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Discuss New water supply pipe what size feed into boiler? in the Plumbing Forum area at

  1. Jamesc1234

    Jamesc1234 New Member

    Hi guys,

    Need a little help here and very new to all of this.

    We have a Victorian house and are struggling with low water flow, although pressure is ok. We want to have a decent thermostatic shower. Currently it is on a common supply pipe and we are house number 5 and it has a combi boiler. The (hopeful) solution we settled on was a new water supply pipe.

    A new 25mm MDPE pipe has been put in and is going to be connected to the water main. We thought it would be best to feed the new water supply into the boiler to deliver it the greatest flow rate.

    The plumber came this morning to fit the internal stop tap and make the connection into the house system.

    He has connected the 25mm supply pipe onto a 15mm copper pipe, which then runs to a T junction and splits into 2 x 15mm pipes, one which supplies the boiler and the other which supplies the rest of the house cold.

    Given that we want maximum water flow to the boiler and also to stop the problem with flushing toilets/taps/washing machine at the same time, should he have run 22mm pipe up to the T junction?

    Have attached a photo. Any advice greatly appreciated.

  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Me personally I would, did you tell the plumber you wanted it in 22mm ?
    • Agree Agree x 2
  3. Last Plumber

    Last Plumber Plumber GSR

    I would have at least run it to a 15 x 15 x 22 tee, yes.

    It is however only very short and it may not make a massive amount of difference in all fairness.

    What's your pressure and flow rate at the nearest draw off point to the stop tap?
    • Agree Agree x 1
  4. scott_d

    scott_d Plumber GSR Top Contributor!!

    What size combi is it?
  5. Chalked

    Chalked Plumber GSR

    Would have been better. But that 15mm main will have more capacity than your combi can use.
    • Like Like x 1
    • Agree Agree x 1
  6. Jamesc1234

    Jamesc1234 New Member

    Worcester Greenstar 29CDi Classic 30Kw.

    Should I ask the plumber to change it to a 22mm pipe? I'm concerned that as it is not supply the maximum flow at the T junction that when we turn on a tap etc, flow to the boiler will drop.
  7. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Cold tap and a hot tap

    Or two hot taps open ?
  8. Jamesc1234

    Jamesc1234 New Member

    Just if the shower was on and then someone turned a hot and cold tap on at another basin, for example
  9. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Do you know your new pressure and flow rate

    Also that's standard for a combi will always do that
  10. Jamesc1234

    Jamesc1234 New Member

    no, we're not connected to the mains yet. tomorrow. potentially could check pressure/flow then. Just really if it needs changing I want to do it before the plumbers leave.
  11. Vee

    Vee Plumber GSR

    It will help if you make sure your shower head is a water saving one.
    Data from Worcester for Greenstar 29CDi Classic
    Hot water flow rate 35°c ∆T (litres per minute) 12.3l/min
    Hot water flow rate 40°c ∆T (litres per minute) 11l/min
  12. Last Plumber

    Last Plumber Plumber GSR

    I would see how it performs. Like I said earlier, I can't see it making a massive difference over such a short length but it depends on the pressure and flow you have.
  13. Jamesc1234

    Jamesc1234 New Member

    Was this the best way to connect the new water supply into my system? Could it be improved?
  14. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Since the rest of the run is in 15mm no but wouldn't harm / would benefit if you could put the boiler feed in 22mm as far as you can
    • Informative Informative x 1
  15. Jamesc1234

    Jamesc1234 New Member

    So I should ask him to change it to 22mm pipe from the 25, which splits into 2 x 18mm pipes at the T junction?
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