Very Bad idea.

First is that it takes an EXTREMELY powerful blower (think 4,200 CFM to acheive 400 CFM actual) to force hot air to go down in a duct, not to mention you'd likely have a lot of static pressure enducing elbows in order to accomplish this. Same concept for why downdraft exhausts are largely useless.

Perhaps a bigger issue though is the potential for grease buildup and fire at the bottom of the duct run.

What styles of cooking do you do? How often? How often do you have a lot of windows open in your home to get fresh air from natural flow through ventilation?

We have a lot of windows and previously had a downdraft which was not that effective but did not seem dangerous.

"but did not seem dangerous."

Yeah, the problem with VOC's, Carcinogens and PM is that they are largely odorless and colorless.

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

@opaone Given significant hood air mixing of plume and blower air, and some duct length to mix in, the hot cooking plume captured by the hood and contained by the baffles should be only warm. I think the devious down and over basement duct path should be achievable with a blower zero static pressure rating of a factor of two, or at most three times the desired CFM. Adequate MUA is assumed.

You've got to go up. Dimensioned drawing please.

I will ask them to try another route.

Thanks

Make them go another route. I've got a gas cooktop on a center island with a downdraft blower. It is ineffective. In some jurisdictions you wouldn't pass the code with this set up. I didn't do it. I didn't install this kitchen. It is one element to an otherwise great house that I will not miss when I sell it. Reconfigure your plan to find another route. Remember, function first, aesthetics second.

This was an induction stove with an overhead hood that got routed through basement — different set up.

We did a full reno/addition during the pandemic and also had huge girder trusses running horizontal to the hood vent. My husband (GC) framed a double wall between the kitchen and the dining room to allow enough space for a 10" duct to slip under the trusses and behind them, and then go up and out through the roof. No angles, just a slight curve. It took a some space from the dining room but allowed the proper venting for my 48" range.

Yeah, the problem with VOC's, Carcinogens and PM is that they are largely odorless and colorless.

I really don't want to start a big debate here, however, I have seen "carcinogens" many times recently in discussions on venting cooking surfaces.

Carcinogen is a money word. Say it in the right places and people will just absolutely throw money at it. However, what is missing in many of these discussions is perspective. Cooking, especially charring, creates VOC's and can/will produce carcinogens... so does farting. Your body is constantly emitting VOC's through breathing, sweating, sneezing, etc. You are a mobile VOC producing machine. As are many other things in our life.

I support making your home as healthy as you can and so please be concerned about how you cook foods, however, take a step back and look at the entire picture of your home and see where your spending is most impactful. Odds are your new cabinets are emitting far more carcinogens in a day than your cooking does. If you are cooking on a Teflon pan, have some stain treatment on your sofa, you have carpets or rugs treated for durability or ease of cleaning, etc. then those things likely produce more health hazards in the average home than cooking does.

Perspective matters... there are no safe levels of friable asbestos and most of California breathes in asbestos with every breath. So please be concerned about your health but also try to do so in a rational way.

ETA: We all have our things and I am not trying to criticize Opaone for their thing. I am on a crusade to get conversion varnish outlawed and compared to the possible health effects in the textile industry conversion varnish is probably minor. However, suitable alternatives have now passed conversion varnish in every reasonable metric. The only thing keeping it so popular in the industry is the capital expenditure and training expense required to make the change.