I'm in the midst of a kitchen remodel and have selected and ordered a hood with external blower. Sized per the salesperson's recommendation at 1200cfm (900cfm would have been the next size down). Their advice was you can always run the 1200cfm lower but you can't turn the 900cfm up. But after reading here and googled posts, I see that makeup air may be an issue with either one of these high cfm beasties. I've been unable to find good info on properly sizing the hood blower itself, let alone means of make up air. I live in Austin, TX, rarely does it dip below freezing, and we have our summers in the high 90's low 100's for several months, then several more in the 80's. So Just putting a big hole in the wall with a damper will probably murder me on cooling costs. Sure I can open a window when I run it, but we cook at home a lot and it will get mighty hot that way.
Hood and external blower are Viking (VCWH3648BK)
and External Blower (DEV1200)
They are being installed over a Thermador 36" 5 burner gas cooktop. (SG3365C13) Total BTU output 58,200. Single largest burner 16,000 BTU
I called Viking and am told I need a 10" duct either way (900 or 1200cfm). The rep wasn't able to help me size the unit. She was quoting some very loose guidelines. They had no suggestions about makeup air.
My vent run will be straight up from the hood, 90 degrees to exit exterior wall of house (duct will go up into ceiling and turn to run in between the floors, no attic access at this location).
The existing original exhaust looks to be a 6" diameter. (See photos). I was thinking I might just leave it as is and install a damper which opened in vs. out on the exterior portion to supply a good portion of the make up air directly behind the exhaust hood (It seems like from what I read commercial kitchen units often work similarly with the return air being ducted to the hood location). The existing duct will not be removed (as the new one will just go straight up to the ceiling going up through the 2x4 wall as the current one does).
Photo 1,3 - Cabinets which will be removed, existing vent duct here.
Photo 2 - close up of the existing duct, you can see what I believe is a 6" round pipe.