flow and pressure query

5 Oct 2015
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United Kingdom
Hi all, this relates to a a job I've had done relating to a previous post problem. I have a new mains inlet that was supplying an unvented cylinder boiler setup - Ideal istor. Problem is it was limited by a 15mm supply pipe and a 15mm PRV so the flow rates were pretty poor - 8/9 LPM. Anyway, I've had this problem sorted - I now have a 22mm copper inlet and a specific 22mm PRV. Flow rates have increased to 15LPM approx as I have 22mm copper running up to the bathroom and ensuite.

However, my cold runs up in 15mm. The cold for the house is not directly off the mains but is after the PRV. So, my hot taps are pumping out at 15LPM and the cold at 10LPM. Because we cant install a separate cold water supply straight off the mains the plumber has advised installing 2 PRV's - one PRV for the hot and one PRV for the cold. The idea being that I could increase the pressure of the cold PRV which would increase the flow. We could increase the pressure of the cold to match the flow rate of the hot.

If we do achieve the same flow rate at the taps by increasing the pressure on the cold would this mean my showers then have a balanced feed? or would the pressures still be inbalanced even though the flow is the same and throw my showers out of syn? My plumber isnt 100% on this but said its worth a shot as he could just set the PRV at the same pressure as the hot one. Just wondered what your thoughts are on the flow/pressure issue? Thanks a lot
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As in your previous post, the question comes back to pressure - You simply need to know what your dynamic pressures are (best and worst) and balance them to avoid issues. Flow isn't relative to a balanced supply, that is controlled by the taps or thermostat.

If you are using the same feed for the hot and cold, i.e. both are after the same PRV then both hot and cod pressures should be the same unless pipe runs are hugely different in length/turns.

Again....test the pressure of hot and cold at the outlets (y)
Hi again Rob, was hoping you would reply. I asked the plumber to check the dynamic pressures and he seemed to brush that off and tell me the 22mm pipe would do the trick - which it has to an extent.

"If you are using the same feed for the hot and cold, i.e. both are after the same PRV then both hot and cod pressures should be the same unless pipe runs are hugely different in length/turns".

would the pressures be the same even though I've got different pipe sizes on the go? The plumber tells me if I want the same flow I need a setup like the image im attaching here. Currently I just have the one PRV but with another he says I can alter the pressure on the 15mm to increase the flow at the outlets. Without me telling the plumber how to do his job what should I be asking him? cheers
plumbing diagram.jpg
Increasing/reducing the pipe size does not change the pressure significantly, it changes the amount of water a given length will flow but that's not what's important when it comes to a balanced supply, it's the pressure that is at the inlet of the appliance that is important. If you add another PRV to the cold and increase that pressure to get the same flow as the hot then you are back to unbalanced supplies, cold now being higher pressure than the hot??

Trying explain the principle in relation to your setup, you say you have 22mm feeding your hot but it must reduce to 15mm prior to reaching the iso's/outlets? That means you do have 15mm pipe that is flowing at least 15L/min......therefore the cold 15mm pipe, if from the same supply/PRV, should be able to flow that at least too, that's a given. Not sure if that explains?

It's not the change in hot pipe size but the PRV change that has probably increased the hot flow due to a pressure/flow increase through the valve or changing the pipe has removed a restriction. The cold pipe length/bends aren't large enough to see such a difference in flows dues to resistance so something else must be restricting the cold flow.

I don't see what's so difficult about your plumber not wishing to take a couple of pressure readings??
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Thanks Rob - basically the plumber said he couldn't take a pressure reading because he couldn't fit his gizmo to my taps or the shower? Not sure how accurate this is as i've not asked him to prove it! Dont think he's be too happy If I did!
He's not necessarily coming back now as I may as well stick with the one valve. Currently, I do have different flow rates coming out of my taps and you are correct in that the 22mm does eventually turn into 15mm once it hits the bathroom and ensuite. The PRV is set at 2 bar so even if I measure the pressure at the outlets is there nothing I can do to increase the cold flow without throwing out the balance?
I've had a look on the net and I've seen a few bits of kit of I can buy and maybe try it myself? If I did get different pressure readings with hot/cold is a case of tough luck seeing as I cant adjust the PRV without upsetting the boiler/cylinder?
Worst case is a pipe from an outlet is disconnected and a gauge applied, shouldn't be rocket science for a plumber.

So the way to understand it is that a 15mm pipe will only flow a certain amount of water for a given pressure whether it's 100mm or 1000mm long, even if it was larger diameter before. Increasing the pressure in the cold to equal the flow of the hot will only create an imbalance.
If I understand your setup correctly (pic below?), your cold should be delivering the same. The flow restriction or pressure reduction then has to be in the pipework after the PRV or at the outlet where the L/Min measurements are being taken.


It's all about how much of a pressure difference that would be created if you were to increase the cold pressure to equal the flow but you won't know that until (copy 'n' paste :p) knowing what the pressure in the hot supply pipe is.
great - and yep that is my current setup but the cold flow rate is slower throughout the house on every tap even though by that stage both hot/cold are on 15mm piping. I just assumed the logical reason for this imbalance was purely due to different pipe sizes delivering the water to the rooms (15mm most of the way vs 22mm) but as you the say the pressures at the outlets will give me a clearer idea. I'll get the gear and have a go at doing it myself save myself some pennies. If the cold pressure is down I'm assuming adding a seperate PRV I could tweak would assist? I'm away for a period now but I'll get this done and post back. I'll post back once I've got some figures and hopefully you can make sense of it - not sure I'll be able to. cheers
Yup, with all the info it's not pointing at the pipe size, 15mm pipe should flow enough, there has to be a restriction on the cold somewhere.

If the cold pressure is down, it's finding out what's reducing it and correcting that.

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