grow tent airflow?

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cell491

Member
Ok so if one wanted to use this grow tent with a 250w HPS + 2x 15w grow spot led's for lighting, how much air flow would be sufficient? I want to only have 2 fans, 1 axial intake fan and 1 inline blower exhaust fan.

I want a slight negative pressure inside the tent (to keep air from escaping without first passing through the inline carbon filter). So i will want my exhaust fan to be more powerful than my intake fan. But I cant decide if i should go with a 4" fan or a 6" fan, most 4" blower fans have a air flow rating of between 140-175 CFM whereas the 6" blower fans are in the 350-425 CFM range, would the smaller fan be sufficient? Would it still be sufficient if later on i moved up to a 400w hps?

I attached a very artistic ms paint rendering of what im considering.

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cell491

Member
ok i found a 4 in blower that does 140 cfm and only draws 22w, the tent is 32.5 ft^3 so at 140cfm... the air would be completely exchanged 4 times every minute... that seems like it would be more than enough for even a 400w but i would still like to know what you guys think
 
purplecream

purplecream

Active Member
hey whats up. Here's a formula i found online to figure out how much airflow you need.
1st. 3.16*the number of watts you are using so in your case 250 watts so it would be 3.16*250=790.
2nd.You divide that number by the degree change you want in your tent. So if room temperature is 70f and you want it to be 80f, divide it by 10, so 790/10=79.
So if you want a ten degree change in your temp use 79 cfm. Hope this helped, let me know if you need me to explain more.
 
Bonzi Lighthouse

Bonzi Lighthouse

Well-Known Member
purplecream said:
hey whats up. Here's a formula i found online to figure out how much airflow you need.
1st. 3.16*the number of watts you are using so in your case 250 watts so it would be 3.16*250=790.
2nd.You divide that number by the degree change you want in your tent. So if room temperature is 70f and you want it to be 80f, divide it by 10, so 790/10=79.
So if you want a ten degree change in your temp use 79 cfm. Hope this helped, let me know if you need me to explain more.
Click to expand...
That is the minimum given a ballest outside the room and not counting for any other Amps in the equation.

I would get the biggest fan i could afford and then dial it down, with a Speedster fan control. Also all the carbon filters I have seen will not work the way you have it. They go on the very end.

http://www.hydrowholesale.com/Ventilation-Accessories/Speedster-The-Speedster-Variable-Fan-Speed-Control.asp
 
jrinlv

jrinlv

Well-Known Member
or you could run the filter, to your hood, out of the hood to the fan, out of the room.

You would only need one 6 inch fan and like someone said before, use a 25 dollar dimmer switch to dial it down a little. It will pull enough new air into the room...Good luck JR
 
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cell491

Member
thanks for the replies guys. yea I know they dont make any carbon filters like that, i was gonna make a custom one using pvc and aquarium grade activated carbon... should be fairly straight forward to build. So you guys think one large 6" blower fan and no intake fan will be sufficient? Ill probably leave one hole open with just a regular air filter on it (to keep bugs and dust out). Will a regular lamp dimmer work to adjust the fan or would i need a special type?
 
C

cell491

Member
purplecream said:
hey whats up. Here's a formula i found online to figure out how much airflow you need.
1st. 3.16*the number of watts you are using so in your case 250 watts so it would be 3.16*250=790.
2nd.You divide that number by the degree change you want in your tent. So if room temperature is 70f and you want it to be 80f, divide it by 10, so 790/10=79.
So if you want a ten degree change in your temp use 79 cfm. Hope this helped, let me know if you need me to explain more.
Click to expand...
so if i wanted to keep it at room temperature I would need a 800cfm fan? holy crap thats a big fan
 
purplecream

purplecream

Active Member
cell491 said:
so if i wanted to keep it at room temperature I would need a 800cfm fan? holy crap thats a big fan
Click to expand...
No i think you'll be fine with less cfm, but remember you're going to have to add cfm to deal with the static pressure from the carbon filter. Oh yea i would go with a passive intake if i was you.
 
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cell491

Member
purplecream said:
No i think you'll be fine with less cfm, but remember you're going to have to add cfm to deal with the static pressure from the carbon filter. Oh yea i would go with a passive intake if i was you.
Click to expand...
word, its cheaper with just 1 fan too. Ill probably pick up one of the 6" 400cfm+ models. Also one last question, if the hood im looking at uses 4" ducting will the reduction in size (4" to 6" adapter on the fan) significantly affect the air flow?
 
purplecream

purplecream

Active Member
cell491 said:
word, its cheaper with just 1 fan too. Ill probably pick up one of the 6" 400cfm+ models. Also one last question, if the hood im looking at uses 4" ducting will the reduction in size (4" to 6" adapter on the fan) significantly affect the air flow?
Click to expand...
No you'll be fine.
 
rzza

rzza

Well-Known Member
i like the filter after the fan, like the original drawing. but you wont need the intake fan, just use the passive intake that your tent provides. the electricity would be a waste becuase the tent will have negative pressure from the exhaust and the passive intakes with suck in the air just fine.
 
mindatlarge

mindatlarge

Member
cell491 said:
Ok so if one wanted to use this grow tent with a 250w HPS + 2x 15w grow spot led's for lighting, how much air flow would be sufficient? I want to only have 2 fans, 1 axial intake fan and 1 inline blower exhaust fan.

I want a slight negative pressure inside the tent (to keep air from escaping without first passing through the inline carbon filter). So i will want my exhaust fan to be more powerful than my intake fan. But I cant decide if i should go with a 4" fan or a 6" fan, most 4" blower fans have a air flow rating of between 140-175 CFM whereas the 6" blower fans are in the 350-425 CFM range, would the smaller fan be sufficient? Would it still be sufficient if later on i moved up to a 400w hps?

I attached a very artistic ms paint rendering of what im considering.

​
Click to expand...
I wouldn't recommend running your "intake" air through your light, unless you don't mind the fact that you're wasting your grow room air (if it's cool air). maybe since you're only running a 250 watter, you don't have to worry about heat as much , but I'd caution about running your humid grow room air through your light bro, probably not a good thing....can i suggest getting ducting for the other side of your light there, and pull that air into the room somehow? Peace, and good luck!
 
rzza

rzza

Well-Known Member
hey mindatlarge.

so i think it is better to pull the [warmer] air inside the tent thru the light and outside. opposed to your suggestion (if i read that correctly) to pull the fresh air thru the light and into the tent.

arent you then warming the air that goes into the tent by letting it pass thru the light first?

im in the middle of RE-arranging a bit and im also looking for the most efficient way to cool the tent.
 
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