You could really do with a more powerful extractor. Your ZHC925 is rated
at 470m^3/h. You can get 750m^3/h and more, e.g.
http://www.elica.co.uk/master.asp?http://www.elica.co.uk/view_products_parent.asp?Designer%20Hoods

Having said that, I'm sure you'll get a usable result if you use round
duct (125 or 150) with the ZHC925.


Unfortunately we've already bought the hood, at a very good price I
might add, as much because SWMBO liked the look of it as anything else.
It's only now that I'm starting to find out the possible drawbacks of
trying to install the damn thing.