What Does CFM Mean in Fans?

When shopping for inline fans, duct fans and other ventilation equipment for a grow tent or grow room you may have been on the spot when your were asked what CFM you wanted, but exactly what does CFM mean in fans?

You will encounter various sizes and capacities in fans. The power of your fan is reflected by its CFM, which stands for cubic feet per minute. Your CFM rating indicates the volume of air which the fan can move within the time-span of a minute, and the right CFM is relative to the amount of space you're growing in.

Here we'll be taking you through a look at the different types of fans you get and what CFM to expect from each.

You'll also see exactly how to calculate the optimal CFM for your grow tent or any other area you plan to use the fan in.

Why You Need a Strong Enough Fan

Why You Need a Strong Enough Fan

Air circulation and a good rate of air flow are essential to growing healthy, high-quality marijuana. As one vents out the hot air heated by your grow lights and fresh air is, in turn, pulled in, you increase your yield, making your cannabis plants thrive. There are simply so many problems that arise from an inadequate airflow that ventilation and exhaust fans become essential.

By making sure that your CFM is right, you know that your air will always be in circulation at an optimal rate. It is not good enough to have a fan blowing on your plant; you need proper ventilation and that starts with selecting and using the right CFM.

How Air Circulation and Ventilation Helps Your Cannabis

The perfect environment can be created using the correct selection of grow lights, inline fans, duct fans, and exhaust fans. When your air circulation is set up and your exhaust system is in place, your plants are protected against mold, bud rot, and white powdery mildew. Heat and humidity within your grow tent or grow room are also perfectly regulated, while pests like spider mites and gnats are far less likely to attack your plants.

As air blows over the leaves of your plant, it carries away moisture. This moisturizes removed after transpiration allows you plant to pull in more water and nutrients from its roots, boosting its health and growth rate.

Calculating the CFM You Need

Unless you are growing in a sealed CO2 grow room, you will always want all the heated, stale air pumped out of your tent or room and is replaced with fresh, cool air every three minutes. This presents a fairly easy formula for calculating the CFM needed for your growing area. First, you need to calculate the total cubic feet of your grow tent, box or room.  To do this multiply the width by the height by the breadth, giving you the total cubic feet.

Divide this by three (as you want all the air replaced every 3-minutes). Now you have a base CFM to work with. For every 1000-watts of HID lighting, 10% of this value is added to your total. Each carbon filter adds another 20% of the initial subtotal to your rating, while every foot of duct tubing adds 1% per measure.

Make Sure Your System is Sealed

Make Sure Your System is Sealed

Once you've selected the proper filters and exhaust fan, you need to keep the entire system running efficiently by making sure it is totally sealed. The whole exhaust system needs to be sealed off which will also prevent smells from escaping.

By entirely sealing your grow space other than the intake and exhaust holes, you create negative air pressure, containing all odors to the area you are growing in. Test your intake hole to see if your system is sealed. Burn incense at the intake and if you see the smoke pulling straight in then you know your negative air pressure is working.

Always Buy Bigger

It is much better to buy a fan with a higher CFM than you need, using an additional variable speed controller to adjust the fan speed in case it's moving too much air. Most grow tents require less than 400 CFM in total when equipped with filters, exhaust, and inlet. Small-sized grow rooms can use up to 700 CFM or more.

Another tip to keep in mind is that your carbon filters can carry a much, much higher CFM than you need; just make sure that they’re equal or less than your fans. By choosing the right CFM fan, you assist transpiration and take care of one of marijuana’s greatest problems – humidity. Only in very temperate areas will you need more than a good exhaust and inlet fan.

Turn to dehumidifiers and humidifiers for when your fans just can't keep your plants at optimal humidity levels.

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Shelby Wahdan - January 18, 2019 Reply

Thanks a bunch for sharing this! Bookmarked.

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