What diameter should pipes from boiler to heating the hot water cylinder be - the same as the pipes used for radiators or larger ?

1 Answer 1


The usual diameter is 22mm in the UK.

Radiators will be 15mm (or microbore which is smaller)

  • 1
    Why are they different, what is the effect if you used 15mm for hot water, or conversely 22mm for radiators Dec 3, 2017 at 23:18
  • I surmise 15mm would either be insufficient to bring the cylinder contents to 60C or would take too long to do so. Upstairs rads are usually parallel to downstairs rads. 2 x 15 cross-reference area (2 x 177) is close to 1 x 22 cross section area (1 x 380)? Dec 4, 2017 at 11:42
  • Thanks, so larger the pipe the quicker in theory the hot water can get to where it needs to go (be it radiators or heating hot water cylinder) , but the downside is that boilers need to heat a greater volume of water to get sufficient hot water for all radiators, and also does the pump need to do more work to pump the water around ? Dec 4, 2017 at 13:18
  • My radiators downstairs are physically parallel to radiators upstairs, but each has their own supply/return loop they are not plumbed in series with the one upstairs. I don't get your point, does it matter where they physically connect to the 22mmm pipe (via 15mm pipe) Dec 4, 2017 at 13:21

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