Hi guys - I am in the final design stage for my grow room and was wondering what size of fans I will need for my ventilation. The room will be 4.5ftx2ftx6ft - I was thinkin gof using 2x120mm PC fans- 1 to blow cool air over my light/ reflector and 1 to extract the hot air from the room. Sound about right, or am I gonna need to go bigger?

it depends on what type of light youll be using?

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Typical stoner!!! Forget to include the most inportant bit of info for the ?!!!
Due to £££, will start off with a 600w MH for the veggie and a 600w HPS for flowering. Once I start to see the £££ benefits of not buying shit every week, will sort out 1000w of each.

Wow, that alot of heat to put in the room. Are you using just a reflector without a cooling tube? because it's going to cook that room
besides cooling the light and room temperature, your not considering your plants if thats all the fans your going to use. You need plenty of fresh air in and exhaust air filtered out. And at least a 6" desk fan to blow on the plants to keep air moving around.

what would you recommend in that case for lights? 400w?

Almost any light over 250w in that small room will make it too hot. I would get a cooling hood so the light can be cooled seperatly.
But you can try a 400W with some fans, but it would be hot and if it gets hot in your area you'll have problems. Just not a 600W without some powerful cooling, put a 500 watt heater in the closet and see how hot it gets, thats what a 600W will do.

my wife always says size doesnt matter,,, oops sorry,, didnt know we were talkin about fans...............

Hello - i use a 1000w light in a space about the same as yours. Rule of thumb is to get a fan that can expel the air out of your grow room about 3 times a minute (once is adequate but about 3 is ideal). so 4.5ftx2ftx6ft = 54 cubic feet so in order to expel air from that space 3 times a minute you need a fan rated at 150 CFM or above (54 x 3)

Definitely use two fans - one for exhaust and the other to cool your light BUT get a reflector that has duct attachments and a glass cover so you can vent all the hot air directly out instead of blowing it around inside your box.

two more things -
don't forget to add some passive intake holes to avoid a vacuum
don't forget to add an oscillating fan or two to increase air movement and help strengthen the stems

PS - i use a 170 CFM fan to exhaust my light and a 440 CFM fan to exhaust my cab - vortex style - not inline

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Cheer guys - room design binned and have bought a grow tent - 1mx1mx2m - got my eye on a crackin wee extraction fan that should more than suffice. Hope to be up and running in the next few weeks, will keep you all posted and prob start my own wee journal on here.
Happy Smoking!!