We are building a covered porch and want to grill outside. While we may build a fancier, built-in grill when budget allows, for now, we'd like to use our nice gas grill (and ARE building the gas into the wall,) but use an outdoor vent that could vent our grill now, and a built-in later. Can anyone suggest a reasonably priced but effective outdoor vent for this situation? Thanks!

Forgot to mention - we are hoping to find hood that are recommended for outdoor use as well that are cheaper than upper $2k (including duct cover if needed). Thanks!

If you don't use a highly filtered recirculating hood such as the Vent-a-Hood ARS, it is unlikely that any flow rate will make much of a difference, in my view. Otherwise, sufficient flow rate is determined by cooking plume upward velocity and should be in the ball park of 90 CFM per square foot of hood entry aperture. The blower rated flow rate needs to be higher, perhaps by as much as 1.5X, depending on system restrictions, in this case by filtering restrictions.

If the outdoor kitchen is located under a covered porch a vent hood is a good idea for fire safety (if you cook like me) and for occupant comfort--at least for non-smokers. With respect to the size of exhaust duct work, what matters is the total pressure losses vs. the fan's performance curve. A transition from a 10" diameter to an 8" diameter and back to 10" diameter is a perfectly reasonable solution to a framing constraint. A mechanical contractor worth his/her salt should be able to calculate an equivalent length and compare it to the manufacturer's recommendations.