Ron Lowe wrote:

To do so, I'd have to bring the existing tundish down from about 1m high
to 30 cm off the floor (is that in itself allowed?), and add a tee into
the 22mm copper below the tundish, connected to a short vertical run
into which the 0.5" plastic drain hose from the dehumidifier would be
tucked. Diagram below.

ISTR there's regulations about the distance from the PT relief valve
down to the tundish.

tundish should be* vertical,
located in the same space as the unvented hot water storage system and
be fitted as close as
possible and *within 500mm of the safety device* e.g. the temperature
relief valve.

OK, so if I moved the tundish downwards, it'd fail the above spec, so
that's out.

Harry's suggestion about the a/c pump is interesting though - I know
nothing about them but if that worked it would mean I could leave the
tundish where it is, and interpose my tee below it. (But wouldn't the
dehumidifier object to the head of pressure on its output?)

Given that the dehumidifier spec is 10L/day, that's 7 ml/min which could
potentially already be trickling down the discharge pipe at such time
when I have my proverbial catastrophic failure, and end up with
mains-pressure boiling water (20 L/min?) pouring down the tundish (ie,
~3,000 times as much as produced ny the dehumidifier) - ie the presence
of the dehumidifier output is totally trivial, right?

So it looks to me that providing I don't alter the height of the
tundish, my idea should be OK...???

David