technically you need a 36" vent hood. 8-> 6 is a huge reduction in cross sectional area.

If this system doesn’t work as is-with the 6” duct, I see about three options to choose from....

A. Rip out the drywall and install new 8” or 10” ducting. This would not be cheap, but the hood is not installed yet so it’s better than if we waited.

B. Use a smaller rated hood in the 400 cfm range with 6” duct standard. This may not be enough CFM for my range, and these hoods tend to be smaller in size (20” deep compared to 24”), and use mesh filters which I’m not crazy about. There are very few hoods to choose from in this category with 6” ducts , but this may negate the need for MUA.

C. Add an in-line blower to help the main motor do its job, but I’m not sure if this would really make a difference to overcome the small duct size.

Or do I just live with a less than perfect system?

For the next build, define all of the appliances and determine their interface requirements before the construction drawings are considered done. Unless money is no object, of course, in which case errors such as installing a mid-19th century duct size can be corrected when discovered.

One can certainly add an inline or exterior blower to increase air flow, but the blower needs to be selected with the anticipated pressure drop accounted for. There are in-line aerospace blowers that can move more than enough airflow through a six-inch duct, but your neighbors will have to appreciate your simulation of an idling F-86 Saberjet.

Supplemental blowers intended for quiet kitchen ventilation will still help, but the improvement will be limited and noise may increase. Years ago we had a Litton cooking center with inadequate ventilation and relatively negligible air flow rate. Adding a larger duct and a roof-mounted down-blast blower helped to supplement the existing blower and get the flow rate to where at least some effectiveness was available.

To evaluate a supplemental blower, first you need to determine the pressure loss in the present system getting air into the house and out of the house. This is non-trivial and will need some study or professional support. Then you can look at the hood's fan curve (these are usually published by Broan) to see what flow rate you will likely have. Then, selecting a six-inch duct suitable in-line blower, such as Fantech probably sells, you can look at its fan curve (Fantech supplies tabular data, but these can be plotted) and find the common flow rate at which the two blowers pressure drop capabilities together equal the overall loop pressure drop. (Alternatively do this with assuming an external blower.) The process can be recursive as you tweak your MUA design to minimize as much as possible the pressure loss of getting replacement air into the house.

Your goal is to achieve at least 90 CFM per square foot of hood inlet aperture actual flow rate for whatever hood you have. If you can accomplish this given your assumptions defining what the MUA blower and filtering are, then you can specify the MUA system.

kas

Thanks for the very informative response Kas!

Yes, you know what they say about hindsight, but our attention before/ during construction was on the myriad of other decisions we had to make. I though by choosing a modest 30” range, albeit a bluestar would negate some of the heavy duty venting. I wish my contractor would have run by some ducting options with me but it was never mentioned.

Im now looking at a few hoods designed for 6” ducting, but there are very few to choose from. I’ve found a couple Broan chimney style in the 450 cfm range but with mesh filters and a slightly smaller capture area. So far the best thing I see is a boxy pro style Prestige 30x24”, 600 cfm with baffles. It’s a bit pricy but seems to be the closest to what I need. Yes, I understand the inefficiency of asking any motor to push a lot of air through a small duct, but this is what I’m stuck with for now.

Anybody have a Prestige hood with 6” ducts?

thanks!

How is the exterior of the duct configured? Can you get an electrical line to it? Alternatively, can you get to the duct in the garage and add an in-line blower.

Once your hood is in place and operational, you can check the outside for apparent flow rate (with some house windows open just to reduce any MUA choking). If it seems as weak as we might fear, you could consider replacing the cap with a supplemental blower or going with an inline blower. It would have to be wired back to a relay that the hood blower power energizes, or else the hood blower control would have to be upgraded to handle both motors' total power draw. This assumes that the hood uses a standard induction motor with one of the usual variable motor controls.

Effectively, the exterior or in-line blower would be lowering the duct internal pressure, thereby compensating somewhat for the pressure losses in the MUA and hood baffles, duct turbulence, etc.

The Broan (I think) scheme of using an air flow sensor to turn on an MUA blower won't work in this case for the supplemental blower because once flow is detected, the controlled secondary motor will keep itself on.

You might be able to double the achieved flow with a supplemental blower. This should make up for using a duct of half the sectional area needed, but that will depend on both blowers' fan curves.

You will possibly end up with adequate performance at the expense of another blower and perhaps a higher than desirable duct air velocity (over 2000 ft/min). In that case, insulating the duct in the garage (if the garage is unheated) will reduce grease collection in the winter.

Before using insulation, check for vibration resonances and if needed cover selected duct sections with sound deadening material, such as the lead-filled [my assumption] sticky-sided plastic (maybe it is asphaltic) sheet used for automotive interior soundproofing. There is also the product here, but I have no experience with it.
http://www.acoustiproducts.us/

Making the differential MUA house air pressure positive would also help the flow rate, but now house wall leakage outward will contribute to moving moisture into the walls from inside in the winter, a problem unless the house is ultra well sealed.

" IOW, if you have two identical hoods with the exception of one hood
having a 6” outlet duct vs the other one having an 8” outlet, can they
still claim both as 600 cfm hoods, or should they advertise one with a
much lower rate, say 340 cfm?"

They are really claiming 600 CFM blowers, generally without the hood even being present. The CFM with the hood depends on hood configuration and filter configuration. The CFM when located in the house also depends on ducting and MUA. (VaH has a slightly different basis, claims wise, so please see this link for my opinion.)
[https://www.houzz.com/discussions/vent-hood-ducting-issue-how-to-deal-with-too-many-turns-and-screws-dsvw-vd~5164570?n=19[(https://www.houzz.com/discussions/vent-hood-ducting-issue-how-to-deal-with-too-many-turns-and-screws-dsvw-vd~5164570?n=19)

The flow rate for a given hood and given blower will pretty much be the same if the hood has a six-inch duct interface, or has an eight-inch interface that you attach a six-inch duct to, with the exception that if the 8-to-6 inch transition is long instead of abrupt, there might be a bit less pressure drop and a bit more flow rate.

Eight-inch ducting should be adequate for blowers rated at up to 1000 CFM and actually moving 600 or so CFM.

If your actual hood entry aperture were 30 x 24 inches, or 2.5 x 2 ft, then you would have 5 square feet of area and require approximately 450 CFM for good containment of the rising and expanding plume effluent that the hood aperture captured. For a 36 inch hood, this would rise to 6 square feet and 540 CFM. Depending on ducting and MUA pressure loss, you could potentially get away with blowers rated 700 and 800 CFM, respectively. Or for less ideal conditions, more rated CFM would be required. Restrictive MUA conditions cause negative house pressure. (See the incipient FAQ for an image listing limits for safe operation of combustion appliances, if applicable.)