In an earlier contribution to this discussion,
Digby wrote:

Thanks guys - all useful stuff !
To be a little more specific about my problem the problem occurred
with two 22mm plastic pipes that run up the wall in the hall. There
was leak where they joined other pipework under the bathroom floor
above. Whilst I took up the tiles in the bathroom to gain access, my
wife chopped out the two plastic pipes all the way down to the under
the hall connections with the idea that I would then replace the
plastic with copper. However in the fun of the moment we didn't pay
enough attention to which pipe from below the hall joined to which
pipe above the hall ceiling (in the bathroom floor).
I have completed all the pipework now, re-filled the system and am now
running it up.
Apart from a lot of airlocks it appears to be working OK but I am
still concerned that I may have reversed those joints. The boiler
(which is situated 30 yards from the plumbing incident ) is firing up
alright and the correct outlet pipe (ie the flow) is getting hot
first.
Would this have happened if I had incorrectly joined the pipes ?
Thanks again for all this valuable help.

It sounds as if you're probably ok.

The rest of the system probably doesn't care too much as long as the boiler
is the right way round. The only exception to this is that some TRVs are
noisy if the flow is in the wrong direction, and - for reasons which I don't
understand - Honeywell say that their 3-port valves *must* be in the flow
pipe.
--
Cheers,
Roger
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