Quote Originally Posted by Havien
This is for a 56" x 56" x 79" tent/cab.
I currently cool 2 600 watt easycool 6" hoods in my flower tent with 1 fan 6" inline fan 400ish cfm (growbright highflow), It goes intake from outside tent > light > light > Fan > exhaust. I have a seperate 6" fan attached to a carbon filter that keeps negitive pressure on the tent, filter > fan > exhuast.
Sounds good. Were you thinking a 3rd 600?

Quote Originally Posted by Havien
My question is, if I could add a third hood/light (intake/light/light/light/fan/exhuast) or if I need a bigger fan and if so what size.
You didn't provide current temps, but I'm inclined to say you should be fine with your present fan setup. The only potential problem could be the routing of the ducting within the space available. Too many bends can drastically lower airflow. Plumbing the 3rd light in with the charcoal filter may be an option, if you find airflow compromised with all 3 on the same route. Do you use a speed controller, or do the fans run full-bore?


Quote Originally Posted by Havien
Second question, I am currently pulling the air through the hoods, I've read on various forums that some people recommend pushing the air through so that the ducting has a positive pressure, intake > fan > light > light > exhuast, if anyone has further input (if cooling temps are similar etc) on this please share, since it does seem like a good idea to avoid leaks, and I do plan on using co2 next grow. I have always thought inline fans were more effiecent at pulling then pushing, hoping someone proves me wrong =). Any ideas welcome and appreciated.

Thanks
The only reason I can think of to push instead of pull is if the ducting is sloppy/leaky. The reason being, if you pull the unfiltered tent air into poorly sealed light ducting, the smell just gets pumped right out. In a small tent set-up, you can go either way, just check the seals. Given a choice, I'll pull. The seals of the AC hoods are designed so that negative pressure improves the seal of the gasket. Pressurizing the hood works against the gasket sealing properly.

BTW: Always double check the seal of the hood, add weatherstripping if needed. I was visiting a friend recently, one of his hoods was leaking so badly it was whistling like a tea kettle. The sound was driving me nuts.
HTH