Grunff wrote:

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

There is more to it than comparing the *free space* airflow figures -
sure you'll likely be heading in the right direction but there are
other factors such as the design of the fan itself as to how well it
performs under (back) pressure (e.g. axial vs centrifugal).

You should be able to get the fan performance graphs for varying back
pressures and do the sums without too much difficulty in order to
ascertain expected performance with different ducting
size/configurations.

Mathew