Every indoor garden should have two kinds of fans: garden exhaust fans and air circulation fans. As you can tell from their names, each kind has it's own special place in the garden. In this section, we will take a look at appropriate exhaust fans, but before we do let us first discuss the difference between exhaust fans and circulating fans, and explain why circulating fans should not be used as exhaust fans in your garden.
Basically, the difference between the two is how much air volume the fans are capable of moving and how efficient the fan is at moving the air. Air circulation fans have a much smaller capacity to move air. This capacity is called a CFM rating, which stands for cubic feet per minute. Anyone who has measured their grow room and gone to the exhaust fan calculator will realize just how much air your garden exhaust fans need to be capable of moving.
The other quality to look for in an exhaust fan, efficiency, is a little harder to figure out if all you have to go by is the information that came with the fan. Efficiency is the ability of a fan to pull air through obstacles. For example, if you attach a small oscillating fan to a long, 6 inch exhaust run, air pressure accumulates in the exhaust run.
Once pressure exists, the blades of the oscillating fan may be turning but very little air is actually being sucked out and exhausted. Oscillating fans DO NOT make good exhaust fans. On the other hand, a good centrifugal fan can develop enough pressure in the system to pull your air through a Carbon filter and still exhaust it. Air pressure is not a problem with centrifugal fans, and one very big reason why they make good exhaust fans.
As a general rule, your exhaust fan (or fans) should be able to exchange the entire air volume of your garden area in less than 5 minutes, although 3 minutes or less would be better.
There are two types of fans that are commonly used as exhaust fans for a garden. These are squirrel cage fans (left) and centrifugal fans (right). Squirrel cage fans scoop air from the side and bat it out the front. You can see from the picture how the air exhaust from the squirrel cage fan is at a right angle to the direction of the air intake. This forces all the air to change directions in the process, which is one reason why they are less efficient than centrifugal fans.
The centrifugal fan on the other hand intakes and exhausts air
in a straight line. While both garden exhaust fans work well
to exhaust an indoor garden area, only the centrifugal fans are capable of
running efficiently at the pressures required to pull air through a carbon filter. So, if you intend to use a carbon filter now or in the future, your best choice would be to buy a centrifugal fan. Some Carbon filters come with a pre-filter of some sort, which greatly increases the life of the Carbon filter.