Where do you live? How often do you normally have windows open? Do you have any gas appliances like a gas furnace or water heater that are NOT sealed combustion? Do you have a fireplace? What type?

Why can't your contractor install proper MUA?

If you are confident that you will ALWAYS have windows open then this should be fine so long as it meets code (most building codes rightly require powered MUA for anything over 400, 500 or 600 CFM but some temporate climates do not). Keep in mind that if you turn on your hood without a window open that you could backdraft any non-sealed-combustion* appliances resulting in inhumane levels of CO, sickness and possibly death.

Your hood (specifically the opening along the bottom) s/b 6-10" wider than your range.

You'll not have an especiallly powerful range so seat of the pants says 800 w/b sufficient. @kaseki might have better input on that.

Personally I would not get that hood. It has a quite small opening that combined w/ the lack of containment volume will make it fairly ineffective (and higher CFM's can't really make up for it).

More info here: https://bamasotan.us/range-exhaust-hood-faq/

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* Sealed combustion will typically have 2-3" PVC pipe for combustion and exhaust air. If either appliance has a larger metal duct for exhaust gases then it is not sealed-combustion.

Some combustion appliances can back-draft on the pressure loss of pulling air in through window screens. More than one open window may be needed, and in any case a CO monitor near these combustion appliances is essential for safety.

Yes, a 42-inch hood would be better for side capture, and given that this hood is only 24 inches deep on the exterior (somewhat smaller on the interior), searing and wok cooking should be performed on rear burners. One cannot more than slightly improve effective capture area with excess air flow.

Is there a reason for using an interior blower instead of an exterior blower? How long is your duct planned to be?

Hi Kesaki, thanks for all the help. I know you strongly recommend external or inline. But in my area, finding people to install exterior is extremely hard thesedays. I searched enough and im giving up.

42 inch hood would had been nicer but its too late. So lets cross it out. But going back to the original question, between 800cfm and 1200cfm- which one would you suggest? (Without MUA, with internal blower only)

The air flow out of the hood will exactly equal the air flow in from your windows, so there is always MUA. In many places it needs to be heated and usually filtered, but your case allows for windows.

I expect that an 800 CFM rated blower in this hood will provide the needed 90 CFM/sq. ft. for good containment of that part of the plume captured by the hood entry aperture envelope. Otherwise, a 1200 CFM blower operated at reduced power to actually flow 540 CFM against restrictions may (!) be quieter, if that is important, than the 800 CFM blower at full power eking out 540 CFM. Again, what is your duct length?

This is a one story house. So its just straight up from ceiling through the attic (shouldnt be more than 8ft) Duct is 10inch in diameter.

Sorry im confused.
You mean basically …. 800cfm at MAX is good enough, but 1200cfm at MEDIUM is also equivalent PLUS quieter? (Knowing that wolf has three power settings)

Seems you will have room for a silencer in the duct path. Just angle the duct as needed in the attic to fit the silencer and to put the exhaust point where you want it on the roof.

I don't see putting a cap on the roof to be significantly less difficult than mounting a blower assembly. All you need in addition is a power supplying cable from hood junction box to motor junction box. This shouldn't be a challenge. Use an electrician if you don't know the code. This is an in-attic project once the blower assembly is mounted over its cutout and properly interfaced with the shingles.

I'd probably go with the 1200 CFM rated blower in this case and between the silencer and under-speed operation, noise should be limited to baffle hiss.

Thanks a lot.. do you have a good link for a silencer that you recommend?

So, our conclusion is = Internal 1200CFM blower (in house) --> 10ft Diameter Duct (attic) ---> Silencer (attic) --> Exhaust hole on roof

Did i understand it correctly? Thanks !

A larger fan running at lower RPM's is often queiter than the same fan but smaller running at higher RPM's. If you look at a movie shoot that needs wind you'll always see a giant (like up to 12' sometimes) fan. A much smaller fan can produce the same visual effect for a lot less $'s but the larger fan is massively quieter.

I'm not sure what purpose a silencer would serve if it's after the blower?

Silencer between hood and blower; not useful if the blower is in the hood. I am recommending using a down-roof blower such as Abbaka, Broan and Wolf sell. It comes with its own damper. You will need a damper at each end anyway.

Search on LD-10 and Fantech in a browser.

Probably 10-inch ducting

Thanks all.
I am not sure what you mean by “under roof blower”. Would it be the same as “in line blower”? Then i guess the highest option for me is 1100cfm.

Inline is inside inline in the duct. A down-roof-blower as @kaseki recommended would likely be a remote blower.

The opening in your hood appears to be quite tiny (and irresponsibly undersized for your range - as in even though your range isn't very powerful I'd very seriously consider a different hood) so 1100 would be more than enough.

I used "down-roof" to distinguish the common residential roof-mounted centrifugal blower housing shape and function from commercial style up-blast and down-blast blower housing designs. I have an ancient (for most of you) NuTone down-blast blower connected to registers over my wall ovens. I also have an empty down-blast style blower housing that is the intake for my MUA system. My hood uses a Wolf (Broan) down-roof 1500 CFM rated blower assembly.

On the roof or attached to an outside wall would come under the Wolf category of remote blower, as @opaone noted. See Abbaka for additional examples.

Wolf hood has 3 power settings, low, medium and high. So if i get a 1200CFM blower, would it be something like 400, 800,1200?

Maybe same thing for 800CFM blower : 200, 400, 800 (?)

Interesting; mine is continuous control (off to full down to minimum), but my hood was actually fabricated for Wolf by Independent. Continuous control is so simple, with a trivial parts count available since the late 1960's, that I see no reason for restriction to three ON settings.

Wolf questions about details can be had from Wolf. They answer their telephones.