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Discuss New Central heating but problems, Please help in the Central Heating Forum area at

  1. Dale Sanders

    Dale Sanders Member


    hopefully someone can help me here.

    I have a recently had a new gas central heating system consisting of 7 rads and a Worchester 36cdi compact located in the lofty controlled by the wave thermostat.

    I used to have electric storage rads and the electric controlled heating with were on tanks.
    after install i have a drop in hot water pressure. if the cold is on then the hot drops and even running the bath takes forever.

    any ideas?
    i want to make sure i have all my facts straight before speaking to the plumber who installed as he did come arounf measureing pressure and said it was about average on the set up up i requested.
    the main in pressure was 3 bar and the flow was 14l/m with the hot being 10l/m.
    thanks for your help in advance
  2. jtsplumbing

    jtsplumbing Plumber GSR

    You need to call back the installer to check water flow your about 3Ltr per min short on hot water delivery,
  3. Scott_1979

    Scott_1979 GSR

    Main stop tap fully opened? Isolaters under the boiler fully open?
  4. Dale Sanders

    Dale Sanders Member

    He checked flow when I was present.
    Everything is open.
    I have 15mm that enter house that run in the loft but then goes to 22mm. Would it help to increase to 22mm from 15mm pipe.
    The flow seems fairly quick on top taps on sinks but very poor on bath.
  5. sammathias

    sammathias Plumber GSR

    All your outlets are 15mm to the hots but on your bath i presume its still old 22mm probably delievers the same litres per minute just seems slower.
  6. Dale Sanders

    Dale Sanders Member

    Is the set up correct? How i can I increase flow?
    What seems to be the problem?
  7. sammathias

    sammathias Plumber GSR

    The flow on your boiler will be preset. Im not 100% familiar with your boiler but id say 12litres per minute. Thus cant be adjusted.
    As for the 22mm to the bath. Ideally it should be 15mm from a may find you have to draw off alot more water before it becomes hot.
  8. AWheating

    AWheating Trusted Plumber GSR

    Tbh it's sounds like there's not much wrong. Your water main is giving 14lpm. Your getting a little less once the flow has been restricted by your pipe work and boiler. If you open the cold then it is sharing the 14lpm with your boiler, probably have less restriction than your boiler it gets more of the share.
  9. Dale Sanders

    Dale Sanders Member

    So if I am getting 14lpm and it enters the boiler at 14lpm why isn't it coming out at 14 or even 13lpm.
    Sure it shouldn't drop 4lpm.
  10. steadyon

    steadyon Active Member

    1. Your boiler will provide 13 lpm at a 40 degree temperature rise, plus or minus 15%. If you're unlucky, and its minus 15%, the maximum hot flow rate will be about 11 lpm. Also, the Compact ERP version has a flow restrictor to limit the flow necessary to ensure that 40 degree, which may also reduce the flow rate.

    2. The hot (and cold) flow rate at the bath outlet will be lower than an open pipe immediately after the boiler, as it has to navigate the pipework and fittings / bends. Friction in the system will reduce the rate of flow.

    3. With a tank system, the hot and cold flow rates are largely independent as both come from large stores of water, usually (in the case of a bath) via 22mm pipes. With a combi, the cold comes off the mains via 15mm pipe, and the hot also comes off the mains via the boiler, again with (usually) 15mm pipe.

    4. It would help to know what the cold mains flow rate is at the point of entry to your property. This would give you the maximum possible flow rate for both hot and cold. Measuring separately at the bath taps doesn't give you a full picture.
  11. Dale Sanders

    Dale Sanders Member

    Thanks for this.

    I have measured all taps and the all cold measured around 14-15 Lpm and all hot is around 9-10lpm. The mains in is on 15mm until it gets to the loft transferring to 22mm it then runs the length of the loft to other side to where the boiler is situated.
    Should I be lose 4-5lpm on flow just going via the boiler?
  12. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    Check the instruction manual for the boiler. It will tell you roughly what flow rates to expect at what temps. But as stated above this is a rough guide only and may not be spot on.
  13. Dale Sanders

    Dale Sanders Member

    Max flow on hot is 13lpm at 40 c + 15%.
    How can I increase flow as only getting 10lpm
  14. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Under sized pipes if it's run from the mains to the boiler in 15mm also all hot in 15mm
    • Like Like x 1
  15. Dale Sanders

    Dale Sanders Member

    Most of hot is in 22mm but mains in is 15mm from floor to loft then transferred to 22mm
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