Can you start with a vertical run of 32mm and then join that to the
23mm? I ask as I seem to recall that you get a much greater flow rate
through a given run of downpipe after the initial section in which the
flow may be especially turbulent. That applies in spades with rainwater
downpipes (which have a greater capacity if they collect water from a
wider inlet than from - say - juts a running outlet.) Might it also
mean you can avoid the issues of siphoning by using a conventional pipe
hooked into the 32mm pipe?
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R