I LOVE HPS lights! Heat question.

Doing a box grow (about 4 feet tall, 3 feet wide) and had 5 plants about 2 months old under 4 100 watt CFLs.

I ordered a 400 watt HPS light and since using them for ONE day they have grown each 2 inches. They also look much fuller and healthier (using foxfarm nutes over MG ones help also) and I am very impressed.

Only issue was that now the box is at a 84 degree temp which I read is too high (humidity is right at 50). Should keep temps below 80 I thought. Right now I have a box fan blowing in cooler air and that is helping a bit. I have not ordered the exhaust fan yet which I was going to put at the top of the box (exit) running to the attic with a carbon filter. Will the one exhaust fan be enough to keep the heat at a reasonable temp? How big shoudl the exhaust fan be? If not what else should I consider?

Anyways thanks for any suggestions. I am pumped about the lights!

Depending on the size of the room, I'm thinking once you get that outtake fan installed you should be golden.

Thanks Zonked. The grow box is in a closet that is about 10 by 10 maybe a bit bigger. But that part of the house usually stays cool.

ya get a intake and get another to blow the hot air in the out room so it stay away from being hot in the room mate

babyboi420 said:

ya get a intake and get another to blow the hot air in the out room so it stay away from being hot in the room mate

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The Exhaust will be blown into the attic above the closet so the air will be getting a long way from the room the closet is in! I need to get a carbon filter but that comes later.

I thought the intake could just be a few vents and doesnt have to be a fan because the exhaust fan will pull the air thru the vents???

blzntd19 said:

The Exhaust will be blown into the attic above the closet so the air will be getting a long way from the room the closet is in! I need to get a carbon filter but that comes later.

I thought the intake could just be a few vents and doesnt have to be a fan because the exhaust fan will pull the air thru the vents???

Click to expand...

That is a passive intake, and will be fine for that space, as long as cool air is drawn in. The thing you haven't described is how many CFM you are drawing for that space.

You should consider some sort of CO2 so that your plants can with hold those temps, just my opinion.

Serapis said:

That is a passive intake, and will be fine for that space, as long as cool air is drawn in. The thing you haven't described is how many CFM you are drawing for that space.

Click to expand...

CFM? Sorry but I have to ask what that is! Still a noob. I did a search and couldnt find anythinig for CFM. Thanks!

THis is what I found on CFM

Minimum Required CFM to ventilate room:

(CFM required for room – step 2) + (CFM required for room – step 2 x 10% (2 air cooled lights)) + (CFM required for room – step 2 x 5% (CO2)) + (CFM required for room – step 2 x 20% (Carbon Filter)) + (Ambient Temperature 0 (Air coming from air-conditioned room)).

= (171cfm) + (171cfm x 10%) +
(171cfm x 5%) + (171cfm x 20%) + ( 0 )

At the moment I just have a box fan that is blowing cool air into the box to keep the air fresh. I plan to get a carbon filter as well to cut down on the smell so I guess this is how I figure out what size fan I need?


= 231cfm – this is the absolute minimum cfm required to ventilate your room.

Here is a good way to find out how much CFM you need to pull for a given space. Sweeet.