Remote blowers to handle high static pressure in kitchen exhaust? – GreenBuildingAdvisor

The kitchen ventilation system in our new build is the bane of my existence. Our engineer specified a 10” duct for the exhaust in order for the system to meet my requirements:

1. Range hood exhaust airflow of ~ 400-450 CFM on high speed (actual, not rated at zero SP)

2. Extremely quiet, ideally 46-49 dBA on high speed (definitely < 55).

Turns out we only have a 6” duct installed because bigger could not fit, despite this being the first thing I ever specified to the architect long before any of the “conflicts” were designed or built. Drywall is done and the kitchen is now installed. We purchased a Ventahood liner insert for custom wood cover but it does not work with the ducting: BH240PSLD https://www.ventahood.com/index.php?option=com_content&Itemid=333&id=361&view=article&modelsku=BH240PSLD

So the Ventahood insert requires an 8” duct for obvious reasons. My guess is SP might be about 0.2” to 0.3” in WC with the 8” duct so the “600” CFM would end up right around 450 CFM according to their chart — perfect! However, with a 6” duct the pressure is probably over 1” in WC and that airflow will be reduced I’m guessing to like 200-300 which is not only unacceptable for cooking based on my research but also it will be loud and destroy the motor which is simply not built for that kind of pressure.

No matter what at this point I think it will be an expensive fix and I intend to fight my GC on this. If we can’t rip out the duct to replace with 8” (I think it does not fit) then we need to put a different range hood and possibly a 3rd party exterior remote blower.

The other range hood insert I saved is the Wolf Pro insert which can be purchased with or without the motor  – https://ca.subzero-wolf.com/en/wolf/range-hood/42-inch-pro-hood-liner-22-inch-depth

Now you have all the facts, is there a blower we could use on the outside of the house with the existing 6” ducting to withstand that extreme pressure and achieve 400 CFM of exhaust while still being super quiet?

Our installer left us a spec sheet for the “300 CFM” version of the same Ventahood insert, which would require no cutting open drywall and no modifications to the cabinetry. Of course, I am the difficult who client who does not want that. I’m not sure how I could explain to them that with a 6” duct and maybe 1” in WC of pressure, I don’t want a range hood performing at maybe like 200 CFM for very well researched and proven reasons, especially considering we are paying several thousand $ for the insert and the custom cover – the whole point of doing this and having an engineer specify the duct size was because we wanted it to actually work.

Little did she know… asking for kitchen ventilation that actually works is totally radical 🤣