So I'm setting up a new grow room and trying to figure out my intake/outgoing fan combo. I'm thinking about a 4" inline at floor level pulling through a hepa filter and blowing in my room for intake, and a 6" inline pulling from my room and blowing through my reflector then a charcoal filter outside the room for outgoing. My question is, is this going to cause a problem with regards to positive or negative air pressures in the room?
Fan selection/positive & negative pressures
- Thread starter hot dawg
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Supernaut
New Member
As long as your pulling out more air than your bringing in, you should have neg pressure.
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6" in and out is def going to be the best way hands down.
Supernaut
New Member
If it were only that simple!
To say "X is best hands down" is a pretty definite answer to a fairly vague description of a situation. There are other factors involved other than fan diameter.
A fan must overcome resistance when pushing air from the fan through the duct to some final outlet. This resistance is static pressure. Static pressure in a typical duct run is caused by the velocity of the air, type of duct material, elbows/fittings, dampers, diffusers, filters etc.
Example:
If your intake is shorter, straighter and pulling through less filter than your exhaust, it's going to have less static pressure in it's line and therefore will be more efficient than your exhaust. If your intake is more efficient than your exhaust you will end up with positive pressure, air will escape the room through any airleaks available which will negate your carbon filter and planned evacuation route.
That means the air from your room isn't going where you want it to go and the whole point in growing indoors is having the ability to control every aspect of the grow.
This above example would be if your fans are the same obviously. A 6" duct fan rated at 250cfm for intake with a 6" inline fan rated at ~424cfm for exhaust would be a different situation.
Ventilation is a vital step in the process and is probably the thing I spent the most time planning and pondering over.
Hey hot dawg! 
to 420 Magazine!
I'm glad you are asking a question about ventilation. Environment is the single most important factor in a successful grow, and proper air flow is often overlooked.
Supernaut is right. I hate when I'm in a grow shop and the guy is saying something like "yeah...you're gonna want a 6" fan to duct that". Different 6 inch fans perform differently. I use Vortex fans... They make two 6" models and one 4" model.
Here's what they move for air:
4" VTX400 177 CFM at 0" static pressure
6" VTX600L 235 CFM at 0" static pressure
6" VTX600 452 CFM at 0" static pressure
These same fans perform much differently when the static pressure is increased in the duct:
4" VTX400 35 CFM at 1.5" static pressure
6" VTX600L 30 CFM at 1.5" static pressure
6" VTX600 169 CFM at 1.5" static pressure
Inch water column or inch water gauge is a unit of measure that measures the difference between the pressure in a duct and the pressure of the atmosphere. To give you a point of reference, a residential gas line after the regulator is usually pressurized between 5"-10" wg.
Different brands of filters are going to offer different amounts of resistance. With the filters I am familiar with, the HEPA filters required a more powerful fan than the carbon filter did to move the same amount of air through it. So that means if I had two identical fans and two identical duct runs with the setup you described I'd have negative pressure in the room.
Every situation is unique, if you get quality fans and speed controllers you can tweak each to get the airflow just where you need it. Also, don't forget that as a filter (especially the HEPA) gets dirty, it becomes more difficult to move air through it.
Hope I helped a little...
to 420 Magazine!I'm glad you are asking a question about ventilation. Environment is the single most important factor in a successful grow, and proper air flow is often overlooked.
Supernaut is right. I hate when I'm in a grow shop and the guy is saying something like "yeah...you're gonna want a 6" fan to duct that". Different 6 inch fans perform differently. I use Vortex fans... They make two 6" models and one 4" model.
Here's what they move for air:
4" VTX400 177 CFM at 0" static pressure
6" VTX600L 235 CFM at 0" static pressure
6" VTX600 452 CFM at 0" static pressure
These same fans perform much differently when the static pressure is increased in the duct:
4" VTX400 35 CFM at 1.5" static pressure
6" VTX600L 30 CFM at 1.5" static pressure
6" VTX600 169 CFM at 1.5" static pressure
Inch water column or inch water gauge is a unit of measure that measures the difference between the pressure in a duct and the pressure of the atmosphere. To give you a point of reference, a residential gas line after the regulator is usually pressurized between 5"-10" wg.
Different brands of filters are going to offer different amounts of resistance. With the filters I am familiar with, the HEPA filters required a more powerful fan than the carbon filter did to move the same amount of air through it. So that means if I had two identical fans and two identical duct runs with the setup you described I'd have negative pressure in the room.
Every situation is unique, if you get quality fans and speed controllers you can tweak each to get the airflow just where you need it. Also, don't forget that as a filter (especially the HEPA) gets dirty, it becomes more difficult to move air through it.
Hope I helped a little...
