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Charley's Greenhouse
17979 State Route 536
Mt. Vernon, WA 98273
1-800-322-4707
Fax 360-873-8264

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360-428-2626

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360-395-3001

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How to Select the Correct Exhaust Fan Size

Greenhouse ventilation is based on a specific formula regardless of your plant types and climate zone. You may vary shading and misting to accommodate plant needs and your local climate.

The exhaust fan should change the air in the greenhouse in 1-1/2 minutes. Fan capacity is rated in Cubic Feet per Minute (CFM). For your correct fan size in CFM, calculate your greenhouse Volume:

Length x Width x Average Height = Volume.

For Average Height, measure halfway up a roof rafter and then measure straight down to the floor. This vertical measurement is the Average Height.

Multiply the Volume by 0.75 for your fan CFM.

Now Select a fan with a CFM rating near or greater than your calculation. Our larger fans can be slowed with a fan speed control.

Caution: A fan that is too small will provide very little cooling.

Some shading will be required in warmer months, and misting for evaporative cooling may also be necessary.

Typical Greenhouse Calculations

Size: L x W x A.H.

= Volume

Fan CFM: V x 0.75

Fan CFM Recommended

8' x 10' x 7'

= 560 cu. ft.

420

585

8' x 12' x 7'

= 672 cu. ft

504

585

8' x 16' x 7'

= 896 cu. ft

672

975

10' x 12' x 7.5'

= 900 cu. ft.

675

975

10' x 16' x 7.5'

= 1200 cu. ft

900

975

10' x 20' x 7.5'

= 1500 cu. ft.

1125

1900

12' x 16' x 8'

= 1536 cu. ft.

1152

1900

12' x 20' x 8'

= 1920 cu ft.

1440

1900

12' x 24' x 8'

= 2304 cu ft.

1728

1900

16' x 24' x 9'

= 3456 cu. ft.

2592

2625

16' x 30' x 8'

= 3840 cu. ft

2888

3000


Shutter Size:

Determine shutter size by dividing your fan CFM by 250. This gives shutter size in square feet. Divide this figure by 2 if two shutters are to be used. (Two shutters are recommended for greenhouses over 100 sq. ft. or wider than 8 feet.)

Fan & Shutter Installation

Fan: Position as high as possible in one end, usually opposite from the door. Install with motor on inside of greenhouse, mounting either from inside or outside as convenient.

Shutters: Install opposite the exhaust fan. Economy type (no motor): Install with vanes opening into greenhouse.

Motorized Shutters: Install with motor on inside of greenhouse, vanes open out.

Frame Opening ("rough opening") should be square and 1/2" larger than fan blade or shutter size. (12" fan = 12 1/2" x 12 1/2" opening)

See Framing and Wiring for Greenhouse Fan Systems