We are installing a new DSC gas range and VentaHood with the dual fans
(600cfm) to go over it. Because of the location, it is very difficult
to run a duct to the outside. Our contractor suggested getting a
back-venting model and running a duct (rectangular?) down through the
wall into the crawlspace and then laterally out. Does this make
sense? It will be about a 17 foot run (4 feet down, 13 feet across).
Do these things have to be vented in the upward direction only, or it
doesn't matter as long as the fan is strong enough and there aren't
too many "bends"?

Thanks for any suggestions.

-Morgan