Supporting local growers since 1994

How to Size Fans & Shutters

Size Fans & Shutters

These simple steps will increase the efficiency of your cooling system

Although it may seem complicated, sizing fans and shutters for your greenhouse cooling system does not require complex mathematical formulas or difficult calculations. With a few simple steps you can choose the most efficient fans and shutters for your cooling system.

First, calculate your total CFMs.

CFMs are the total cubic feet of air that should be exchanged every minute with mechanical ventilation.

  • If you have a 15' Catamount style greenhouse, take the length and width (15') and multiply by 5 to get your total CFMs.
    Ex: A 15' × 96' greenhouse will be 15 × 96 × 5 = 7,200 CFMs per minute of air flow.
  • If you have a 22', 26', 30' or 34' greenhouse, use the same principle as above, except multiply by 7
    Ex: A 30' × 96' greenhouse will be 30 × 96 × 7 = 20,160 CFMs per minute of air flow.
  • If you have a gutter connect greenhouse, multiply width times length times (gutter height plus three feet)
    Ex: A 21' × 96' gutter connect greenhouse with a 10' gutter height will be 21 × 96 × (10 +3) = 26,208 CFMs per minute.

Next, use your total CFMs to decide what size fan you will need by using the chart below.

For larger houses, use two fans so that you can stage your cooling in two or three different levels.

For example, A 30' x 96' greenhouse would be 20,160 CFMs. Using two fans, each fan would be required to move 10,080 CFMs of air. Using the chart, you will see that two 36", 1/2 hp fans would meet the requirements.

Fan Size CFM's at .05 Static Pressure Angle Wall Housing (H × W) Motor HP
24″ 5,800 33 ½″ × 33″ 1/3
24″ 6,760 33 ½″ × 33″ 1/2
30″ 7,890 39 ½″ × 39″ 1/3
30″ 9,430 39 ½″ × 39″ 1/2
36″ 11,325 45 ½″ × 45″ 1/2
36″ 13,105 45 ½″ × 45″ 3/4
42″ 15,865 51½″ × 51″ 3/4
42″ 17,515 51½″ × 51″ 1
48″ 19,240 57½″ × 57″ 3/4
48″ 20,665 57½″ × 57″ 1

Now calculate shutter size

To calculate shutter size, take your total CFMs and divide by 600. The number is an air velocity number that means that the air is traveling at 600 feet per second. Smaller shutter openings will create a higher air velocity and create more of a breeze; larger openings will create a lower air velocity and create less of a breeze.

For example, A 30' × 96' greenhouse with two 36″, 1/2 hp fans will equal (10,308 × 2) 20,616 cfm's of air movement. Divide by 600 and it will equal 34.36 square feet of opening required. Therefore, you can use two 51” shutters, or you can use two 45” shutters and one 30” gable shutter.

Use the chart below to determine what size shutter you will need.

Model Number Size Air Flow Capacity (CFMs)
ACLRW27E 27″ 3,000
ACLRW33E 33″ 4,500
ACLRW39E 39″ 6,300
ACLRW45E 45″ 8,400
ACLRW51E 51″ 10,700
ACLRW57E 57″ 13,400
ACLRW6024 60″ × 24″ 6,000
ACLRW6036 60″ × 36″ 8,900
  • All shutters are motorized. Non-motorized shutters are available. Please call for pricing.
  • Add ¼" on each side for rough openings.

Stage Your Greenhouse

By using two large shutters and one small gable shutter, you can stage your cooling so that your greenhouse is properly cooled year round.

Stage 1

The lower speed of a two-speed fan turns on, and the small gable shutter opens. This is ideal for winter cooling.

Stage 2

The large shutters open in addition to the small gable shutter, and the higher speed of the two-speed fan turns on. This is ideal for spring and fall cooling.

Stage 3

The second fan turns on, which is a single speed fan. Now that both fans are running you have full ventilation with one air exchange per minute. This is ideal for summer cooling.