Jason, what you described is what we did. Passive MUA. I bought the kit recommended by the hood manufacturer, and we have dampers. Our city inspector said that the diameter of duct going out has to match the one coming in with fresh air. I also crack a window open a bit if I'm wokking etc. My 48" range has a 1200 cfm 48" chimney hood.

Thanks. Do you like your hood, and which hood did you go with? How about the MUA kit - any idea which kit you used?

Any details you can share would be great!

I'm not certain we can do 10" ducting for MUA, but it doesn't seem like MUA is required here in San Diego anyway. I figure SOMETHING is better than NOTHING, even if we can only fit a 6-8" duct in the ceiling joists.

Thanks!

Where duct equivalence is a requirement, two 8-inch ducts could be used to match a 10-inch duct.

In any case, searching this forum should have led to commentaries on determining requirements before picking hoods. Decide how much cooking plume effluent you want to capture by how much the hood overlaps the possible cooking pans and locations. (Assume a 10-degree from vertical plume spread.) Then use the area of the resulting hood to determine needed air flow rate when performing hot cooking (even if it is on only one burner) by multiplying the area in square feet by 90 CFM/sq.ft.

Then, for the case of going directly out the wall with free air flow MUA, assume a factor of 1.3 for baffle pressure loss. This will raise the rated CFM needed for the blower by 1.3 such that the rating should be 1.3 x 90 x area in sq. ft. (Note that this is less than the 1.5 I usually suggest for normal duct lengths, less than perfect MUA, and a duct silencer.)

If the hood or insert is a VaH without baffles, use 1.1 to compensate for minor losses such as from the outside cap loss and imperfect MUA.

Example: 45 ft x 2 ft = 9 sq. ft. Desired flow is 9 x 90 = 810 CFM. Rated flow for conventional baffled hood then should be 720 x 1.3 ~ a nominal 1000 CFM.

While equal blowers mounted in the hood or just outside via a minimal duct length will have roughly the same noise, outside mounting may allow a larger blower that one can run at less than full speed. This will greatly reduce blade tip turbulence noise, which otherwise dominates.

Abbaka and Wolf and Broan, for three examples, make down-roof radial blowers that should be adequate to mount on a wall. Exterior aesthetic considerations may influence inside vs. outside, but a low pressure loss cap will not be much larger than a motor housing, and you get a damper with the housing.

Thanks kaseki!

So do you think if I go with an external blower such as the Broan 335 (1200 cfm) mounted on the other side of the kitchen wall that I'd notice a difference in noise vs. a wall cap and built-in blower of similar CFM?

I welcome any advice you can offer as to steering me towards the right hood insert!

I think that external 1200 CFM rated blower operated at 1000/1.3 CFM actual flow rate would be quieter than an in-hood blower rated for 1000 CFM operating at full power (1000/1.3) CFM.

I do not recommend particular brands of hoods or inserts, except for the particular hood I own (Wolf Pro Island hood). My comments are directed toward requirements. This may lead to criticism of particular hoods that deviate far from meeting normal requirements, but for hoods that provide the basic characteristics of a residential approximation of a commercial hood, the actual selection is up to the buyer. Others here with hoods or inserts that meet your needs may comment from their experience.

Thanks @kaseki. I will go with a Broan 335 1200 cfm external blower then and pair it with an insert that can be purchased without an internal blower (i.e. Imperial 54" or similar).

Appreciate your help here as well as numerous other posts I've read from you when searching here for hood advice!

I'm all ears if you have any further advice on a simple/inexpensive MUA solution which allows unheated fresh Southern California air (typically 50-90 degrees during times of cooking) into the room when the hood CFM gets above a certain threshold. Even if I went with a smaller 6-8" MUA system due to ductwork space constraints, I assume this is better than nothing. I think that I read a post where you indicated that the MUA ducting would need to vent air pretty far from the kitchen however as to not create a draft in the kitchen, correct? This would complicate things greatly given our 2-story home however.

One wants the MUA to approach the cooktop in as non-turbulent a manner as feasible without going to the test kitchen approach of a perforated wall many feet away. Options include a diffuser down a hall aimed at the kitchen, or a diffuser on a kitchen ceiling or soffit aimed away from the hood. Toe-kick registers have also been used, but need to be placed such that the air stream not reflect off of a nearby island/peninsula/whatever and rise turbulently into the cooktop area. So it should be across the room, or perhaps directed across the room. Directed onto the cook's legs from under the cooktop cabinet would be ill advised.

Another approach is into a basement and thence to the kitchen. Basement and entry/mud-room schemes allow for using a local room heating scheme to moderate the air instead of a duct heat exchanger or heating coil, where heating is needed.

I would use two 6"-8" ducts. I would feed them to a diffuser (see Hart & Cooley web pages for examples) or at worst a register under a box or boxes in the joists (see your local HVAC supply house that does sheet metal) that are sized to fit a furnace filter. Or the filter can be upstream before the duct(s). You can use electrically coupled dampers or, I suspect, lightly spring loaded passive dampers. (Consider external wind and whether stronger closure is needed.) We have already determined, I think, that slight depressurization is not an issue, safety-wise.