ventilation for 4x4 tent

Hi all, I am starting my first grow in a 4x4x7 tent and plan on using a 400 watt MH with standard reflector. Can anyone suggest how much cfm I will need? The ambient temp outside tent will be around 65 -80.
I have two tents and will set up the other one with HPS once I get this one going.
Thanks for your help.

So the hood is not air cooled? That would be my first suggestion, get an appropriate air cooled reflector. The temps outside my 4x4 tent are 72 degrees and I can run a 600w HPS in it with a 365CFM fan connected to my carbon filter. I don't have any temperature problems.

When I use a 400w (particularly hot days) I don't think it's more than 4 degrees warmer in the tent than outside of it.

Get a pretty big fan and you'll have the options to do some pretty big things.

If the fan is for odor control between 180-220 cfm would be plenty. If the fan is to cool your lights you should have enclosed reflectors and the same fan would work. If I had that size tent I would use a 1000W with a large air-cooled capable reflector. I saw a new reflector at my local store that the employee was raving about, it distributes heat/light more evenly and allows for very close placement.

Im using the same size tent and am running one 600w with a Dayton 465 cfm blower. It's about 3 times the flow than recommended but Im not using an air cooled hood, got one of those handy adjust-a-wings. At mid-summer when the temps would climb to 85 outside the tent, the negative airflow created by the big cfm fan never allowed my tent to climb above 80 as long as the tent was kep closed to keep the negative air pressure.

Now that fall is coming with cooler temps I have to watch it to make sure it doesnt get too cool in the tent. Sounds like you are running a similar setup as what Im doing but like others it depends on how much control you have over the the ambient temp. Im indoors with a controlled climate surrounding the tent so it's very easy to maintain cool temps. Sucked over the summer on the hot days when we had to turn on the AC but you do what you must for the sake of the buzz. Hope this helps.

I certaiy wouldn't go less than 300CFM, particularly if you are going to connect it to a carbon filter.

I'm planning on running a 1000w light in mine when the summer finally dies off and the night time temps get low enough to cool it. Right now it's all in a second bedroom and the summer heat has been a problem and I've had to use my A/C to keep things cool. Still cheaper than buying sacks though.

If you stay with a 400MH you don't need a large (200 cfm or less) fan or air-cooling. Carbon filters are rated for different cfm and having smaller components would save you money and keep the noise down. If you go big (1000W) you should run a small (200 cfm or less) for odor control and a larger fan (300+ cfm) dedicated to air-cooling your bulb. A 240 cfm fan would replace all the air in your tent in less than 30sec which is more than adequate to keep the smell down or nonexistent.

Great info, thanks. That helps because I didn't think it would need that much fan. Since is a first grow Ithought I'd stick with a 400watt this time, but I will look at the a/c/reflectors. Money is tight right now but I want to do it right. If I get an a/c reflector would I need two ex.fans or could I suck room air thru the light and outside using one fan?

mavs215 said:

Great info, thanks. That helps because I didn't think it would need that much fan. Since is a first grow Ithought I'd stick with a 400watt this time, but I will look at the a/c/reflectors. Money is tight right now but I want to do it right. If I get an a/c reflector would I need two ex.fans or could I suck room air thru the light and outside using one fan?

Click to expand...

If you're on a budget (like myself) just get the fan, filter and an adjust-a-waing. The light spread is amazing and it does allow the plants to get closer to the light with the way it spreads heat throughout the garden instead of directly below. Thats all I use and havent had any problems at all except for low humidity. In fact we arent using a filter at all at the moment and were 6 weeks into flower. Helps with lemony flavored plants and some home made diy odor control.

I wouldn't use less than a 200cfm fan. You may run in to problems now and you certainly will if you expand to a larger light. You have 122 cubic ft. A 200cfm fan will change it less than twice a minute. I'm able to keep my grow room @ good temps with a 600w light and don't need A/C. A larger fan isn't a huge cost difference, buying it twice becasue your first one was adequate sucks balls.