Can water flow rate be measured at the supply pipework?

[Mdljk]

Can yous sketch us the pipework arrangement? I've done a sketch of how I assumed it was, based on your initial post, but sounds like I got the wrong impression. Apologies for frankly appalling sketch (and appreciate I've shown hot and cold taps as separate for diagramatic purposes: I am aware what you have are mixers).View attachment 60349

 

[Mdljk]

View attachment 60351

Here is my truly appalling sketch but I hope it gives the info you want

 

[Mdljk]

View attachment 60351

Utility may not return on itself but kitchen definitely does

 

[John.g]

By throttled state, do you mean the mixer is still set to give a mix of waters, and the cold has been isolated at the isolator? I wonder if this is what mdljk did?

Question: does this tap have separate controls for hot and cold, or, as I assume, is it a single lever mixer?

Yes, that's what I mean, leave the mixer in its mixed position and isolate each supply separately. I have a very simple mixer using two hoses connected via a "Y" to a hand held shower head for dog washing, hot is gravity fed and cold ~ 2 bar mains and the hot flowrate is very close to its full flow rate rate, I leave the hot tap full open and just throttle the cold, the throttled cold is then practically the same with hot isolated as the calculated volume.

 

[Ric2013]

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.

I just want to recap. The OP has 12l/m cold (which is acceptable) and 4l/m hot (not awful for a kitchen mixer, but definitely on the low side of acceptability and may not even comply with the Water Regulations requirement to have 50°C water available at the tap within 30 seconds (depending on how much dead-leg there is in the pipe run)).

If I assume 1.5 Bar cold working pressure and 1.44 Bar hot working pressure and use your pressure/flow equation above, and assume the mix the manufacturer suggests is 50/50, then I get 16 point something l/m combined flow, which is not far off the 14.5 l/m measured flow. Since the working pressure of the hot is certain to be at least slightly less than the 1.44 Bar (which is the standing pressure), but not massively less, I can't see that there is anything to suggest that the tap itself is defective.

I'm going to assume this is a lever mixer and that Mdljk has tested the hot and cold flows not as John wanted, but by putting the lever to fully hot and fully cold positions. If so, I would explain the fact that the combined flow is less than the sum of the fully hot and fully cold flows by the fact the this kind of mixer design tends to throttle both sides to some degree when in mixing mode and only opens the individual ports fully when in 100% cold or 100% hot mode. Presumably this is to ensure that (on balanced pressures), the flow is roughly consistent across the entire sweep of the hot-cold spectrum.

Back to where this leaves the OP: If (1.) the tap incorporates single or double check (anti-backflow) valves (or similar device - may be worth asking Blanco) or these have been fitted to both the pipes below the tap and (2.) with the dead leg from the cylinder to the tap fully cold (eg. first thing in the morning), the hot water reaches the tap in under 30 seconds (tap fully opened in 100% hot mode) and (3.) Blanco does not specifically state that the tap MUST be installed on a 'balanced pressure' (i.e. equal hot and cold pressures) system, I suggest the installation is acceptable from a technical point of view and if Mdljk wants better performance, this needs to be an 'extra'. On the other hand, if these 3 requirements have not been met, then the tap is not suited to the plumbing system at the house, the installation is illegal under the Water Supply (Water Fittings) Regulations 1999, and either the plumbing system needs to be modified or the tap needs to be replaced with one suitable for use with a traditional British mains cold, gravity hot system.

 

[Mdljk]

I just want to recap. The OP has 12l/m cold (which is acceptable) and 4l/m hot (not awful for a kitchen mixer, but definitely on the low side of acceptability and may not even comply with the Water Regulations requirement to have 50°C water available at the tap within 30 seconds (depending on how much dead-leg there is in the pipe run)).

If I assume 1.5 Bar cold working pressure and 1.44 Bar hot working pressure and use your pressure/flow equation above, and assume the mix the manufacturer suggests is 50/50, then I get 16 point something l/m combined flow, which is not far off the 14.5 l/m measured flow. Since the working pressure of the hot is certain to be at least slightly less than the 1.44 Bar (which is the standing pressure), but not massively less, I can't see that there is anything to suggest that the tap itself is defective.

I'm going to assume this is a lever mixer and that Mdljk has tested the hot and cold flows not as John wanted, but by putting the lever to fully hot and fully cold positions. If so, I would explain the fact that the combined flow is less than the sum of the fully hot and fully cold flows by the fact the this kind of mixer design tends to throttle both sides to some degree when in mixing mode and only opens the individual ports fully when in 100% cold or 100% hot mode. Presumably this is to ensure that (on balanced pressures), the flow is roughly consistent across the entire sweep of the hot-cold spectrum.

Back to where this leaves the OP: If (1.) the tap incorporates single or double check (anti-backflow) valves (or similar device - may be worth asking Blanco) or these have been fitted to both the pipes below the tap and (2.) with the dead leg from the cylinder to the tap fully cold (eg. first thing in the morning), the hot water reaches the tap in under 30 seconds (tap fully opened in 100% hot mode) and (3.) Blanco does not specifically state that the tap MUST be installed on a 'balanced pressure' (i.e. equal hot and cold pressures) system, I suggest the installation is acceptable from a technical point of view and if Mdljk wants better performance, this needs to be an 'extra'. On the other hand, if these 3 requirements have not been met, then the tap is not suited to the plumbing system at the house, the installation is illegal under the Water Supply (Water Fittings) Regulations 1999, and either the plumbing system needs to be modified or the tap needs to be replaced with one suitable for use with a traditional British mains cold, gravity hot system.

Excellent info Ric2013 - the tank is 4.4m above the tap outlet so 0.44 OP rather than 1.4? Interesting about the Blanco perhaps including check-valve technology. The tap is double lever, separate lever for hot and for cold (nothing thermostatic). You seem highly knowledgeable on the tech calcs side. Does the 4.4m head alter you conclusion above about being acceptable. Havent measured yet but first thing in the morning draw-off of hot water is certainly well in excess of 30 sec. Any further thoughts gratefully received.

 

[Ric2013]

Excellent info Ric2013 - the tank is 4.4m above the tap outlet so 0.44 OP rather than 1.4? Interesting about the Blanco perhaps including check-valve technology. The tap is double lever, separate lever for hot and for cold (nothing thermostatic). You seem highly knowledgeable on the tech calcs side. Does the 4.4m head alter you conclusion above about being acceptable. Havent measured yet but first thing in the morning draw-off of hot water is certainly well in excess of 30 sec. Any further thoughts gratefully received.

Yes. Sorry. 1.4 bar is a typo. I intended 0.44!

I'm not highly knowledgeable on the tech calcs - I'm just fairly good at maths and followed what John suggested, but thanks anyway.

I'm not saying the Blanco tap does incorporate check valves, but it's worth asking the question. The Water Regulations would generally require a tap that allows in-body mixing (as opposed to one that has separate internal waterways) to have check valves fitted if the pressures are not balanced (for various reasons that I have had explained to me by a Water Regulation officer of Dwr Cymru, but that seem a bit tenuous in your circumstances): still that's the law. However, if the manufacturer of a WRAS-approved tap (and I expect Blanco taps sold in the UK are all WRAS-approved) states that separate external check valves are not needed with unbalanced pressure installations, then the manufacturer is saying they have got around the need some other way (such as adding incorporated check valves, for example) and this workaround is legally accepted. External check valves are a nuisance as they WILL restrict flow.

If double lever, I can't understand why your mixed flow is LESS than with cold only. This is weird. So you have the cold fully open, and when you also open the hot, the flow of water issuing from the tap drops instantly (and visibly?). The effect is instant, I take it? If instant, I'd talk this through with Blanco and see if they can understand what their tap is doing as it sounds quite clever. If not instant (say a 30-second lag), then I can only assume that as your cistern starts to refill the use of cold water to do this is then competing and dropping the pressure at the tap (and if so, check your mains stopcock and external stopcock are both fully-open: sometimes people leave them half shut for some reason).

I'd suggest the 30-second rule applies unless there was a previous kitchen tap that broke the rule in which case I'd say a plumber has worked with due care if the new tap is no more non-compliant than the old one.

Out of interest, what is the exact name of the tap, as I'd like to see what we're discussing.

 

[Mdljk]

Yes. Sorry. 1.4 bar is a typo. I intended 0.44!

I'm not highly knowledgeable on the tech calcs - I'm just fairly good at maths and followed what John suggested, but thanks anyway.

I'm not saying the Blanco tap does incorporate check valves, but it's worth asking the question. The Water Regulations would generally require a tap that allows in-body mixing (as opposed to one that has separate internal waterways) to have check valves fitted if the pressures are not balanced (for various reasons that I have had explained to me by a Water Regulation officer of Dwr Cymru, but that seem a bit tenuous in your circumstances): still that's the law. However, if the manufacturer of a WRAS-approved tap (and I expect Blanco taps sold in the UK are all WRAS-approved) states that separate external check valves are not needed with unbalanced pressure installations, then the manufacturer is saying they have got around the need some other way (such as adding incorporated check valves, for example) and this workaround is legally accepted. External check valves are a nuisance as they WILL restrict flow.

If double lever, I can't understand why your mixed flow is LESS than with cold only. This is weird. So you have the cold fully open, and when you also open the hot, the flow of water issuing from the tap drops instantly (and visibly?). The effect is instant, I take it? If instant, I'd talk this through with Blanco and see if they can understand what their tap is doing as it sounds quite clever. If not instant (say a 30-second lag), then I can only assume that as your cistern starts to refill the use of cold water to do this is then competing and dropping the pressure at the tap (and if so, check your mains stopcock and external stopcock are both fully-open: sometimes people leave them half shut for some reason).

I'd suggest the 30-second rule applies unless there was a previous kitchen tap that broke the rule in which case I'd say a plumber has worked with due care if the new tap is no more non-compliant than the old one.

Out of interest, what is the exact name of the tap, as I'd like to see what we're discussing.

Ric2013
They are Blanco Arti -specified as low-pressure. Here is the link

ARTI

ARTI_LP_MS_uk

www.blanco.com

Will repeat the separate vs combined test again tomorrow as my data does seem strange.
I shall update you tomorrow

Thanks for all your help
Mdljk

 

[John.g]

I have a similar two lever mixer downstairs, the unit is a Franke but don't know if the mixer is, it was all installed 10 years ago as one unit, the hot gravity (0.45 bar) flow is 5.0 LPM and the mains (2.4 bar dynamic) cold is 11.1 LPM so very close to yours, I would be a little skeptical of their claimed 7.9 LPM at 0.3 bar

 

[Mdljk]

I have a similar two lever mixer downstairs, the unit is a Franke but don't know if the mixer is, it was all installed 10 years ago as one unit, the hot gravity (0.45 bar) flow is 5.0 LPM and the mains (2.4 bar dynamic) cold is 11.1 LPM so very close to yours, I would be a little skeptical of their claimed 7.9 LPM at 0.3 bar

Hi John G. It is very helpful to see that similar comparison. I am indebted to you for your doubtless having performed that test just for my needs. Thank you

 

[Ric2013]

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

Hi John g

so I tested hot alone - 4.0 Lpm (0.4 bar head) Cold 12 Lpm (mains pressure) and when both were on together it was 9.5 Lpm. Was that the method you expected?

Confused: combined flow rate is 14.5, or 9.5 l/m? If 14.5, you can ignore #22 paragraph 4!

 

[John.g]

Ok, I think your taps are designed to flow ~ 3.9/4LPM each @ 0.3M head.

You got 4LPM hottap & 12LPMcoldtap, now, based on my own tests, you will not get 16LPM combined with both taps fully opened but you should ( I did) get 12 LPM, you won't get 16 LPM, you certainly shouldn't be only getting 9.6LPM except that the cold water is flowing back towards the HW cylinder, I would suggest leaving both levers fully opened, isolate the hot supply somewhere upstream of the hottap lever (leave this fully opened) and measure the flowrate.

Re mixing, again from my own tests, if you separately match the cold and hot flows by throttling the cold (hot full open) then you will get close to double the hot water flow, I got 85 to 90% of the hotflowX2.
So depending on the water temps you should be able to use up to (almost) the full HW flow, ie if you had 60/10/40(mixed) you will get a max of 6.7LPM@40C, if the temps are 70/10/40, then you should get a Very max of 8.0 LPM@40C.

 

[Mdljk]

Ok, I think your taps are designed to flow ~ 3.9/4LPM each @ 0.3M head.

You got 4LPM hottap & 12LPMcoldtap, now, based on my own tests, you will not get 16LPM combined with both taps fully opened but you should ( I did) get 12 LPM, you won't get 16 LPM, you certainly shouldn't be only getting 9.6LPM except that the cold water is flowing back towards the HW cylinder, I would suggest leaving both levers fully opened, isolate the hot supply somewhere upstream of the hottap lever (leave this fully opened) and measure the flowrate.

Re mixing, again from my own tests, if you separately match the cold and hot flows by throttling the cold (hot full open) then you will get close to double the hot water flow, I got 85 to 90% of the hotflowX2.
So depending on the water temps you should be able to use up to (almost) the full HW flow, ie if you had 60/10/40(mixed) you will get a max of 6.7LPM@40C, if the temps are 70/10/40, then you should get a Very max of 8.0 LPM@40C.

Hi John g

Your data and predictions are close, as I have re-performed the tests with a second person helping me to swap out measuring jugs. The figures I got are as follows.

Hot only (from a 4.4 m head and perhaps 8 to 9 m of 15mm pipework including bends etc) = 4.2 Lpm (Your expectation of 4.0 Lpm @ 0.3 bar is probably equivalent including pipe runs and fittings to 0.3 Bar?)

Cold only (mains pressure which is good, same pipe-run set-up) = 11.0 Lpm.

Hot and cold running consecutively = 10.Lpm ( your expectation = 12.0 Lpm

The Company’s Director has offered to swap-out the two standard 15mm ball service valves for 15mm full-bore valves. I have done this myself in the past for poor upstairs hot water to the bathroom sink but it had no-effect.

So John g - would you agree with me that (despite the fact that the monobloc taps in the Utility room less than 3.0m away have good flow-rates) the contractor is likely to say that they have no control over my existing water pipes, they have fitted low-pressure taps, they will minimised service -valve resistance, that is all we can do.

It’s a mystery.

 

[Mdljk]

Confused: combined flow rate is 14.5, or 9.5 l/m? If 14.5, you can ignore #22 paragraph 4!

Hi Ric2013

At your prompting have re-run the test with a second person to swap out measuring jugs

Hot only = 4.2 Lpm
Cold only = 11.0 Lpm
Both = 10.0 Lpm

So the ‘combined’ flow rate is less than the sum of the individual flow rates and even Less than the Cold-only rate

 

[John.g]

Hi John g

Your data and predictions are close, as I have re-performed the tests with a second person helping me to swap out measuring jugs. The figures I got are as follows.

Hot only (from a 4.4 m head and perhaps 8 to 9 m of 15mm pipework including bends etc) = 4.2 Lpm (Your expectation of 4.0 Lpm @ 0.3 bar is probably equivalent including pipe runs and fittings to 0.3 Bar?)

Cold only (mains pressure which is good, same pipe-run set-up) = 11.0 Lpm.

Hot and cold running consecutively = 10.Lpm ( your expectation = 12.0 Lpm

The Company’s Director has offered to swap-out the two standard 15mm ball service valves for 15mm full-bore valves. I have done this myself in the past for poor upstairs hot water to the bathroom sink but it had no-effect.

So John g - would you agree with me that (despite the fact that the monobloc taps in the Utility room less than 3.0m away have good flow-rates) the contractor is likely to say that they have no control over my existing water pipes, they have fitted low-pressure taps, they will minimised service -valve resistance, that is all we can do.

It’s a mystery.

Generally speaking, I would say yes, those flows are more or less what I expect and similar to mine but swapping the ball valve to full flow can only improve things but I would be surprised if the cold flowrate would double, did/can you measure the hot flowrate only from the Utility?

Its a mystery though as you say that you get 25LPM from the Utility cold main will require 5 times the deltaP, for example, 9M of 15mm pipe will have a pressure loss of 0.1bar @ 12LPM but 12M of 15mm pipe will have a pressure loss of 0.5 bar @ 25LPM.