The response I got from OSO is surprising - they're actually in the process of changing the orientation of the flow and return in their installation manuals so that flow will now be plumbed to enter at the top of the coil and exit at the bottom.

When I pressed them as to why they said it was to be consistent with the rest of the industry.

Which leaves me feeling I've been sold a pup, to some extent, despite platitudes from OSO that it'll work "perfectly well" either way round.

Hmph!

--
Simon