# How/where to measure incoming water flow rate

#### JackHennety

I'm in the process of considering an unvented cylinder to replace the existing gravity system. I know that unvented systems are only as good as the pressure/flow rate that supply them, so I'm anxious to make sure these are adequate before I give the go-ahead.

The pressure has been measured at between 3 and 4 bar, which is fine, but the flow rate is what I'm questioning.

The potential installer has measured flow at around 16 litres/min. He thinks this is on the low side for unvented and that a case could be made for a high-end combi as an alternative (the risk with unvented being the loss of pressure in cases of multiple demands for water).

From my own research I can see that 16 l/min is on the low side but my question is how accurate is this, given it was taken by using a tap in the downstairs utility? From what I can see, flow rate is governed by the diameter of the thing the water exits through, meaning that the flow rate from a 1/2" tap would be less than say, the flow rate from a 3/4" tap, given equal pressure.

Googling suggests that your standard 1/2" sink or utility tap will have a 15 l/min flow rate as a maximum. If that's the case, what's the point of using that tap to determine flow, given you're unlikely to see anything over 15 l/min, and how does one measure the 'real', unrestricted flow rate that's coming into the house?

Sorry if this is painfully ignorant, learning as I go by Google!

1. a 15mm pipe will flow much more than 15L/Min, it's just it gets noisier as the flow increases. If you've ever seen an open ended 15mm pipe @ >6 bar you'll know what I mean. A 15mm pipe's potential delivery capacity in L/min probably wouldn't be reached with a standard mains dynamic pressure of 2-3bar. A 15mm tap should be able to give a good indication of the mains potential dynamic output. Most outlets used these days are 15mm too, therefore it will also tell the installer what he needs to know, what you'll get at multiple outlets at the same time.

2. It's usually much more difficult to connect a gauge to a 22mm pipe as they would usually be harder to get to at/around the mains stop tap area, under sinks/floor etc. You could have a pre made adapter/jig setup To cut into the 22mm mains with a 3/4" connector and a 22mm tap for the gauge and outlet to measure flow and dynamic pressure.

1. a 15mm pipe will flow much more than 15L/Min, it's just it gets noisier as the flow increases. If you've ever seen an open ended 15mm pipe @ >6 bar you'll know what I mean. A 15mm pipe's potential delivery capacity in L/min probably wouldn't be reached with a standard mains dynamic pressure of 2-3bar. A 15mm tap should be able to give a good indication of the mains potential dynamic output. Most outlets used these days are 15mm too, therefore it will also tell the installer what he needs to know, what you'll get at multiple outlets at the same time.

Thanks very much for the reply Madrab. So I think you’re confirming that the installer’s test is valid, though I still don’t quite grasp how. Was I right that a standard 1/2” tap would generally not deliver more than ~15 l/min when attached to a 15mm pipe? And if so, how does it give a good indication of the mains potential? If it tops out at 15 l/m, who’s to say how high it would be if the tap itself weren’t restricting it?

The first thing to do is politely ask your water provider to measure the flow and pressure at their stopcock coming into your boundary/property.

Then you can evaluate if its your own domestic supply that is causing the restriction and make enquires about having that replaced with 25mm mdpe... If you have an old 1/2" lead main, it'll be throttling the hell out of your supply.

a 15mm pipe will flow much more than 15L/Min

It all depends on how it's measured too ..... depending on dynamic pressure and flow from the mains suppling it, it could be an old restrictive mains pipe, a standard old style full bore 1/2" tap has the potential to deliver a lot more than 15L/Min. I've just been out to my outside tap and I'm getting 22L/Min through a weir gauge.

More often than not, the only test points available to an installer on an assessment visit are kitchen cold tap (usually a mixer with restrictive hoses), outside bib tap (if your lucky) and a washing machine/dishwasher isolator valve... Which can mean having to move the appliance, which may be built in

Best two for "dynamic" testing are 2 and 3 above.

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