Calculating CFM (Cubic Feet per Minute) for your Marijuana Grow Room Exhaust Fan

Your marijuana plants need fresh, clean grow room air.

You need to send odorless air and heat out of your grow room so it dissipates outdoors.

That’s why some of the most important marijuana grow room factors are air exchange, exhaust fan cfm (cubic feet per minute), and odor control.

Unless you’re running a sealed grow room and/or adding carbon dioxide (C02), you want to remove all the air in your grow room every 3-5 minutes and replace it with fresh air.

To exchange grow room air, remove smell, and exhaust grow room air, at minimum you need an exhaust fan and scrubber.

In most cases, you’ll need flex tubing material to connect your exhaust fan to the scrubber, and/or to connect the fan to its terminal exit, which is best in a soffit rather than cutting a hole in the roof to make a roof vent.

Many growers add a silencer or muffler into the system to decrease noise.

In most cases, the fan generates the most noise.

Using a decibels meter with the major brands of exhaust fans sold at hydroponics stores, I’ve discovered the quietest brand of fan is the Hyper Fan Stealth.

So here’s how you calculate the CFM rating for your fan:

• Multiply the width and length of your grow room times the height to get the total cubic feet.
• You want to exhaust all the air every three minutes, so divide your total cubic feet by three.
• For every 1000 watt of HID lighting, add 10% to your CFM rating.
• If you use a carbon scrubber, add 20% to your CFM rating.
• Add 1% for every foot of duct tubing you use to connect your exhaust fan to the scrubber.

Here’s a grow room example so you can see this formula in action:

Let’s say you have a grow room that’s 12 feet by 12 feet, and 8 feet tall, which equals 1152 cubic feet.

Your base CFM is 1152 divided by three = 384.

You’re running two 1000-watt HIDs, so add 10% each. That’s 20% of 384 = 77 added CFM.

If you’re running double-ended HID bulbs, which tend to be hotter than regular HPS, you might add 14% each instead of 10%.

On the other hand if you were running 1000 watts of LED grow lights, you’d add only 6-7% of extra CFM percentage (LED grow lights are about 40% cooler than HID lights).

You’re running an eight-inch carbon scrubber, so that’s 20% of the original 384, another added 77 CFM.

You’ll use four feet of insulated tubing to connect the scrubber to the ceiling-mounted exhaust fan, and another foot to take the exhaust air out the soffit.

So that’s five total feet at 1% added CFM per foot, so that’s 5% of 384 = 19.

Now you add your base CFM (384) plus 77 + 77 + 19, for a total of 557 CFM.

In this situation, I’d choose the Hyper Fan Stealth 710 CFM model.

Extra CFM capacity is good. For example, you might decide to use a muffler to dampen noise.

The carbon scrubber would be another 10-20% of added CFM, which would bring you over 600 CFM, well within the capacity of a 710 CFM unit.

You could match a Phresh carbon filter to the fan, such as this 750 CFM model.

You want the capacity rating of your carbon filter to be pretty much the same as the CFM rating of your exhaust fan.

One thing to watch out for when you’re running an exhaust fan is it removes humidity from a grow room, which could alter your vapor pressure deficit conditions.

If you’re adding C02, it can remove that too.

Some growers manager their exhaust fan using monitors or controllers that turn the fan on and off depending on temperature, humidity, and pre-sent timed cycles.

Monitors and controllers may help you ensure that your exhaust fan doesn’t suck all the added C02 out of your grow room.

The exhaust fan might also cause a negative pressure draw that creates pressure gradients that interact with air conditioning.

In some cases outside air and contaminants such as mold and fungi will be sucked into the dwelling via air gaps around windows, doors, and roofing if your exhaust fan is sucking too much air out of your grow room too fast.

Please watch the YouTube videos embedded in this article so you have an even better understanding of CFM and carbon grow room filters.

Take a look at this video about what can happen if your house smells like marijuana…