How to Calculate CFM Greenhouse Specifcations

Greenhouses must be ventilated and, in most cases, cooled.

Every plant species has optimal growing temperatures. Growers use greenhouses to start seedlings or cuttings, create new plant species, grow exotic species or provide plants for consumers year-round. Regardless of size and location, all greenhouses need proper ventilation and most require cooling. Ventilation or exhaust fans provide passive cooling as well as providing ventilation. Passive cooling permits temperatures at the plant level to rise more than 20 degrees Fahrenheit higher than the air temperature. Manufacturers rate fans based on the cubic feet of the estimated amount of air moved per minute (CFM).

Things You'll Need

• Calculator
• Pen and paper
• Fan catalog

Instructions

Calculating the Space

1. Measure the width, length and height of your greenhouse, or use the intended specifications of one under construction. As an example, assume the greenhouse is 12 feet wide, 24 feet long and 10 feet high.

2. Multiplying the dimensions in feet provides the volume of air within the structure. The air volume provides the information necessary for determining the proper ventilation fan. In this example, the greenhouse dimensions total 2,880 cubic feet of volume.

3. According to the presentation "Greenhouse Cooling Concepts," environmental factors impact the greenhouse's performance. Air removal must increase as the elevation rises. You must determine the geographic location's average temperature and elevation. Known as F-house, you correct for these environmental factors by multiplying the feet of elevation by the location's average temperature and the greenhouse's light intensity.

   To determine the greenhouse's light intensity measurement, you have two choices: use a footcandle meter or use a mole lighting map. According to the Greenhouse Product News website, mole is now the standard for measuring the light falling on greenhouses. The site states, "Light levels measured at the bench or floor inside a greenhouse are typically 40 to 70 percent of the levels measured outside the greenhouse."

   The example greenhouse is located in Chicago, Illinois. According to the IDcide website, Chicago is 596 feet above sea level, with an average winter temperature of 20 degrees Fahrenheit and approximately 25 to 30 moles of light per day. Multiply 30 moles by 0.40 for an estimated 12 moles inside. This adjustment is 596 x .20 x 12, or an adjusted F-house of approximately 1,430.

4. Multiply your F-house by the air volume for your total CFM requirement. The sample specifications would be 1,430 x 2,880, or 4,118,400 CFM.

5. Decide the number of fans necessary for the greenhouse. Fans should be evenly spaced. Divide your CFM by the number of fans required for the space. Select fans that, when combined, provide the CFM rating required by the greenhouse.

   If you use six fans in the sample greenhouse, each fan must have a CFM rating of at least 686,400 if you use environmental adjustments, or 480 CFM each without adjustments.

Tips & Warnings

• For best results, place exhaust fans on the greenhouse's leeward side, or the side most protected from the wind.

• Place the fans at the height of the plants.

• Cover fans with screening to protect wildlife and humans.

• If you plan to raise sensitive plants, it is best to contact a horticulture professional to assist you in planning ventilation and/or cooling. The ventilation and cooling balance is critical.

References

• Slideshare: Greenhouse Cooling Concepts
• Dr. Paul A. Thomas: Greenhouse Cooling lecture
• Greenhouse Product News: Mapping Light Across the United States
• Rutgers University: Some Thoughts on Supplemental Lighting for Greenhouse Crop Production
• IDcide: Chicago, Illinois