12 feet of intake ducting, do I need a fan?

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completeretard

Member
I'm doing an attic grow with a 80x80x160 cm tent. I have 4 plants under a 400w HPS. I'm using a 4" fan to exhaust into the surrounding attic.

The intake is passive, there's 12 feet between the source of air and the tent, and the ducting has to round a 90 degree turn.

Should I add an intake fan? Or will the exhaust fan create enough suction to pull the intake air through the 12 feet of ducting?

Thanks for the help.
 
T

TheLoneRaptor

Member
I guess it would depend on the CFM of the fan, but if your just trying to cool one light with that small of ducting I would think it would be fine, but I also advise against venting to the attic unless you have a carbon filter also hooked up b4 venting to that because I have seen cases where smell leaks out from the attic to your neighbors with even just a 1 light flower set up. You also have to remember that FLIR cameras pick up on heat that escapes from your house, which is mainly spotted leaving your attic. Then again it all depends on where you live and if you have to worry about all this stuff :D
 
C

completeretard

Member
Hey thanks very much. I have a carbon filter, given that I'm exhausting into the attic anyway I suppose I could just exhaust into that.
 
joe blow greenthumb

joe blow greenthumb

Well-Known Member
TheLoneRaptor said:
I guess it would depend on the CFM of the fan, but if your just trying to cool one light with that small of ducting I would think it would be fine, but I also advise against venting to the attic unless you have a carbon filter also hooked up b4 venting to that because I have seen cases where smell leaks out from the attic to your neighbors with even just a 1 light flower set up. You also have to remember that FLIR cameras pick up on heat that escapes from your house, which is mainly spotted leaving your attic. Then again it all depends on where you live and if you have to worry about all this stuff :D
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You people are funny worrying about flir. If you're not growing a dispensary grow or larger then Uncle Sam has better things to do than bust your little operation. Not only that, they can't use flir to arrest you because it's just like a search without a warrant.
 
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TheLoneRaptor

Member
It doesn't stop them from harassing you, replacing your meter, or still coming to your house trying to trick you into letting them in. I try to avoid all avenues of problems. And you have to remember that cops are stupid and think there is some record breaking promotional worthy operation going on behind the door of every house they suspect. Its in their nature to suck at life.
 
Twitch

Twitch

Well-Known Member
joe blow greenthumb said:
You people are funny worrying about flir. If you're not growing a dispensary grow or larger then Uncle Sam has better things to do than bust your little operation. Not only that, they can't use flir to arrest you because it's just like a search without a warrant.
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its not uncle sam i am worried about it is the regular law enforcement
 
C

completeretard

Member
I have no fear of FLIR cameras, I don't think it's a policing priority where I live. I doubt the police have the resources to be flying around in a chopper pointing cameras at my roof looking for my four plant grow.
I do have a fear that 12 feet of ducting is just too long for my passive intake though! To clarify, I'm not concerned about cooling, I'm concerned about a lack of fresh air intake...
 
4

4020 team

Member
Where is fresh air coming from, are you just drawing from attic or is it ducted to outside? Always best to have some sort of inline filter before entering into your grow area. Carbon filter after that for smell ect is fine.
 
ogkush970

ogkush970

Active Member
What is your exhaust setup, if your gonnna use a fan make sure its roughly 25% less than your exhaust. A booster fan will help move the air through that 90°. Like 4020 said use a filter.
 
C

completeretard

Member
So I decided to get an intake fan. My room is 36 cubic feet and the exhaust fan is rated at 235cfm, the intake at 180. I definitely don't need them running all the time, but what is optimal?
 
Twitch

Twitch

Well-Known Member
if ur tent is sealed besides the exhaust and the intake it will pull just as fast and as much as it is pushing out

i dont think u will need an intake fan, but if you feel you need more air upgrade your exhaust fan
 
B

budballer

Well-Known Member
that's a decent amount of static pressure, id say an intake fan with a slightly lower CFM than your main fan certainly couldn't hurt. However i'm going off the cuff here. If you look around there are ways to roughly estimate how much fan power you need for length of duct + carbon filter etc.
 
C

completeretard

Member
Cheers. It may not be strictly necessary, but I think there's so much distance between the source of air and the tent (and a 90 degree bend) that it's worth the investment. Using my exhaust fan tends to reduce the humidity by ~30%, and the intake fan should push air with a higher RH in to compensate for that.

Also I've worried about it to the point where I'm buying the fan just so I don't think about it any more. I'm hooking both fans up to my thermostat controller, and they'll both kick on when the temperature goes above 77 at the tops. I might run them on a timer later.
 
C

completeretard

Member
Twitch. said:
let us know if that works
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It's been working very well. The short fan on times due to the thermostat have been keeping the humidity around 70%. When I switch to flowering next week I'll be running both fans 24/7 to achieve the opposite effect, and leaving the heater on the thermostat so that the temperature doesn't drop too much at night.
 
wolftickets500

wolftickets500

Member
i would put a small booster fan in a straight run of ducting would be ok but when u make a 90 deg turn you loose about 25% of the suction
 
echelon1k1

echelon1k1

New Member
completeretard said:
I'm doing an attic grow with a 80x80x160 cm tent. I have 4 plants under a 400w HPS. I'm using a 4" fan to exhaust into the surrounding attic.

The intake is passive, there's 12 feet between the source of air and the tent, and the ducting has to round a 90 degree turn.

Should I add an intake fan? Or will the exhaust fan create enough suction to pull the intake air through the 12 feet of ducting?

Thanks for the help.
Click to expand...
Yeah intake fan would help... You lose half your CFM on a 6m duct run alone and that's with minimal bends...

You want to refresh the air in your tent at least 3 times a minute and a 6" inline will do that and more in a 80x80x160 space...
 
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