Hey, I'm sure people are reading and relishing what you've written so far. I still need to re-read all that you've said, but right off, I'd like to know how enclosed light-hoods affect your CFM ventilation calculations.

I made a spreadsheet with formulas so that I could goof with variables: room dimensions, feet of duct-work, number of 90 degree turns and such. Seems that your equations assume that the grow light is open to the room, rather than being hooded and ducted on its own, or in conjunction with the room's exhaust. I'd expect that if the heat did NOT leave the lamp and run all over the room, that it would be less of a problem to exhaust. It should be exhausted at its source (at the hood). This may lower the need for so many CFMs.

I'm planning two tiny veg and bloom closets (2.5 x 3 foot each, with 6-foot ceilings).

I'm going with a suggestion from stinkyattic I believe, to have my two grow rooms heat/cool each other by setting the light cycles out-of-synch between the veg and bloom rooms and exhausting one room into the other. Correct me if I'm wrong, but unless I get some solenoid-driven dampers to switch flow around as the lights cycle, it might be easier to just have an dedicated exhaust fan in each room, pushing one room's exhaust into the other. Of course I'd have some light-blocking feature so that the light would not bleed over between the veg and bloom rooms.

I'm thinking of just having both veg and bloom rooms exhaust directly through their cooled lamp hoods continuously, with their own dedicated can fan directed into a separate carbon filter. The outflow will be directed into the other room, and I'd adjust it if I see the temp in either room rise above 78F. The extra cost of duplicate fan and carbon filter will allow for some redundancy (e.g., in case one fan gives out), and the two carbon filters will last longer since there are two of them. But, two fans will use more electric power and add some heat on their own.

The other possibility is for me to include ductwork INTO both enclosed hoods and exhaust them without any carbon (odor) filtering. This would draw clean air into the light hood, past the lamp, out of the hood and out of the grow room. I'd ONLY filter the air from the ROOM itself (the air that is in contact with the plants). This is more complicated than I wanted to get, but is an option if I get some consensus here or impetus to pursue it.

What do you think? How much would directly handling the heat from the lamp/hood change the required CFM to properly ventilate a grow room (or specifically, a veg and bloom room exhausting into the other room)?

I'm more concerned about the NOISE of the fans rather than the added cost of two fans to exhaust the two rooms. If I think long enough, maybe I could figure out how to do it with one big can fan instead of two smaller ones. But again, stealth dictates for me to go with less noise.