On Saturday, 28 December 2019 21:28:22 UTC, wrote:
My cooker sits in what was once a fireplace and there is insufficient
height for a conventional "chimney-style" extractor. I've picked-up an
ex-display extractor and dismantled it, with the aim of fixing the S/S
rectangular part (with filters) over the cooker and remote-mounting the
centrifugal fan unit. The question is whether to put the fan in a box on
the outside wall (so it's sucking the 800mm from the filter box) or to
make some sort of "power bulge" in the 5" ducting (nearer to the filter
box) and blowing the rest of the way.
Before I do some experiments, does anyone know how well centrifugal fans
"suck" compared to blowing? I have a vague feeling that they suffer from
blade stall with even slightly reduced pressure, but I can't find any
useful info.

Make sure the ductwork can be cleaned out.