Central heating pipe size?

Discussion in 'Engineers' Talk' started by Norman Collins, Nov 28, 2018.

  1. Norman Collins

    Norman Collins New Member

    I am looking to replace the heating system in my bungalow. The plan is to use 28 mm copper into loft then down to 22 barrier pipe flow and return. Then off the 22mm to 10 mm down to radiators. My problem is I keep been told by others this is not a good practice. As the pipe been smaller is easier to block although I find this hard to believe has my last property had this system from new and never gave a problem. Any advice would be great thanks all.
  2. Dam0n

    Dam0n Screwfix Select

    Really I would skip the 28mm all together and use 22mm and 15mm for the rads.
    Allsorts likes this.
  3. Allsorts

    Allsorts Super Member

    Is this a sealed - pressurised - system, Norm? If so, there is - especially so - no reason for it to cause problems, even if you use 10mm 'microbore'.

    As Damon says, tho', 22mm is perfectly ok for your main rails (unless you have a huge system).

    Fit a Magnaclean on the final return pipe and add a good dose of inhibitor (X100, MC1 etc) and you should have no problems.

    Pipes only clog up with sludge when something goes wrong - water leaks out, air gets in repeatedly, no inhibitor has been added etc etc over a longish period - ie completely avoidable issues.
  4. Pollowick

    Pollowick Screwfix Select

    Why 10mm? That is very small when the wall thickness is taken into account. 15mm would be a lot better and fittings easier to obtain as well as being cheaper.
    KIAB likes this.
  5. KIAB

    KIAB Super Member

    Do as much as you can in 22mm, keep 15mm for drops to radiators,not keen on microbore, I would avoid using it.
    Agree with Allsorts fit Magnaclean Pro 2 filter,among the best.
  6. Heat

    Heat Screwfix Select

    Microbore pipe is limited to very short runs of not much more than 3 or 4 metres.
    Plastic microbore is even worse due to having a smaller bore (because of thicker pipe walls).

    Your pipe runs from boiler need to be able to cope with the house heat requirements. The run distance is also a factor.
    Even if your boiler has 22mm connections, the system might need 28mm.
    In severe winter days, if your pipe runs are below the full load of the heat required, you will find a lot of the rooms will not reach proper heat.
    But 28mm is more expensive and 28mm copper is better installed by a plumber experienced in copper imo.
    If you do need to use 28mm, to do it properly you should first tee off it to take some of the system load before just reducing to 22mm.
    On an open vented system it was known to use 22mm for up to approx 18kw and 28mm up to near double that.
    Sealed systems (like with combi boilers) will perform better and 22mm will do a higher kw
    WillyEckerslike and KIAB like this.
  7. Norman Collins

    Norman Collins New Member

    Hi it will be a pressurised system when done. Thanks for the reply.
  8. Norman Collins

    Norman Collins New Member

    Hi the only reason for 28mm is the cylinder is pre plumbed and flow and return on this is 28mm and the 10mm is easily insulated and chased in to walls.
    Otherwise i would agree and have 15 mm to rads. Thanks
  9. Norman Collins

    Norman Collins New Member

  10. Norman Collins

    Norman Collins New Member

    Hi it’s a sealed system and I have explained in previous post the cylinder is pre plumbed and flow and return on cylinder are 28mm so I plan to run this into loft and then covert to plastic. Working with copper is not a problem for me having done 25 years in the commercial refrigeration. As you say it’s costly.
    Thanks for your reply
  11. Heat

    Heat Screwfix Select

    Okay, I just mentioned 28mm copper as difficult to install because few plumbers are capable, or have a 28mm stand bender.

    The cylinder being pre plumbed is irrelevant I would have thought.
    On pumped pipes to a domestic cylinder coil you only need 22mm maximum - and that even needs reduced down further using a balancing valve.
    You just simply pipe to the cylinder in 22mm and use soldered reducers.
    Any motorised valve to go on cylinder will also only need to be 22mm. Makes it easier also.

    Using 28mm to the cylinder would be a very obvious rookie looking mistake and would cause serious balancing problems if left full flow. (As would unbalanced 22mm to cyl.)

    The coil in a copper cylinder is usually 28mm for heat transfer purpose obviously, but only on gravity primaries pipes of stoves would they need to be 28mm.

    I have recently overhauled a very old system of 36kw (rads and cylinder) - it has original 2 of 28mm f&r piped to rads and also 2 of 28mm pipes on gravity to cylinder.
    I have installed new boiler and rads and converted it all to a fully pumped and sealed system.
    But the original ‘28mm’ (actually imperial one inch) pipes that go to cylinder, I reduced to them in 22mm.
    KIAB likes this.
  12. Norman Collins

    Norman Collins New Member

    Thanks for this I have sine checked the cylinder and although manufacturers paper work says 28mm the flow and return are 22mm only flow and return from boiler to cylinder 28mm. Sorry my fault for not checking before posting
    Thanks again
    Heat likes this.
  13. kiaora

    kiaora Guest

    My rule of thumb, 22mm from Boiler, until Heating load is less than 4kw, then 15 mm to radiators.
    May be over engineered, but you don’t get any trouble with heating system.

    KIAB and Heat like this.
  14. Heat

    Heat Screwfix Select

    And that makes the system better balanced due to good flow at end of circuits.
    On open vented systems the old rule was 15mm copper pipes would have a limit of 20,000 btus - near 6kw.
    But that would be pushing it.
    So your limit of below 4kw seems sensible
    KIAB and kiaora like this.
  15. shekhar

    shekhar New Member

    it would be better to use 22 mm copper pipe
  16. MGW

    MGW Screwfix Select

    This is what I did with my house some 30 years ago, and because it is piped that way, it does not matter if the lock shield valves are not set, it still works, however this was before the advent of the TRV, or modulating boiler.

    I tried in vain to get info on how the anti-cycle software in a modulating boiler works, likely it changes make to make, however as the TRV's shut the remaining radiators need to take the extra flow or the by-pass valve will open, if the by-pass valve opens you get hot water returning, which will modulate the boiler.

    I simply don't know how this will affect the system as a whole?

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