Pipe size help?

Discussion in 'Plumbers' Talk' started by Owdotzz, Aug 7, 2021.

  1. Owdotzz

    Owdotzz New Member


    I’m wondering if anyone can help.
    Iv got two main questions. It might be worth me doing two different threads but I’ll try here first.

    first question.

    what size mm pipes do I need for the sink, shower, bath etc around the house. Will 15mm be sufficient or should it be 22mm main then spur off to 15mm?

    second question.

    I’m changing from a one pipe system to a two pipe flow and return system.
    I have a combi boiler and I’m just wondering.

    firstly. Can I keep the same boiler? And secondly should I run this flow and return in 22mm with 15mm spur off to each radiator?
    ALSO I have 4 radiators upstairs and 2 (currently) downstairs as I’m planning on having UFH for the rest. But if this doesn’t happen then I will have 8 downstairs.

    is it ok for the heating to run upstairs first and then have drops into each downstairs room rather than running two seperate(ish) circuits as a lot of the upstairs radiators are above the downstairs ones.

    thanks for any help

    sorry if these seem stupid questions but appreciate it either way.
  2. terrymac

    terrymac Screwfix Select

    Combi boilers domestic hot water output / distribution pipe is 15 mm ,no point in having larger than that going to taps etc. Combi can be utilised with two pipe ( flow & return) c/ heating set up.
  3. teabreak

    teabreak Screwfix Select

    15mm fine if running a combi.
    Depends on the condition of the boiler and it's output. If fitting bigger rads it might not cope.
    Use as much 22mm for flow / returns as you practically can.
    Drop systems are fine but beware of creating air traps where they drop down.
    A one pipe system must be pretty antiquated, I would avoid use of old pipework where possible and ensure the boiler is thoroughly cleaned and flushed before starting any upgrade.
    Might well be better to bite the bullet and get a new system professionally installed.
    terrymac likes this.

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