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Discuss 3 radiators on 15mm branch? in the Central Heating Forum area at

  1. Drip

    Drip New Member

    I was wondering whether it would be a good idea to replace some 15mm pipe with 22mm. I currently have 3 radiators on a single 15mm branch. Small, medium and large, approx 10,000BTU's at 50C in total. The room with the large radiator is always cold in the Winter, partly due to insulation but obviously a bit more heat output would help. If I were to put each radiator on its own branch, would there be any significant benefit?

    I have smart heating (Evohome) so on a cold day the rads will typically be on low (or off) during the day and then high shortly before I arrive home. It would be good to minimise the time it takes to heat up as the rooms aren't always fully warm when I get home, particularly in the room with the large rad.

    From what I've read, I think my setup is probably a bit borderline and not ideal. Going to 22mm would probably slightly reduce the load on the pump and have the rooms warm up a bit quicker. I'm in a position where the flooring is up and the job would be quite straight forward. I can do the work myself so the cost is minimal.

    What do you think, does it sound like it would be beneficial or not? Do you always like to have each rad on their own branch when possible? Would you make the change if it was your own house?
  2. Rob Foster

    Rob Foster Top Contributor!!

    I tried 7kw off 15mm in my extension a few years ago, no good, so certainkly go for it.
    • Like Like x 1
  3. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    Definitely. Needs taking back to the 22 mm carcass and each rad on its own feed from there
    • Agree Agree x 3
  4. scott_d

    scott_d Plumber GSR Top Contributor!!

    Get it sorted while the floors up
    • Agree Agree x 2
  5. doitmyself

    doitmyself Well-Known Member

    10,000BTU is approximately 3kW, which is well within the capacity of 15mm pipe. Don't forget that it is only the first section which has to carry the full 3kW.

    The attached file will explain why.

    Attached Files:

    • Like Like x 1
  6. king of pipes

    king of pipes Trusted Plumber GSR

    Upsize it you have more to gain than you have to lose . Kop
    • Agree Agree x 2
    • Like Like x 1
    • Disagree Disagree x 1
  7. Jimmy18

    Jimmy18 GSR

    No need to upsize for 3kw. Try turning up the pump or replacing it for a larger one. Cheaper than running in new pipework
  8. doitmyself

    doitmyself Well-Known Member

    I doubt very much if turning up the pump or a larger pump will be necessary, if you balance the system correctly.
  9. EvilDrPorkChop

    EvilDrPorkChop GSR

    You've not actually mentioned whether the rads get hot or not? If they aren't getting hot then upsize them. If they're getting hot but the room isn't getting warm then you need to look at rad sizes/insulating the room more.

    Also in your Evohome Zones, have you got the HR92s for each rad on one zone for that room, and if so have you selected 'Multi Zone' in the zone settings. As otherwise it'll only be using one HR92 as the temp sensor for that zone.
  10. Drip

    Drip New Member

    I'm a bit confused/unsure as I've had conflicting advice in this thread. However, the replies from plumbers seem to think it's a good idea, so I'm going to give it a go.

    The rads are all in different rooms/zones and they do get hot. The pump runs really well but reducing the load and noise would be even better.
    • Like Like x 1
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