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Discuss Flow rate of 20mm MDPE in the Plumbing Forum area at

  1. rich1

    rich1 New Member

    Not sure if this is the right forum but i wanted advise on flow rates coming into a house.

    I have an incoming blue MDPE 20mm pipe that terminates into some kind of connector that goes to 15mm copper. From here into a stop tap and then to the rest of the house. Testing the flow rate at different points in the day I get a constant 6-7 litres per minute.

    1) Whats the maximum flow rate through 20mm MDPE?
    2) Whats maximum flow rate through 15mm copper?
    3) Forgive me ignorance but how does pressure effect the flow rate? (I havent been able to test the pressure yet)
    4) Would moving from 15mm copper to 22mm make a difference in flow rate.

    My main issue is that as soon as you try and use 2 cold outlets at once the flow rate goes down the pan!

  2. Craig Watson

    Craig Watson Plumber GSR Top Contributor!!

    Your maximum flow is determined by your pressure and how much resistance there is in the pipe. Resistence is affected by the bore of the pipe, the length of the run, how many bends and restrictive fittings are on the run and the height. A 20mm mdpe pipe can give you flow rates of over 30ltm depending on the pressure feeding it. So let's say at your boundary stop tap you have 30ltm, the length of the pipe from the boundary to your house will add resistance and slightly reduce that flow rate, it will be further affected once the pipe is reduced down to 15mm copper, it will then be reduced further by the stop tap. From the stop tap to the first outlet will add further resistance and then depending on type of isolation valve used and the tap itself will also increase the resistance. This method follows on down the line to each outlet and the further away from the stop tap the outlet is, the more resistance there is as you can now guess, the flow rate is reduced.

    As for your problem, have you first checked that both the internal and boundary stop tap are fully open? Also just because you see a 20mm blue pipe, doesn't mean the whole run is done in the same pipe, it may have just been added onto either a lead or cast main. The pipe may also be kinked or you may have an external leak.

    After checking if both stop taps are fully on, if your still suffering, check with your neighbours to see if they too have poor flow rates. Then ask your water supplier what the expected flow rates and pressure are in your area. If these rates are well above what your getting then you know you have a problem that can be fixed.
    • Informative Informative x 3
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  3. rich1

    rich1 New Member

    Thanks for the reply Craig. The street stop cock was replaced 2 weeks ago by SS water (same low flow rate before and after) I have checked and the new street stop cock is fully open (90deg turn type). The house stop cock is also fully open. I think the main pipe run is 20mm MDPE all the way, as I can see this at both ends, house and street. Hopefully I will catch a neighbour this week and see if they have the same low rate.

    Thanks again
  4. CBW1982

    CBW1982 GSR Top Contributor!!

    Hi Rich, I would certainly try and check the neighbours flow rates. However, your internal stopcock could be broken depending on the type, so you may think it’s open?
    A bit unusual to be able to see both ends of MDPE, is it definitely connected? Have seen lots of aborted MDPE jobs so just tails sticking out.
    • Like Like x 1
  5. rich1

    rich1 New Member

    Hi checked with a neighbour and they get around 11-12 litres minute. I have tried a pressure meter on the the outside tap which comes out just at the top of the attached picture. I get 5 bar here with nothing else running. As soon as another tap is opened the pressure drops to 0.5 bar ish.

    I was going to replace the stop cock in the picture to see if this is the problem. If not what about upgrading the incoming pipe to 22mm?

  6. CBW1982

    CBW1982 GSR Top Contributor!!

    Hello Rich, that stopcock doesn’t look fully open in that picture, are you sure it’s open all the way then back a quarter turn? Could be the stopcock. Upgrading to 22mm will not resolve a pressure/flow issue if it’s not the pipe causing it - if that makes sense?
    • Like Like x 1
  7. rich1

    rich1 New Member

    Thanks for the advice. Have just had another go at the stop tap and the result is now around 9 litres a minute. Better than before but still less than what the neighbours are getting. I think i will change the stop cock and rule that out( guessing its been in since the early 80's).

    Next one if i change the stop cock and it is still the same is there any benefit to upgrading to a 22mm pipe? Sorry if im missing the point here but if the pressure is good then 22mm will allow much more flow through than 15mm
  8. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    How is your garden tap plumbed in is it on one of those stupid cut into the pipes type attachments
  9. rich1

    rich1 New Member

    It is a full 15mm feed off a tee junction
  10. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    Have you tested your kitchen tap as well
  11. Rob Foster

    Rob Foster Top Contributor!!

    Pressure and volume is often mis understood they are inter related but problems need to be analysed. Do a running test with a quantifiable container at all times of the busy and quiet to see what volume you achieve in litres per min
    then report back
    • Agree Agree x 1
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