Exhaust fan placement slightly inside the grow cab?

It seems to be the standard for the exhaust fans in grow cabs to be fitted right onto the back (or top) panneling of the cabinet. I was curious, since my stealth cab would be less stealthy sticking too far out from the wall, if it were plausible to have the exhaust fan sit a couple inches inside the cab to reduce the distance my ducting + carbon scrubber will take up behind the cab. Should i expect any efficiency loss this way as long as i can "afford" the room im losing on the inside of my cab?

I'll be using a DIY carbon scrubber which is smaller than average (scotchbright towels sewed together filled with activated carbon.) I'm also hoping this makes it easier to do scrubber maintenence since It'd be a pain to get behind my cabinet without removing everything inside it first. For what it is worth my grow cab is about 4 feet long, 1 and 2/3rds feet wide, and 5 and 2/3rds feet tall. I plan to grow up to 4 plants in 5 gallon DWC bubble buckets. The cabinet is slightly angled against a wall so that one side sticks out 7 inches from the wall and the other side 2 inches.

Lastly this is just a PC fan carbon scrubber outtake fan, not a cooltube. I hope I gave as much information as would be needed, but to be honest i haven't seen any designs that had the exhaust fan placement anywhere other than flush with a wall of the cab.

*Edit* I may be using an inline duct fan for higher CFMs on the outtake, unless I find a local PC dealer that sells powerful enough PC fans.

What kind of light do you plan on using

250w MH for veg, then 250w HPS for flowering. Here's a cab temp test i did yesterday with no exhaust (looks promising to me.)


71.6 – 50% humidity 2:42 PM
79.1 – 46% humidity 3:00 PM
82.9 – 42% humidity 3:15 PM
85.2 – 39% humidity 3:45 PM
87.0 – 35% humidity 4:00 PM
87.0 – 34% humidity 4:40 PM

Doesnt look promising to me, looks more like an oven.

You only ran your light for two hours and it already got over the limit, 87 is too hot. And like i said you only ran it for two hours. I dont think running any kind of HPS without proper ventilation is wise.

That was with no exhaust or intake fan though, and I thought that as long as it was below 90 the temperature wouldn't hinder growth too much. I stopped testing because after 40 straight minutes the temps didn't increase at all. You think I should retest it for longer? Regardless i'm trying to get information on exhaust fan placement I am not growing anything at the moment.

Also is it unrealistic to expect a few degree temperature drop after adding in intake / outtake fans? I always thought that was one of the primary methods to drop cab temps. The current fan I have for intake is 78 CFM, and im still searching for a strong enough fan for the exhaust.

Anything over 85 will hinder growth unless you co2 enrich the room.

Test your set up for a full 24 hours and check it often and record temps. Maybe that part of the house is hotter at a certain part of the day?

Cool, then I'm going to stay optimistic since my exhaust system won't have to cool the cab down too much. It may be possible that the room temp fluctuates, but my apartment is mostly underground with central air so it shouldn't very much. Worst case scenario I can always keep my night cycle during the times that the room may be a degree warmer.

So does anybody have any insight or links to grow journals where the exhaust fan wasn't flush with the cab walls? I know it is atypical, but I want to be sure i'm not shooting myself in the foot by making it significantly less effective.