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Pipe sizing

Discussion in 'Plumbing and Central Heating' started by Multibez, 6 Feb 2018.

  1. Multibez

    Multibez

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    i know I know this has been asked loads of times but I can't find a difinitive answer/answers.

    Here goes....

    My house is a shell at moment, and having a reputable plumber fit me a new Worcester boiler.

    Water supply coming in from Street via plastic pipe is 22mm or something like that not measured properly but def bigger than 15mm at 14Lpm.

    I'm replumbing the whole house in plastic to include d/stairs WC, family bathroom, en suite, kitchen, utility room.

    So do I continue the 22mm throughout the house and tee off with 15mm to basins and toilets etc or do I use 10mm to washers/dishwashers and toilets for the tees to stop pressure loss.

    Next question, I'm seeing people speak about a "manifold system" what is this?????

    Cheers for help.
     
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  3. John D v2.0

    John D v2.0

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    Generally for hot you want the smallest pipe you can get away with to stop losesl in the pipework every time you finish using water and the volume of the pipe cools. Also reduces three time to come through got.
    For showers and kitchen sink you'd want 15 but for basin 10 is ok.
    Regarding cold is l of an issue, but still 15 would be fine for most things.
    Separate dedicated pipe for the shower can help reduce pressure fluctuations when someone flushes the loo.

    Manifold is just a large pipe to a central location and dedicated pipe to each room or each appliance. It would reduce the effect of loads on each other but uses more pipe.
     
  4. Hot&Cold

    Hot&Cold

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    you need to test or have the incoming water service pipe tested to ensure a good flow & pressure is available,also check if the water service pipe is shared with your neighbours.
    At this early stage its worth splashing some cash to get decent water flow & pressure.

    14lpm is on the low side (n) for your quoted needs.
     
  5. Madrab

    Madrab

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    You do need to know your starting point though as @Old&Cold mentions, what is your static and dynamic pressure?

    Ideally use copper and insulate it all to minimise heat soak/heat loss. If you have suitable dynamic pressure then run 22mm as a primary and make the split to 15mm to the outlets as short as possible to maximise flow and minimise any drop at the outlet.

    Where possible try and avoid 10mm if you can to any outlet/appliance that uses a 15mm inlet. Copper installed correctly will probably outlast you.
     
  6. Wildcat1961

    Wildcat1961

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    10 mm supply for basins? :)

    OP is referring to a combi so all outlets will be mains fed.

    Can you explain what this manifold is that supplies hot and cold water to all outlets?
     
  7. John D v2.0

    John D v2.0

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    Evening wildcat! You always like to keep me on my toes;)
    Yes correct, 10mm will be ample assuming reasonable pipe runs 5m or so and reasonable flow rates for hand washing/cleaning teeth etc e.g. 4l/m.
    Correct. Same setup as ours.
    Big pipe in, small pipes out.
    https://www.monsterplumb.co.uk/ jg-...SzVOJsrmu1_RdYFPAMM3rHOsp3EXJPGxoC75kQAvD_BwE
     
  8. Wildcat1961

    Wildcat1961

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    Would you mind sharing the calculations (pressure/flow rate/pipework volume) as to how you derived at a figure of “5m or so” in order to achieve a “reasonable flow rate”?
    Bare in mind some of the members who frequent this site are qualified plumbing and heating engineers who have served their time and posses the relevant qualifications/certificates/knowledge.
    Some may suggest that your above statement was either plucked out of a hat or equally out of thin air.
     
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  9. John D v2.0

    John D v2.0

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    Oh gosh I didn't realise I need to do detailed calculations before every post before I help the OP to avoid smart comments! You don't need a certificate to plug some figures into a formula, and having long experience just means you are providing a service people are happy to pay for over a long period of time.

    Anyway since you're keen, let's say 4l/m would drop 0.4bar per metre in a pipe 7mm i.d. so 5 metres would drop 2 bar and 3 bar average mains pressure would leave 1 bar.
    That pipe has 40ml/metre so 200ml to run though ie 3 seconds to purge the pipe.
    Velocity would be 1.6m/s which is ok.
    Screw fix most popular tap in the basin category is this one
    https://www.screwfix.com/p/watersmi...te/4067p#product_additional_details_container
    Needs 0.5 bar at 4l/min.
    QED
     
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  11. Wildcat1961

    Wildcat1961

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    5m length of 7mm tube @ 3 bar pressure = 12.4 litres/minute.
    5m length of 7mm tube @ 0.35 bar pressure = 4 litres/minute.
    5m length of 4.5mm tube @ 3 bar pressure = 4 litres/minute.
    Your figures seem to differ with the above?
     
  12. John D v2.0

    John D v2.0

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    So you're saying 10mm speedfit is unsuitable for basins because the flow rate would be is too high?

    I don't understand your point
     
  13. Wildcat1961

    Wildcat1961

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    No, I’m saying the information you gave regarding flow rates/pressures/velocity’s is complete nonsense.
    FYI the velocity through a 10mm OD pipe @ 4 litres/minute is around 1 metre/second and not the stated 1.6 m/s.
     
  14. tolly123

    tolly123

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    I wouldnt overthink it too much, if its a combi your having installed then you wont see any benefit from running DHW in 22mm, all it will do it give you a longer time for HW to "pull through" ( arrive at the outlet). IMO stick to 15mm for the whole DHW carcass. 10mm speedfit is horrible stuff. Use copper where possible. If you use plastic make sure all fittings are accessible (i.e no fitttings behind tiled walls or under solid floors)
     
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  15. John D v2.0

    John D v2.0

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    I didn't give any in the post you first complained about, I just said 10 will be fine for a basin. In my experience it is (on a 5m run to a basin with reasonable mains pressure). You don't seem to disagree.
    OP is talking about plastic not copper.
     
  16. John D v2.0

    John D v2.0

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    Anyway I don't think this is helping the op, you can let him know your thoughts about design of his system based on your experience, which I don't doubt is far more extensive than mine! I can only talk based on my more limited experience but I'm happy to help where I can.
    I'll leave this thread now, best of luck to the op.(y)
     
  17. Dan Robinson

    Dan Robinson

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    Unlike this work of art?

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    :ROFLMAO::ROFLMAO::ROFLMAO::ROFLMAO::ROFLMAO::ROFLMAO::ROFLMAO::ROFLMAO::ROFLMAO::ROFLMAO:
     
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