What size is your round duct to the outside?

What is "some kind of wall assembly"?

You need something that goes inside a built hood, which is called either insert, power pack or blower. Usually blower is the term for the motor only and the other two terms are for kits with switches. The higher you place it inside the hood the better it works to collect grease and smoke.

Look at the following companies if you are certain that you only want 400 CFm: Broan/Best-by-Broan, Faber, Zephyr, VAH, Miele, Bosch, Elica Thermador.

Assuming you have a straight duct or one with few bends, you can get a quiet operation. Your ducting determines the resistance which the fan motor has to push against, and this increases raw air friction noise. The quietest ones will be solid well-built centrifuge blade motors in a simple assembly and probably from the companies that talk a lot about quiet, too.

Search on VAH in the forum-specific search field at the bottom of the list of threads. Using VAH as the term brings up good threads. Many other people with tons more knowledge can tell you a lot more.

HTH
David

Dear David,

Thank you. Your information helps. We haven't installed the duct yet but it is a direct run on an outside wall so it will be shorter than 2 ft. I think I understand we need a power pack and a metal frame assembly for the power pack to fit into our Kraftmaid wooden hood. We like the ones with halogen lights in them so are they okay in gas ranges?

Again, thanks for your knowledge. We appreciate it.

Jane and Tom

An insert of just the right shape would go high enough to let you have it as you wish : "would prefer not to have anything show below the wood." I agree this is a good thing to do, both to clear the field of vision, and to hold smoky air that rises into the hood's collector space.

The right size duct is whatever the device calls for. Or larger. Not smaller. A size larger is good, to reduce noise.

Since you have now told us you have a gas range, you can expect the next million comments to be about your wanting 400 CFM. This is too low, according to everyone who cooks with gas.

Lights can be part of the setup, or you can add your own lights there too yourself.

J+T, have you searched and read a bit about kitchen vents in other threads?

David

Little tip about the noise level: a higher-power blower will usually be quieter at its lower speeds, so if you're looking for quiet, it may be a good idea to get a slightly higher-power blower & run it on its 1st or 2nd speed.

For example, an 800CFM blower running at 1/2 power will give you the same 400CFM but be quieter than a 400CFM blower "on full blast".

Keep in mind that a lot of places only show the sone rating -at the lowest speed- (of course!).

Make sure to find out which speed gets you how many CFM's and at what sound level - or at least a range, like "From 1.0 to 4.5 sones". Also, sones are not the same as decibels.

Seconding davidro's advice: installing the insert/liner/powerpack a couple of inches higher than the edge of the hood will create a "capture area" (like an inverted box), where the dirty air can gather until the blower can get it out.

That's about it. Good luck!