At the risk of receiving a lot of conflicting advice, I am seeking
opinions on the best compromise for installing ventilation ducting for
my Zanussi ZHC925 cooker hood. The installation instructions state
that I should use 150mm or 125mm ducting and that the maximum length
should not exceed 3metres, to be reduced by 1m for each 90degree bend,
otherwise efficiency could be impaired. I have read various threads on
this newsgroup suggesting I should avoid using PVC flexible ducting and
that I should not use flat channel pipe, but those threads don't take
into account the specific problems I have.

One of the main reasons for using a cooker hood is to avoid a build-up
of steam in the kitchen. For this reason alone, I don't believe
using the hood in recirculation mode would be effective in any way.

The best installation would be to take the ducting straight up through
the hood chimney(approx 0.5m) , turn through one 90 degree bend and
then run it horizontally between the 1st floor joists to the outside
wall (approx 2.4m). Unfortunately I cannot actually get through the
wall at this point because there is a large steel lintel in the wall at
this point and it would actually end up venting into the conservatory
rather than outside. In order to reach a point where I can vent
externally, the route I would need to take is up 0.5m, 90degree bend to
bring it under the floor joist, another 90degree bend to turn between
the floor joists and then the 2.4m run to the external wall.

The advantage of using flat channel is that it is only 60mm high so the
visible impact in the kitchen, as it passes beneath the joist is much
less than if I was to use 125mm round pipe.

The advantage of using flexible ducting is that I should be able to
form much shallower bends, although I still have concerns about the
visible impact in the room.

I can of course use any combination of the three, the most obvious
being round pipe for the vertical run, round pipe for the horizontal
run and flexible ducting to link the two.

Finally, where can I buy a decent length of 150mm round pipe? Screwfix
only seem to do 350mm lengths of 120mm and I would assume that each
connection itself has some impact upon the airflow, or am I wrong?

Thanks.