What you need for MAU depends on the laws in your area. They are different for many places so there is no one answer to if you even need MAU or not.

Phil

Until recently, 600 CFM did not require MUA in most locales. Only cold northern states required it at that level. These days more places seem to be requiring it - you need to check you local codes.

I believe most locals are requiring make up air for any hood with 300+ CFM.
Anything under 300 CFM usually doesn't require it but as stated above check your local codes

Yes, you could put a passive damper, or a hood flow rate sensor controlled damper in the former downdraft path (once degreased and delinted), assuming that the path is damper compatible and large enough for the anticipated flow rate.

The cold exterior air (in winter) will be imported and distributed below your stove, and this affects two considerations -- the cook's tolerance for cold feet, and any limitations on MUA dumped under the unit by the unit manufacturer.

kas

Based on your handle - I am assuming mid Atlantic and very likely required MUA - check your local ordinances but realize that they may be interpreted differently but different inspectors...

My hood goes up to 600; we were not required to put in make up air in our remodel. I'm mid-Atlantic, too, but not PA. Wonder if remodel vs. new construction makes a difference. No gas appliances here, just electric.

I will say I am in the burbs of Chicago and they told me during our remodel that now MUA was necessary for our Zephyr Monsoon DCBL II insert, I think it is rated over 700 on max. I did not look up any codes myself, but my builder said they did and said it wasn't an issue.

I don't know if they were BSing me or not, but it all passed inspection.

Phil

Phil
It depends on the inspector. They used to only enforce it for new construction and then started requiring for all but some inspectors were being very persnickety.
We installed ours after inspection (did not require one to pass- duh) but went with one mostly in compliance 350 but boosts to 450)

We could not get a clear answer on smoke detector requirements even after reading the code and trying to talk to the city, so we requested the inspection expecting to fail and get the answers. It passed, so we still don't have the answers - mostly relating to requiring 110 hardwire and all go off. We want to use our alarm system to connect but doesn't match power requirements.

On the subject of MUA it may be useful to repeat that there is always MUA. If an extremely tightly sealed house will leak only 30 cfm at the negative pressure that the hood blower can manage, then only 30 cfm will be exhausted at the hood. From a code inspector's perspective, the unremoved cooking grease is not the issue, the issue is what does this negative pressure do to combustion appliances that may be present.

Gas fired hot water heaters, oil burners, dryers, etc., are combustion appliances, and for the purpose of this note, fireplaces should also count. A choked off hood blower can usually manage -0.1 inches of w.c. pressure in a sealed house, but only around -0.03 inches is needed to back-draft fireplaces and water heaters. Even oil-fired boilers will start leaking exhaust into the house past -0.06 inches or thereabouts.

So, to avoid this there are four options:
1) Have a leaky house. The fresh air is good for you. 1950's houses could probably leak 100 - 300 cfm at -0.03 inches depending on the house's size
2) Build an appropriate MUA system, the subject of many threads in this forum
3) Isolate the combustion appliances from the kitchen air such that they have their own effective MUA
4) Have no combustion appliances.

kas

Great post, kaseki. I loved answer no. 1 - I had one of those, a 100-year old house with its own built-in MUA!