Without getting into how much ventilation is enough, I would recommend that you choose your future vent hood, or at least a representative vent hood, read its installation instructions (if available on-line), and have a duct cap appropriate for the recommended duct size installed.

kas

300 CFM is not enough. Not nearly enough. Any vent that will have the appropriate size CFM will need at least an 8" duct or larger. Narrow down your future hood purchase NOW, as it is very difficult and expensive to retrofit a larger duct after your roof is already completed.

Agree with Kaski, but would recommend 600 CFM. My 600 CFM hood requiers 8" duct. The larger the duct the quieter. Also a higher CFM hood will not need to run at full power all the time so it will be quiter than the lower CFM hood the majority of the time. If you wok cook or like to pan sear steaks, go with the higher CFM.

I wound up getting an 8" roof vent. I called Bluestar and they said for a 30" range, a 6" vent would be fine with 600cfm. So to give me a bit more flexibility, I got an 8" fantech brand roof vent...model RC8.

The metal damper seems a bit heavy. Do hood fans push out this much power to lift these dampers?

The pressure drop needed to open a damper is not large, and once open the pressure drop is even less. Check to make sure that none of the moving parts are binding.

Variable motor controls typically progress from off to full on to reduced power. This aids both motor starting and dealing with such loads as initial damper opening. For systems with large motors and long spin-up times, it can be useful for the operator to "dawdle" a bit changing from full on to the desired lower setting.

kas

Thanks Kas...makes sense.