely in plastic apart from final part for service valves etc. Plumber tee-d off supply in loft (single storey offshoot) took it down then wall, along wall before rising up. He has taken the horizontal run ‘back’ along the direction of flow in loft above which did disappoint me but it has to be no more than 1.2m/4 ft. So thanks for this. You’ve proven that the new pipework is not restricted.
 
Messages
87
Just bear in mind that flow is proportional to the sq.root of pressure so the increase in flow at 0.44bar from 0.3bar is a by a factor of 1.21 and not 1.47.
One would think that valves that have internal mixing (either thermostatic or manual) would tend to mix at the lower pressure, or possibly like explained in post #3 above, you might just set your mixer to your normal temperature, measure the individual flowrates with hot/cold isolated, add them and compare it with your actual combined flow and that will give a interesting comparison.
Weird - separately hot is 4Lpm and cold is 12Lpm. Combined DOES NOT give 16 Lpm as might be expected but rather it gives 9.5 Lpm. How can that happen?
 
Messages
1,760
Don't know really but assuming hot water at 60C, cold at 10C and mixed at 40C then the mix is 0.6hot to 0.4cold = 5.7hot & 3.8cold (9.5 LPM), so available cold is almost all used as expected and the hot is throttled to give 5.7 LPM but strange that it flows 12 LPM while still in the throttled state with just the back pressure of the cold removed, assuming tested in this manner.
 

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