I wrote this article this morning for my blog and thought I would share it here, too.
A Basic Understanding of Adequate and Proper Air Circulation in the Hobby Greenhouse
I know when I built my greenhouse, I had very little knowledge or appreciation for what it takes to have a successful greenhouse. Success is not as easy as one would assume, or I assumed! One area I am learning the importance of is adequate and proper air circulation. Good air circulation is crucial to good plant health and growth. There is a lot more to it than opening a vent or throwing in a fan to accomplish these ends. Too little fan capacity or poor spacing and location are common mistakes. Temperature variations, excess moisture, and low available carbon dioxide are three plant hazards that can be alleviated with good air circulation.
Without circulation there can be wide temperature swings between different sections and levels within the greenhouse, as much as 10-15 degrees, especially at night. These different temperature "pockets" affect plant growth and quality.
In addition to having a more uniform temperature, proper circulation reduces the incidence of foliar diseases. Moving air removes moisture from the plant canopy resulting in a drier microclimate. When leaf temperatures cool much below the air temperature, the dew point is reached, and condensation occurs harboring disease organisms. Proper circulation will reduce this.
During the day, photosynthesis depletes the carbon dioxide levels around plants. Adequately moving the greenhouse air will replace the carbon dioxide depleted air around the plants with fresh air having a higher carbon dioxide content and encourage photosynthesis to continue throughout the day.
Conquer these 3 deficits: temperature variations, excess moisture, and low available carbon dioxide, and you are on your way to a much healthier greenhouse environment.
So, what is the optimal flow rate and placement of greenhouse fans? The total fan capacity should be about two times the floor area. If your greenhouse is 10' x 15', your flow rate should be 10 x 15 x 2 for a total of 300 cfm. If you are growing tall plants or have obstructions like hanging baskets, you'll need slightly more. Horizontal positioning of fans is preferable since air layers in horizontal levels. The goal is to break up and disturb these layers.
Placement of fans should be such that you are not just blowing air around but circulating it, breaking up the layers. Using two properly sized fans at opposite ends is best, placing one fan blowing across the length of your greenhouse above the plant canopy and the other blowing back in the opposite direction at a lower level. This will generate the needed circulation. (Note the root of the word "circulation" is "circle".) If you have smaller fans or a wide and long greenhouse, you may need additional fans either in line or spread across, but the placement should be the same to achieve adequate circulation throughout.
Many hobby greenhouse growers are concerned about the high cost of electricity when it comes to maintaining their greenhouses, but you can minimize your cost by using small, low horse power fans (1/10 to 1/15 hp). These fans work well in providing the air movement needed. They move the air with little resistance and at a lower cost than using exhaust fans for this task. Exhaust fans have to "pull" the air and overcome the resistance of louvers or vents. Exhaust fans certainly have their place, but air circulation is not their forte.
Each greenhouse is different, with different plants, conditions and needs. A basic understanding of proper air circulation to reduce temperature variations and excess moisture as well as promote good photosynthesis is key to designing and maintaining an adequate and proper air circulation system for your successful greenhouse growing.
