Heat questions

I am planning out a cab design- the dims for this cab would be 38 cubic feet. 56"H x 4'W x 2'L. I have a 6" 250 CFM in-line fan, and that would exchange the air in the cab 6.5x per minute-on paper at least.I would cut 2 intake ports in the cab, 6" diam. on the opposite side of the cab, toward the bottom.My HID is a 400w HPS/MH conv. with a basic hood with no glass and no direct exhaust fitting. I am going to put the fan in at just above where the hood will be when plants are max size.

Will this be a sufficient ventilation system for a six-pac of plants in 10" pots?


When plants are max size I will only have 12-14" of room between the bulb
and the 2ft. tall plants.Is that enough room so the plants don't get fried by the light?

Thanks in advance.

put it this way- Ive got a 1k hps and my plants are about 8-10in away at most.. just keep the air moving OUT.

shamegame said:

I am planning out a cab design- the dims for this cab would be 38 cubic feet. 56"H x 4'W x 2'L.

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Well 4 x 2 is fine, 4.5 foot high? Too low in my opinion. The light and reflector will take 6-12 inches, the pot 12 inches and you need it 12 inches away from the top of the plants. That leaves you with a plant height of between 1.5-2 feet in height.

If you're growing clones in a SOG that should be fine, if you're growing seed plants you may run into problems.

shamegame said:

I have a 6" 250 CFM in-line fan, and that would exchange the air in the cab 6.5x per minute-on paper at least.

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4 foot x 2 foot x 4.5 foot = 36 cubic feet. To exchange the air in this space once every minute, you'd need a fan size of 36cfm. In reality you need a fan size, depending on the heat build up of two to three times that amount to take into account the resistances to the airflow. So you're looking at a cfm fan of about 72-100cfm. Your 250cfm would therefore be sufficient in theory.

shamegame said:

I would cut 2 intake ports in the cab, 6" diam. on the opposite side of the cab, toward the bottom.

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The optimal passive port sizes for passive intake cabinets is somewhere between 2-5 times the exhaust port hole size. So a 6" intake is 28.26 square inches. You want a passive intake of between 56 square inches and 140 square inches. My advice is to go for the 5 times intake size for small sized cabinets.

shamegame said:

My HID is a 400w HPS/MH conv. with a basic hood with no glass and no direct exhaust fitting. I am going to put the fan in at just above where the hood will be when plants are max size.

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In my opinion a 400w HPS is going to generate faar too much heat in that small space to control. Sure the exhaust fan will remove the inductive heat, but it won't remove the infra-red radiated heat and if allowed to build up will cause damage to your plant.

You'd be far better off looking at a 250w HPS for that area or large wattage CFL's (125, 200w).

shamegame said:

Will this be a sufficient ventilation system for a six-pac of plants in 10" pots?

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In my opinion, with a 400w HPS, no.

shamegame said:

When plants are max size I will only have 12-14" of room between the bulb and the 2ft. tall plants.Is that enough room so the plants don't get fried by the light?.

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Again, in my opinion no.

Thanks for the feedback.I could use half a closet instead of a cab, I was just trying to figure out how to vent it while being able to keep the light out during dark time. The closet doors are mirrored and made of metal so I cannot make any cuts or alter them in any way. I do not need to worry about security that much , but I cannot cut holes in the ceiling of the closet up into the attic. The dims for the cab are based on a framed shelf system inside the closet I was just going to buy a bunch of plywood and enclose it.

So if I made the cab another foot or so taller would that help? I wont be able to make the footprint bigger than 2x4ft.I could
make 2 x 8" wood spacers and put the fan and intakes in them( so both closet doors would be openand and these would fill the space) but it will take some doing....