My contractor told me a fart fan was all I needed for exhaust for my farm. “Those fans only cost like $15!” he said. I was stoked. But, as I was walking through the aisle at Lowes with him and we got to the bathroom exhaust fans, I realized I was going to have to rethink.
Building grow rooms for people taught me the importance of proper ventilation.* Ventilation essentially refers to how much air moves in and out of a given space. The unit for measuring that is CFM, or cubic feet per minute, and it refers to how much air a fan can push or pull.
Looking around the internet, there are a few recommended rates for horticultural applications. The one I had used in the past was that you wanted a complete air change in a room once every three minutes.**
Let’s walk through the calculations on that for my space. The side walls are 16 ft long and the ends are 8 ft long. The ceiling is 7 ft high (a little low so the structure fits under the garage door when it’s up). To find the cubic footage, you multiply those numbers.
If I want an air change every minute, I would need an exhaust fan with at least a 896 CFM rating running constantly. If I want every three minutes, I’d want approximately 299.
So what’s the problem? The fart fans my contractor pointed me to were only 50 CFM.
On top of that, the more I clicked around the internet and called some local connections, the more varied the formulas for calculating the appropriate CFMs were getting. Farmtek (a farm supplier) told me that I wanted a high enough CFM to change the air in a minute but that I’d only need to do that once every 10 minutes. That’s good news because it would mean I wouldn’t have to run the fan constantly, saving me energy.
My local hydroponics store said I wanted a complete air change every minute. This site says once every 5 minutes. Bright Agrotech has a completely different formula: “A general rule is that you need 2 CFM (cubic feet per minute) per square foot of ground space in your greenhouse. (So if you have a 100 square foot greenhouse, you will need a fan with the capacity for 200 CFM.)”
The same ambiguity exists for intake. Intake refers to pulling in fresh air. A lot of times that comes from the negative pressure created by removing air with an exhaust fan (think about a vacuum). Normally that means air coming in through cracks and things like that. But that wouldn’t work in a room as highly insulated as mine (an r value of approximately 30), so I’d need either a large piece of wall cut out (usually in a low corner on the opposite end of the room from your exhaust fan) or another powerful fan at a little more than a third of the strength of the exhaust fan.
So, what am I going to go with? I can’t let the perfect be the enemy of the good and need to go with something.
I think I’m going to end up going with a fan capable of exchanging my air once every minute and then I’m going to figure out how often I need to run it as I go along. This will make sure I don’t have to go through the work of ever needing to install or buy another fan and will save me a little on energy from not having to run it all the time. One benefit I’ll be missing from having a fan run constantly is that I won’t have as much protection from pathogens/contaminants coming in through where the fan leaves an opening, but since I won’t be running a completely sterile operation, that shouldn’t matter too much.
I was between two models: Hurricane Inline 12 In and a 12 inch Direct Drive Exhaust Fan. I decided on the second option; inline options just come with a little bit of extra installation headache where you have to setup a duct system.
Look to spend around 225 for a fan like this.
*Let me know in the comments and I can get more into why proper ventilation is important
**Low budget, tiny square footage grows