# two inline fans on the same duct? or parallel?



## haole420 (Feb 27, 2011)

i have two 6" 400cfm inline fans and need advice on how to most efficiently mount them.

currently, the flow looks like this: carbon filter >> duct >> 6" fan >> duct >> air-cooled reflector with 600watt HPS >> 2nd air-cooled reflector with 600watt HPS >> duct >> vertical 8" cool tube with two 600watt HPS >> duct >> 6" fan >> duct >> another carbon filter (as a backup for when the first one gets spent).

this setup does not seem efficient. pressure builds just ahead of the first fan in the reflectors and leaks out small gaps in the glass seal at the corners. seems like the room runs cooler when just the one fan closer to the exhaust is on.

the horizontally mounted air-cooled reflectors are both on while the vertical cool tube and its two 600watt bulbs haven't been fired up at this time.

would i be better off putting the two fans closer together toward the exhaust end of the duct so they're working together to pull the air? is there any benefit to having two pulling on the same intake? i'm assuming that'll get me closer to 400cfm actual flow.

alternatively, i could split the two setups and run one fan for each pair of 600watt HPS bulbs. but that would require another carbon filter and another hole in the ceiling for exhaust.

any ideas?


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## Mother's Finest (Feb 27, 2011)

Mounting two fans inline with each other increases the "torque" of the airflow but does not increase the maximum unrestricted flow. If you put two of the same fans facing the same way on either side of a 2' tube, the airflow would be roughly the same as if there was only one. If you put a filter on one end of the tube, then two fans inline will move more air past the restriction (up to one fan's maximum air velocity) because of the aforementioned torque increase. The best way to set it up will depend on how restrictive the system is.

Some small amounts of increased flow will be seen using multiple fans inline with each other because of things like an increase in pressure at the second fan's intake. So, the above is a general explanation and not a firm rule.


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## kingofqueen (Feb 27, 2011)

Yeah I,m with him .Two benefit from 2 fans parrallel they would have to y into a larger duct for maximum airflow. I set mine up like you showed except my fan is ahead of my light pushing . I taped up any leaks around the glass in my hood .


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## haole420 (Feb 27, 2011)

Mother's Finest said:


> Mounting two fans inline with each other increases the "torque" of the airflow but does not increase the maximum unrestricted flow. If you put two of the same fans facing the same way on either side of a 2' tube, the airflow would be roughly the same as if there was only one. If you put a filter on one end of the tube, then two fans inline will move more air past the restriction (up to one fan's maximum air velocity) because of the aforementioned torque increase. The best way to set it up will depend on how restrictive the system is.
> 
> Some small amounts of increased flow will be seen using multiple fans inline with each other because of things like an increase in pressure at the second fan's intake. So, the above is a general explanation and not a firm rule.


given all the twists and turns involved (several 90-degree elbows/turns) and carbon scrubbers on both intake and exhaust, i'm assuming i've got a somewhat restrictive system. should i just stick with the inline setup as it currently is? or would there be any additional increase in torque if i put both fans closer together to pull from the exhaust end?


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## Mother's Finest (Feb 27, 2011)

haole420 said:


> currently, the flow looks like this: carbon filter >> duct >> 6" fan >> duct >> air-cooled reflector with 600watt HPS >> 2nd air-cooled reflector with 600watt HPS >> duct >> vertical 8" cool tube with two 600watt HPS >> duct >> 6" fan >> duct >> another carbon filter (as a backup for when the first one gets spent).


Correct me if I'm just stoned but you don't seem to mention the growing area. Where does the air in the grow are fit into this? If the first filter is in the grow area then where is the intake for the grow area's air? Insufficient intake venting for the room can restrict any exhaust you try to push out of it.

Imo, it's best to have the light's cooling air system separate from the plants' breathing air system. The lights get air from a cool source like outside, under the house, etc. and that air is exhausted outside. This can keep the lamps very cool for at least most of the year. The grow area then gets its own fresh air through just a vent that is pulled through and out by a carbon filter-fan-duct-outdoors system. An oscillating fan works well for air blowing onto the plants.

For your current question, if you intend on keeping two filters in the system then your best bet with two fans may be to put one on each filter. The last fan pushing air through the filter will likely do better pulling it. If it's not too inconvenient, you might try moving the first fan to after the first two lamps and seal everything extra well with name brand duct tape. You could then try having the first fan exhaust outside, creating two separate systems.


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## haole420 (Feb 28, 2011)

Mother's Finest said:


> Correct me if I'm just stoned but you don't seem to mention the growing area. Where does the air in the grow are fit into this? If the first filter is in the grow area then where is the intake for the grow area's air? Insufficient intake venting for the room can restrict any exhaust you try to push out of it.
> 
> Imo, it's best to have the light's cooling air system separate from the plants' breathing air system. The lights get air from a cool source like outside, under the house, etc. and that air is exhausted outside. This can keep the lamps very cool for at least most of the year. The grow area then gets its own fresh air through just a vent that is pulled through and out by a carbon filter-fan-duct-outdoors system. An oscillating fan works well for air blowing onto the plants.
> 
> For your current question, if you intend on keeping two filters in the system then your best bet with two fans may be to put one on each filter. The last fan pushing air through the filter will likely do better pulling it. If it's not too inconvenient, you might try moving the first fan to after the first two lamps and seal everything extra well with name brand duct tape. You could then try having the first fan exhaust outside, creating two separate systems.


intake is coming from central a/c vent boosted with 220cfm fan as well as keeping the door to the room wide open during the day.

good point on keeping the cooling system separate, but that involves another intake, another exhaust. climate in my area is too hot/humid to use outside air 6 months of the year.

still a month or so before i fire up the vertical setup. i'm thinking two separate, parallel systems would be better: shorter ductwork, fewer turns, probably at least 2X the flow i'm getting now.


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## haole420 (Mar 3, 2011)

Mother's Finest said:


> You could then try having the first fan exhaust outside, creating two separate systems.


After experimenting with several different configurations, decided on 2 separate systems, as you suggested.

the twists and turns in my ducting for the air-cooled lamps is restricting my airflow. going to bag the idea of the vertical setup and just set that tube on its side with a reflector. that way, air flow is in a straight line, at least between the 2 horizontal hoods and the dual tube. so carbon, duct, hood, hood, tube, duct, fan, out to attic.

the second system for ventilation will be super short with carbon filter near ceiling, fan, out to attic, maybe a muffler somewhere in there. theoretically, at 400cfm, the 6" fan should be able to exchange all the air my 10x10x10' room in 3 minutes or so. assuming i have adequate air intake, this should be sufficient for 1000cuft, right?

as far as intake, i have a puny 5" central A/C duct with a 250cfm rating when the A/C is on (160cfm when off). planning to add passive air intake but need some advice as to size of opening.

i read somewhere that passive air intake should be at least twice as large as the exhaust. so 6" fan means 3" radius, so 3in^2 x 3.14 = 28.26sqin. double that you get 56.5sqin. so (56.5sqin / 3.14)^0.5 = 7.3in. so would an 8" passive intake be sufficient?


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## jonnynobody (Apr 20, 2012)

I know this thread is old but it has helped me immensely. I think ventilation is one of the most difficult things things to optimize in a first time growers garden.


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## + WitchDoctor + (Apr 20, 2012)

Did you already take out the second carbon filter? Blowing through a carbon filter is usually inefficient. Might be part of the problem.


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## SammyBerne (Apr 20, 2012)

an 8" passive intake did not work for me with a 6" exhaust

I have an 8" can fan (500cfm) from home depot pulling fresh air into the room through a straight 5' section of 8" duct. My exhaust all 6" is a home made carbon filter>duct>air coold hood>duct>active air fan (400cfm)>duct

and at this point the exhaust is still kicking the intakes ass, anyone have some suggestions?


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## barrybiddle (Jan 25, 2013)

SammyBerne said:


> an 8" passive intake did not work for me with a 6" exhaust
> 
> I have an 8" can fan (500cfm) from home depot pulling fresh air into the room through a straight 5' section of 8" duct. My exhaust all 6" is a home made carbon filter>duct>air coold hood>duct>active air fan (400cfm)>duct
> 
> and at this point the exhaust is still kicking the intakes ass, anyone have some suggestions?


Maybe using a dimming controller to adjust the speed manually of the unit you need to slow down.


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