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How Do I add Ventilation to my Grow Area, What Can I Do about Smell?


New Member
How do I add ventilation to my grow area?
There are a couple of considerations to observe when planning your ventilation, they're pretty simple concepts; but they are often overlooked.

· First, remember that warmer air will naturally rise to the top of any container, and that cooler air will naturally settle towards the bottom.
· Also remember that when ventilating any space, the volume (VOLUME, in cubic feet or cubic meters... [L x W x H]) of air that goes IN, also has to come OUT.
· You can't expect to ventilate a grow space by simply forcing air in, and not providing an exhaust vent.

Since the object is to remove as much warm air as possible, and replace it with cooler air, it will be most efficient to place the exhaust as close to the top of the space as possible, and place the intake as close to the bottom as possible.

Should I place the fan in the exhaust, intake or both?
The fan should be placed in the exhaust, and the intake should be a simple hole (or light trap, if light getting out is a concern).
This type of system is known as an Active Exhaust, Passive Intake System.
Mounting the fan in the exhaust, sucking air out of the room accomplishes a couple of things...

· Since the exhaust is at the top of the area, the fan will suck the hottest air out of the area first.
· The fan is actually lowering the air pressure inside the area. Any incidental pinholes or leaky seams will simply draw air in. If the fan were blowing IN, those pinholes and leaks would allow potentially smelly air OUT.

How big should the passive intake be?
It should be slightly larger than the exhaust. Remember, the volume of air being blown out, will be replaced through the intake. Using a bigger intake hole allows the incoming air to be at a lower velocity (speed), which minimizes mixing up of the air in the area. It will also allow the fan to operate more efficiently.

How big should the fan be?
Fans are rated in either cubic feet (CFM) or cubic meters per minute in North America. In Eurpose, metric fans are rated in m3/hr - cubic metres per hour (m3/hr).

That means a 70CFM fan will move 70 cubic feet of air in one minute.
Your fan should be big enough to move the volume of your area 2 to 3 times every minute. A 70 CFM fan would be adequate for a 35 cubic foot area, and would be optimal for roughly a 23 cubic foot area.

· To figure your area's cubic volume, multiply (in feet) the length by the width by the height.

What if I have more than one fan? Should I use one to blow air in and one to suck air out?
Not if the object is to provide as much ventilation and cooling as possible.

· If you have two 3-inch diameter fans, and you mount one in the intake, and one in the exhaust, you have a total intake area of one 3-inch hole and a total exhaust area of one 3-inch hole.
· If you use both fans as exhausts, you have TWO 3-inch exhausts and two 3-inch intakes (actually, two 3.3 inch intakes. They should be bigger than the exhausts, remember?).
· Twice as many holes, twice as much airflow.

Enhanced Blower Mods

Timer Options
If you find that the "Lights off" temps are lower than you'd prefer, you can simply run the fan from the same timer as the light by using a multi outlet power strip connected to the timer. Plug the lights and the fan into the power strip, and the fan will turn on/off with the lights. If you're using more than one fan, you could connect some or all of them, remembering that the more fans you have running, the lower the temps will be.
* SAFETY NOTE: The timer must be able to handle the additional electrical load, or an additional timer must be used. SAFTEY FIRST.

Sound Suppression
Making the ventilation system quieter can be an important consideration, and it's important to remember that the air moving through the intakes and exhausts make noise, as well as the fan itself. Some of the fan noise from vibration can be overcome by mounting the fan in a non-rigid manner. The fans can me mounted using rubber grommets to help dampen the vibration. Self-adhesive foam rubber window insulation can also be used. In some installations, it can be mounted by threading a bungee cord through each mounting hole, then attaching the other ends of the bungee cord to the exhaust hole.

· Generally, air moving through ductwork or tubing can become noisy, particularly if the air has to move at a higher velocity. More, larger diameter intakes and vent tubes will generally be quieter than fewer, smaller diameter intakes and vents. The fans also don't have to work quite as hard.
· Finally, although popular and easy to use, flexible "Accordion" type hose, commonly used to connect clothes dryers to external vents are not always the best choice, as they cause a great deal of drag, (making the fans work harder) and generally air flowing through them is noisier than smoother ductwork.

Filtering\Odor Control
Connecting a Carbon scrubber is a good method of controlling the odor that can be a dead giveaway to an otherwise stealthy installation. A carbon scrubber is simply an expansion chamber (box) into which the smelly air from a flower chamber is pumped. The chamber has a large exhaust vent, which is covered by an activated carbon air filter. The chamber must be big enough to provide a damping effect of the incoming air. If too small a scrubber is used, the fan will not be capable of pushing air through the filter. Here, you actually want the exhaust vent to be considerably bigger than the intake.

· There are also several DIY Odor killers available, which work to varying degrees to provide an "Odor Cover-up."
· Remember, the term "Low Odor Strain" is relative.
· Even the low odor strains generally still stink pretty badly near the end of flowering.

Now go grow something!

Author: Smokey D Dope


so when you say the fan..you mean an inline or can fan right? not simply a small 6" regular fan? right? i also have a question concerning air cooled venting with the lights...air cooled implies that you have a fan in the vent on the side of the light sucking the air out correct? so you would need a fan for both the light heat ventilation (b/c u have a piece of glass between the bulb and you plants) and a seperate one for GR ventilation ...right?

im considereing all my options in what to do with my current situation so any answers will help greatly and be very appreciated!


Well-Known Member
Yes...the original writer of the above article was referring to inline or can fans. We normally aren't talking about the desk top oscilating model although there are times when we mean those too. Yes when running a sealed air cooled light you need two seperate fans. Some growers try to leave one end of the reflector open and use one fan to suck room air through the reflector and out of the room eliminating one fan but it requires a much bigger fan and more heat is released into the room that way. Unless your fan is rated for high heat it's best to blow air through the reflector but very few of us do it that way and instead suck the super heated air through the fan heating the bearings which causes a faster fan death.


New Member
"That means a 70CFM fan will move 70 cubic feet of air in one minute.
Your fan should be big enough to move the volume of your area 2 to 3 times every minute. A 70 CFM fan would be adequate for a 35 cubic foot area, and would be optimal for roughly a 23 cubic foot area."

Is this correct? a 70CFM fan can clear 70 cubic feet in one min... so couldnt it clean a 350 cubic ft room in 5 mins?

BWC BayArea

Member of the Month: 3rd Place Winner
When smell is a factor, this simply won't do. Better to spend the $$ up front, than to risk discovery or a sub par crop. Ur fan should be able clear the room of old air in a min. Ur intake should be able to supplement ans support the rate of air transfer. Best thing I've used for smell is an activated charcoal filter. U can see my fan and filter setup by clicking the link to my grow journal in my signature.


New Member
I have a question as I am big time upgrading my CA 100% legal, setup to service my private non-profit patient run co-op.

I saw all this good info here and figured my question belonged in this thread. Any assistance would be great.


I am building a 4x8x8 flower tent inside of a 11x14x8 Room. I will be using a 1000watt HPS, HydroFarm Ballast and Inline air cooled Hood in the tent. I will only be using 4x4 or so right now, but I am leaving room for expansion.

The Veg/Clone/Mother Area will be 100% T5 Flourescent. Probably between (4) and (8) 48" T5 tubes total.

A 9000 BTU AC will keep temps down throughout the room once the light is up. This is essential as I am in Southern California (though close to the ocean so relatively cool (75 to 88 degrees outside temperature compared to 85-105 inland).

This is an apartment building and smell is a concern.


Trying to save money wherever possible, I was considering pulling all the air from within the tent, through the light hood, out some 6" ventilation to a 424 CFM fan, through an active carbon filter for smell.

Would this be sufficient to ventilate & cool the tent while avoiding smell issues?


I was considering this following fan/can filter combo

Looks like a killer deal. I wouldn't trust it except their name brand stuff is really cheap too and they have a high approval rating.

Gotta keep cool enough, gotta keep the smell down. Do you think this will do the Job?


My Mother/Clone/Veg area is not inside of a tent. At any one time, There will be 2-4 Mother plants and up 20 clones, and 15 babies vegging 2 weeks. It is in the room with the AC and I do not have any smell control planned for these guys. I have never had this many plants before and I am concerned about the smell of this many seedlings.

Any help in this matter as I have never had to deal with this stuff on this scale in the past.

Will I be stinkin' up the place with my little seedlings?

Thanks guys. I've been lurking forever.


BWC BayArea

Member of the Month: 3rd Place Winner
Veg room, clones, and moms won't smell bad. The flowering area is the area of concern.
To answer question #1 U should have a larger fan for that sized area. It can work as long as the area is sealed. My setup is similar, but U will need at least around 275 CFM intake or U will have problems with the air pressure. I've addressed similar issues in my journal (click the link in my signature). Question #2 What happens if u get a cheap or used filter? Its not like u can take it back or get it resolved that day. Never heard of this brand fan, is it quiet running? Things to think about. On top of that, $230.00 isn't quite a steal. Y not check out a dro store that buys and sells equipment if u really wanna save some $$?

BWC BayArea

Member of the Month: 3rd Place Winner
whatever happened to the once evry 5 min? You guys are calling up 2-3 a minute, that just seems to be a lot. :yummy:

The thing to remember is this. In ur grow do as u see fit. If there's no smell, ur good. If there is, then i may wanna consider some else's advice.


New Member
Re: smell comimg back through inlet fan

[how do i stop smell comimg back through inlet fan

You can get a "backdraft damper", basically a collar that prevents air from moving backwards through the duct when the fans are off. It has two "wings" on a weak spring that open easily when the fan is on, and then snap shut against a foam ring when there is no air flow. Hope this helps.


New Member
ive been cruising this site a lot lately and cannot seem to find the answer to my question, so i will just ask.

is having can fans really that necessary if smell is not a concern?

i just built a grow house in my room. 8' long x 7' tall x 3' deep. i have my license so im not entirely concerned about my smell.

right now i have a small a/c unit that blows in the bottom of one end, and 2 computer desk fans screwed to the opposite wall blowing air out the top. during the light cycle i leave the front of the grow room open so they get plenty of fresh air during the day. (also have a large oscillating fan that sits at my doorway in attempts to bring cold air from the rest of the house)

so far none of my ladies have died and all seem to be growing well. would they grow that much more if i invested 200 bucks in a fan?

random info: room temp avg. 77F humidity 39%
using all CFL for now (400w hps in mail for flowering)

growing: Romulan, Blueberry, Jack Herrer, Maui Waui, Pineapple Trainwreck, and AK47xSour D


Active Member
Excellent post, one mistake: room clearance. If you have a 10 x 12 x 8 foot grow room, you need one hell of an inline fan to clear 2000 - 3000 CFM.

I think the general take on this subject is to evacuate your room once every three minutes or so.


New Member
Great info! This helps me out considerably with my first indoor grow. ++Rep!


New Member
Rule of thumb - match the cubic feet per minute (CFM) rating of the inline fan to the cubic feet of your garden area. Use the formula L x W x H to find the cubic feet of your garden area, that is the fan CFM rating you want approximately. If you are using a filter you want the filter CFM rating to match the fan CFM rating.

There fore you would need something along the lines of 960cfm?
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