Intake and Exhaust For Indoor Farming

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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.

ventilation-for-indoor-grows

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.

(16)(8)(7)=896

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.

898/3=298.667

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.)”

Greenhouse Exhaust Fan

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

11 thoughts on “Intake and Exhaust For Indoor Farming

    • Yes and no – humidity as a function of air flow is a concern. You don’t want the room to be too humid or not humid enough. At the same time, too much air movement around the plants will lead to too much transpiration. All in all, it’s a balancing act. My feeling is that if the room is properly ventilated, air flow around the plant will necessarily be appropriate.

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  2. You need to consider your heat or cooling requirements with your exchange air. I don’t know how you are planing to address this, but the first step is to lessen the issue with a heat exchanger. Look at FanTech heat exchangers made to supply air tight houses with fresh air. You will find something of an appropriate size. They may seem expensive but you will save a lot on energy costs. As a retired architect, I can probably give you a lot of advice.

    • Thanks, Bill. Heating and cooling is definitely something else I’ve thought through. I wanted to focus on ventilation here in particular and jump into heating/cooling later. Right now the plan is to use a 2-in-1 heat pump since it’s such a small face. I hadn’t been recommended a heat exchanger before so I’ll have to look into that more, thanks. It seems like because it’s such a small, tight space that the 2-in-1 will work just fine without an exchanger. It would make more sense for larger square footage.

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    • That’s true – definitely a factor. I sort of took it for granted as part of getting fresh air in as I’m not going to supplement my CO2.

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