Ventilation Part II -- Choosing An Inline Fan

A breath of fresh air

By Scott Misener

All right people, let's talk about air. Plants and mammals have a symbiotic relationship when it comes to the air we breath. When we take a breath our lungs take in oxygen. The byproduct of our gas exchange is carbon dioxide. Luckily for us, plants do the opposite. That top surface of a leaf really is nothing more than a solar panel protected with a waxy layer. All the action takes place on the underside of the leaf. It's this part of the leaf which is riddled with microscopic mouth openings opening and closing independently. Each tiny mouth opening takes in a gulp of carbon dioxide before breathing out oxygen. It's almost like those lovable plants and all of us crazy mammals were made for each other.

Don't get carbon dioxide confused with the poisonous gas carbon monoxide. There is a big difference. I once talked to a guy that wanted to take his garden to the next level. To achieve this, his plan was to run a tube from the tailpipe of his car to his garden. Well, he would have taken things to the next level all right. This is a perfect example of someone shooting from the hip. That works if you're a gunslinger, but can prove to be fatal when your dealing with poisonous gases and high wattage light systems. If you're not sure how something works, ask someone who knows what he or she is doing! What he meant to do was set up a carbon dioxide (CO2) emitter system to raise the parts per million (ppm) of CO2 in the air. A very good idea when you have all the other kinks in the garden worked out. Since CO2 emitter systems are worthless unless your lights, temperature, humidity, and nutrients are running at optimal, consistent levels, extra CO2 won't make any difference. So we will tackle the topic of CO2 later after we cover everything else.

What I consider to be a ventilation system may differ from others' idea of what ventilation is, so let's make sure we are on the same page. To ventilate a room, you need air exchange. This involves an exhaust fan moving air out and a point where fresh air comes in. Everyone's gardening area is unique to that area. What works for one location may not work for another garden across town. There are way too many variables to have a one-size fits-all explanation. There are, however, guidelines to follow. The first thing to do is find out the total cubic feet of the area you are working with. To get this number, multiply the length of the area in feet by the width and height. For example, an area sizing up to 8x8x8 would have a total volume of 512 cubic feet. This number is the total amount of air sitting in your garden at any given moment. Now you've got to move that air out.

Inline fans are sealed when capped off on both ends with the appropriate size of ducting. This creates a tight, focused vacuum, which moves large amounts of air in a short period of time. Inline fans are superior to other style of fans such as cheap inductors and blowers commonly referred to as squirrel cage fans. These fans cost a lot less than inline fans, but you end up sacrificing a lot of usable CFM, or cubic feet per minute.

CFM is an important number to pay attention to. All fans are rated by CFM. This is their workload. How accurate that number is for that fan once you install it in your garden is another story. A fan rated at 747 CFM may only be performing at 550 CFM once twenty feet of ducting and a carbon air filter for air purification is attached. That's twenty percent loss, or draw, on your fan! Don't undercut your ventilation and buy a rinky-dink fan that just doesn't do the job. You'll just waste your money on something that's not going to work.

The real question is what fan is right for you? Take the number you came up with earlier as your total cubic feet and divide that number by two for average application. Average applications would include an average size home garden with a couple of lights. Your new number is the CFM required to ventilate your garden in two minutes. For more ambitious applications involving wall-to-wall high voltage in a small space, you will want to vent that space every minute. So match up the CFM of the fan with the total cubic feet of your space. That fan will vent the space every minute.

To make an educated decision on a fan that will take care of all your ventilation needs year-round; it is a good idea to buy a fan that is slightly bigger than required for your space. Ace sells inexpensive fan speed controls for about nine dollars that, when wired into your fan, gives you the ability of controlling the speed of your fan. In the summer it gets hot, so dial it up. This time a year when an atrium or greenhouse gets a little frosty and heat is not as much of an issue, turn that thing way down. Be sure to buy a fan speed control, not a light dimmer switch. They are different.

Congratulations, you are now qualified to make an educated decision on an exhaust fan. That's half the battle. Next week we'll talk about forced verses passive air intakes and those ever so important oscillating fans. Now we're getting to the meat of this thing! Until next week, love your plants!

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