How to Select the Correct Size of Exhaust Fan

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How
to Select the Correct Size of Exhaust Fan


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 (except
2-speed) 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′

= 762 cu. ft

572

585

8′ x 16′ x 7′

= 896 cu. ft

672

760 or 800

10′ x 12′ x 7.5′

= 900 cu. ft.

675

760 or 800

10′ x 16′ x 7.5′

= 1200 cu. ft

900

1095

10′ x 20′ x 7.5′

= 1500 cu. ft.

1125

1250

12′ x 16′ x 8′

= 1536 cu. ft.

1152

1250

12′ x 20′ x 8′

= 1920 cu ft.

1440

1860

12′ x 24′ x 8′

= 2304 cu ft.

1728

1860

16′ x 24′ x 9′

= 3456 cu. ft.

2592

2830

16′ x 36′ x 9′

= 5184 cu. ft

3888

2 speed 3985/3760

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)


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