I'm installing a Zephyr Monsoon I (600 CFM) which has an 8" duct. The wall cap is 7" and instead of replacing the cap, I'm thinking of adding a reducer just before the wall cap from 8" to 7".
The ducting is short as the vent hood is up against an exterior wall. There's a 3 ft vertical run, 90 degree turn and a 2 ft run to the wall cap.
Will doing this impact performance of the vent hood from a noise and airflow perspective?
Using a smaller outlet will reduce performance and increase noise.
You're doing the best possible thing by putting the reducer at the last available moment. That will result in the least negative performance impact. It might be reasonable to do the installation, then see if the performance is reduced enough to justify redoing the cap.
It's hard to predict the effect on noise, and it's a legitimate worry. You just have to try. If you make the reduction gradual (use some of the final 2 foot run) it's less likely to create noise.
If you are way under the device's documented limits on pipe length, number of bends, and you are using mostly 8" duct, a short and minor constriction won't reduce performance in a meaningful way. In absolute terms it might reduce it a tiny bit ... but you'll only know that if you do careful before/after measurements.
A good way to improve performance of a range hood is to crack open a window in or near the kitchen so the range hood has a good defined way to create air flow. Especially if you have well sealed house.
The minimum losses will be with about 6 degree taper - a nice exercise with differential equations :)
So, make a tapered section on the 3 ft length as that should be closest to optimal. Any sharp reduction from 8 to 7 will cause more losses ie a vena contracta.
