Exhaust fans for grow room?

[RandyL]

I've got my grow room all set to bring in some clones (planning to start 20), but we are having a problem with the temperature. The room is 8' by 8' with a 9' ceiling, and 2 400 watt MH industrial lights are in the room.

Lighting is good, but temperature is too high. Once the room warms up, temps are in the 85-90 range.

My neighbor is helping me with the room, as he has done this before. I think the problem is in the exhaust fan, which is a $15 8" 3 speed ordinary fan from Home Depot. We have it blowing air from the top of the room (about 7 feet off the floor) into a 25' long 6 inch flexible dryer duct to the window.

Today I got a second 25 foot 6 inch diameter dryer duct so my A/C unit can blow cooled air into the room. The volume of air coming into the room is much greater than what the 8" fan blows out.

I can't guess how much CFM the exhaust fan is rated for, but I'm thinking that any fan at a store with a cfm rating on it will be more powerful.

To test the cooling system, we let the room get to 90 degrees, then turned the a/c fan and the exhaust fan on. In about an hour, the temperature will drop to 84-85 degrees, but if we back off on the exhaust fan from its high speed setting, temps start to go up again.

Any ideas for a good exhaust fan?

 

[Screwnuts33]

Put your fan on the other end of the duct so it sucks the air out instead of trying to blow it out. If that does not do it get a higher CFM fan. You are dealing with 576 cuft of space so you will need a big fan to exchange that much air. Something is 1200 cfm range would be a good start.

 

[RandyL]

Unfortunately, I need to keep the fan on the inside. The only available window to get the air out of the apartment overlooks the parking lot and anything out of place in my window will be VERY visible. I went and got a 2 fan window fan unit off of Craigslist just to hide the ducting. I have the 6" dryer duct taped to the window fan so one of its fans can help pull air out of the room and exhaust it outside.

The lights I'm using have big semicircular dome reflectors on them (like the lights you'd see in a warehouse), with the MH bulbs pointing straight down. I was thinking of getting 2 ventilated reflector housings that hold the bulbs horizontally, so the duct can be directly connected to them to get the hot air out of the room better.

I've also seen inline 6" ventilation duct fans in hydro stores. $60 for the cheapest one, but we need to improve the exhaust before we start growing.

This morning, I turned the lights and fans on in the room. In under an hour the temperature rose from 73 degrees to 83. It WILL go up more in the next hour.

From what I read, the good temperature range for marijuana is about 70-85, but we are too close to the upper end of that range.

 

[Hogdady]

fans are essentially an air pump. fans are more efficient at pushing air than pulling it. you need a 6" to 8" inline fan and carbon filter to pull air from the room and push it out the window. you are gonna have an odor problem....

 

[plantpot4fun]

i got a 750 cfm fan from ebay for 90$ im really happy with. just search for vortex fan. shipping was like 10$

 

[RandyL]

Got a barrel fan at a hydro store yesterday. 465 cfm, model Active Air ACF 465. It moves a lot of air! I've got it blowing into a 6" duct that run to the window, and so far it has helped a lot. It has 2 intakes, 1 on each side of the barrel fan, so I'm going to run 4" duct from those intakes to the lights to get the heat out of the room more before it heats the whole place up too much.