Ignor_Amus
New member
I've seen several questions posted about pushing or pulling air through air cooled reflectors. The answer given here most often is to push the air through the hoods. However, the unconventional approach of pulling air can be very advantageous if it is done properly and incorporated early during your room design. The first few paragraphs of this will seem rather basic to many of you, but bear with me.
The types of fans typically used in grow rooms are better at blowing air (having resistance at the exhaust side of the fan) and not sucking air out. Certain types of fans are very good at pulling air (squirrel cage fans with backward inclined blades) but they are normally used in industrial type settings. Reverse incline blade fans are also much noisier than we'd like.
It appears that (most) everyone nowadays is using axial flow or mixed-flow fans. One thing to consider is that these types of fans pulling air from the hoods will be subjected to high temperatures which will adversely affect the motor's life. If there is inadequate airflow it could potentially trigger the thermal overload protection, which would turn the fan off and leave the hoods to cook up like an oven. Not good! (But you should have a positive airflow detection scheme in place anyways - you want the lights off in case of fan failure!)
Another related consideration is the volume of air that will be passing over your carbon filters. (The ones used to clean the air leaving your grow op, not recirculation filters.) Obviously the less volume the better - both for better odor control and longevity too.
Okay, with that in mind, let's get on with it. All of the following is assuming you are running a sealed room and using fresh outside air to cool your hoods.
Pushing: This seems to be the answer that most people are giving and getting. However, air cooled hoods are not completely sealed - they have leakage. When you are pushing air through the hoods, invariably you will be adding a small amount of positive pressure to your sealed room. That's obviously not good unless you have a separate room depressurization fan which has enough draw to cover that. Usually that depressurization fan is blowing through a scrubber, which is being subjected to warm, moist air.
The advantage of pushing is that the hot air exhausted from the hoods will not pick up any odors, therefore it does not have to pass through an exhaust scrubber.
What I'd do under those circumstances is run the hot hood exhaust air into a large duct that is already handling your room air exhaust - after the scrubber. This dilutes the hot hood air with the already cleansed room air - this leaves your overall exhaust air cooler. Just make sure your final exhaust ducting is large enough to not create excessive back pressure.
The downside of this type setup is the moisture from your room air that you will be sending through your carbon scrubber. Some will say "so what", but it is preferable that air passing through your scrubber is of lower relative humidity and lower water vapor content per volume. And yes, there is a difference between the two.
Pulling: It can be done, and it has it's benefits. I've done it using a belt-driven squirrel cage fan, which isolates the motor from the hot airflow. (No, I didn't use a reverse incline blade type, I just investigated specifications and sized it carefully & properly.) Also, belt drive fans are much quieter than direct drive fans. And the (mechanical) noise they do generate is more of a rumbling low-frequency sound, which is far less obtrusive than than what you're familiar with. You can also mount it on soft rubber isolation pads to further reduce noise transmission.
What I'm going to describe allows you to use one large fan for your entire operation, including an additional function that I'll get to later.
When you are pulling air, the leakage from the hoods will suck air out of the room, which helps depressurize your room. But the downside is it will pick up odors, so you now must run the hood exhaust air through a carbon scrubber - and it will be a large volume. But the air will be low relative humidity due to the heating and the dilution with outside air, so your scrubber will be more effective than when you're dumping high relative humidity room air into it. If there is not enough hood leakage to adequately depressurize your room, you simply create an open air bleed port to enhance room depressurization. Use a manual damper to fine tune the amount of flow needed.
How to: Use the fan to depressurize a large diameter duct (or custom sheet metal airbox/plenum) that acts like a manifold from which you take branch hoses to the hoods. (Use manual dampers to adjust and equalize airflow through the hoods.) The fresh outside air feeding the hoods can be handled in the same manner. Size your fresh air intake ductwork properly (big!) - you don't need an intake fan!
Here's an additional benefit: You can plumb in an electrically controlled airflow damper to the depressurized plenum/duct and another one on the fresh air side. They will remain closed while the lights are on. When the lights go off and you'd normally have a big RH spike, you simply open both dampers and you'll be pulling fresh air through the room, removing excess humidity and providing a nice fresh air exchange. Hook the damper controls to a photostat, or if you already use one to control your CO2 generator you can just wire in a double-throw relay to put power to the dampers when you cut power to the CO2 generator.
If outside temperatures are unfriendly, you purge the room just long enough to do what you want, then close down the dampers and let your AC or heat or dehumidifier do it's thing during the dark phase. A humidistat tied into an appropriate repeat-cycle timer (on for "X" minutes, then off until the next power cycle) all connected to a photoeye would do the trick.
You can also plumb in a spring-loaded balance type damper that will automatically open up when there is enough airflow potential. What this means is that when you open the door to your room, additional air is sucked into the room, helping avoid the smelly air from leaving through the doorway. (You still have to minimize the door opening pathway and the time it's open - just squeeze through quickly and shut the door.)
With this system, you'll have one of the most stink-proof setups imaginable, using only one (very reliable, very powerful, and very quiet) fan that:
1.) Vents your hoods,
2.) Depressurizes your room, and
3.) Provides a fresh air purge when you want it.
If you're running two flowering rooms on a flip-flop, it can all be adapted with dampers to run both rooms on one fan and one exhaust.
I know this was a big mouthful of words, but I hope this can be helpful to some of you.
The types of fans typically used in grow rooms are better at blowing air (having resistance at the exhaust side of the fan) and not sucking air out. Certain types of fans are very good at pulling air (squirrel cage fans with backward inclined blades) but they are normally used in industrial type settings. Reverse incline blade fans are also much noisier than we'd like.
It appears that (most) everyone nowadays is using axial flow or mixed-flow fans. One thing to consider is that these types of fans pulling air from the hoods will be subjected to high temperatures which will adversely affect the motor's life. If there is inadequate airflow it could potentially trigger the thermal overload protection, which would turn the fan off and leave the hoods to cook up like an oven. Not good! (But you should have a positive airflow detection scheme in place anyways - you want the lights off in case of fan failure!)
Another related consideration is the volume of air that will be passing over your carbon filters. (The ones used to clean the air leaving your grow op, not recirculation filters.) Obviously the less volume the better - both for better odor control and longevity too.
Okay, with that in mind, let's get on with it. All of the following is assuming you are running a sealed room and using fresh outside air to cool your hoods.
Pushing: This seems to be the answer that most people are giving and getting. However, air cooled hoods are not completely sealed - they have leakage. When you are pushing air through the hoods, invariably you will be adding a small amount of positive pressure to your sealed room. That's obviously not good unless you have a separate room depressurization fan which has enough draw to cover that. Usually that depressurization fan is blowing through a scrubber, which is being subjected to warm, moist air.
The advantage of pushing is that the hot air exhausted from the hoods will not pick up any odors, therefore it does not have to pass through an exhaust scrubber.
What I'd do under those circumstances is run the hot hood exhaust air into a large duct that is already handling your room air exhaust - after the scrubber. This dilutes the hot hood air with the already cleansed room air - this leaves your overall exhaust air cooler. Just make sure your final exhaust ducting is large enough to not create excessive back pressure.
The downside of this type setup is the moisture from your room air that you will be sending through your carbon scrubber. Some will say "so what", but it is preferable that air passing through your scrubber is of lower relative humidity and lower water vapor content per volume. And yes, there is a difference between the two.
Pulling: It can be done, and it has it's benefits. I've done it using a belt-driven squirrel cage fan, which isolates the motor from the hot airflow. (No, I didn't use a reverse incline blade type, I just investigated specifications and sized it carefully & properly.) Also, belt drive fans are much quieter than direct drive fans. And the (mechanical) noise they do generate is more of a rumbling low-frequency sound, which is far less obtrusive than than what you're familiar with. You can also mount it on soft rubber isolation pads to further reduce noise transmission.
What I'm going to describe allows you to use one large fan for your entire operation, including an additional function that I'll get to later.
When you are pulling air, the leakage from the hoods will suck air out of the room, which helps depressurize your room. But the downside is it will pick up odors, so you now must run the hood exhaust air through a carbon scrubber - and it will be a large volume. But the air will be low relative humidity due to the heating and the dilution with outside air, so your scrubber will be more effective than when you're dumping high relative humidity room air into it. If there is not enough hood leakage to adequately depressurize your room, you simply create an open air bleed port to enhance room depressurization. Use a manual damper to fine tune the amount of flow needed.
How to: Use the fan to depressurize a large diameter duct (or custom sheet metal airbox/plenum) that acts like a manifold from which you take branch hoses to the hoods. (Use manual dampers to adjust and equalize airflow through the hoods.) The fresh outside air feeding the hoods can be handled in the same manner. Size your fresh air intake ductwork properly (big!) - you don't need an intake fan!
Here's an additional benefit: You can plumb in an electrically controlled airflow damper to the depressurized plenum/duct and another one on the fresh air side. They will remain closed while the lights are on. When the lights go off and you'd normally have a big RH spike, you simply open both dampers and you'll be pulling fresh air through the room, removing excess humidity and providing a nice fresh air exchange. Hook the damper controls to a photostat, or if you already use one to control your CO2 generator you can just wire in a double-throw relay to put power to the dampers when you cut power to the CO2 generator.
If outside temperatures are unfriendly, you purge the room just long enough to do what you want, then close down the dampers and let your AC or heat or dehumidifier do it's thing during the dark phase. A humidistat tied into an appropriate repeat-cycle timer (on for "X" minutes, then off until the next power cycle) all connected to a photoeye would do the trick.
You can also plumb in a spring-loaded balance type damper that will automatically open up when there is enough airflow potential. What this means is that when you open the door to your room, additional air is sucked into the room, helping avoid the smelly air from leaving through the doorway. (You still have to minimize the door opening pathway and the time it's open - just squeeze through quickly and shut the door.)
With this system, you'll have one of the most stink-proof setups imaginable, using only one (very reliable, very powerful, and very quiet) fan that:
1.) Vents your hoods,
2.) Depressurizes your room, and
3.) Provides a fresh air purge when you want it.
If you're running two flowering rooms on a flip-flop, it can all be adapted with dampers to run both rooms on one fan and one exhaust.
I know this was a big mouthful of words, but I hope this can be helpful to some of you.
