What Size Exhaust Fan(s) Do I Need?

Greenhouse exhaust fans and shutters help release the spent air from the building and replenish it with fresh air. Improper sizing of fans is a major cause of poor greenhouse ventilation. During the warmest months of the year, your exhaust fans should be sized to provide at least one greenhouse volume air exchange per minute. This will result in approximately a 9°F rise in internal temperature from the intake louver to the fan.

Fan capacity needs to be able to exchange one square foot of greenhouse air per square foot of greenhouse area, 14-16’ tall gutter connected greenhouses usually need slightly more capacity. Fan efficiency can be improved by using fan cones placed outside the greenhouse.

Fans are a major part of a greenhouse ventilation system. Exhaust fans are used to draw air in or push air out of the greenhouse, they are designed to force air through shutters. To work efficiently, the greenhouse must be sealed so that air can only enter and exit through shutters or vents. The shutters and the exhaust fan are usually wired into a thermostat to make sure that the fan runs when needed and the shutters open when it runs.

Calculating Greenhouse Exhaust Fan Size

For precise measurements, you’ll want greenhouse length, width, sidewall height, height from top of end wall to peak, and greenhouse frame type. Sizing your intake shutters properly will allow you to avoid static pressure and maintain efficient airflow.

The rough formula to get the recommended exhaust fan CFM for your greenhouse is Length x Width x Height, but this rough number gives you more CFM than you need.

For a 30ft by 100ft semi-gable greenhouse with 6ft sidewalls and 10ft top of end wall to peak, your total CFM would be 33,000 although there are a number of factors that can influence what size fans you should get.

Your fan sizing ultimately correlates with your intake louver sizing, which in turn is affected by insect screens and light traps.

Just how much fan capacity is needed?

Two-speed or multi-speed fans will allow for variable ventilation rated to optimize seasonal needs.

Summer operation at least 1 air exchange per minute
Spring operation at least ¾ air exchange per minute
Winter operation at least ¼ air exchange per minute

greenhouse exhaust fan diagram

Types of Greenhouse Exhaust Fans

If you are looking to buy greenhouse exhaust fans, here are some models to consider:

J&D Manufacturing Typhoon Slant Wall

slant-wall-exhaust-fan

The Typhoon slant wall exhaust fan has a heavy-gauge galvanized housing and X-frame motor mount to provide a solid structure
Includes an aluminum shutter with tie bar and galvanized shutter clips to hold the shutter tight to the frame for less vibration
Designed for high performance, low maintenance, and low operating costs

J&D Manufacturing’s Wall Master Exhaust Fan

J&D Manufacturing’s Wall Master exhaust fan offers high output and efficient operation
Heavy-duty 18 gauge galvanized housing is strong, compact, and easy to install
Galvanized X–frame motor mount provides solid motor mounting for increased motor life and quieter operation
Aluminum shutters with a tie bar to prevent flapping and locking open

Other Exhaust Fan Options to Browse

Steps for Calculating for Sizing Greenhouse Shutters and Fans

Calculate the total CFMs needed. CFMs needed are the total cubic feet of air the should be exchanged each minute when using mechanical ventilation.

For a 15’ Greenhouse: Total CFMs needed = greenhouse length x greenhouse height x 5

Example: A 15’ x 96’ greenhouse needs 15 x 96 x 5 = 7200 CFMs per minute air flow.

For a 22’, 26’, 30’, or 34’ Greenhouse: length x width x 7 = Total CFMs needed

Example: A 30’ x 96’ greenhouse needs 30 x 96 x 7 = 20160 CFMs per minute air flow.

For 10’ gutter height gutter connected greenhouses: length x width x (10 + 3)

Choose fans based on CFMs needed. Multiple fans may be used to add up to the total CFMs needed.
Calculate the shutter size by dividing the CFMs needed by 600 to determine air velocity. Higher air velocities (more breeze) are achieved with smaller shutter openings
Example: A 30’ x 96’ greenhouse with two 36” fans needs (10308 x 2) x 20616 CFMs ÷ 600 = 34.36 square feet of shutter opening needed. Two 51” shutters or two 45” shutters with one 30” gable shutter would achieve 34.36 square feet of shutter opening.

Maintenance for Greenhouse Fans

Perform regular preventative maintenance to maintain efficiency
- Clean fan blades and motor regularly and frequently to prevent overheating and low performance
- Remove any grass or weed growth around shutters
Squeaky belts are either loose or worn and need to be checked for proper tension by pressing in the center of the belt; the belt should not move more than a distance equal to its width
Replace fan belts with frays or cracks along the edges
Keep bearings and leavers lubricated and be sure shutters move smoothly and have not been bent
Invest in high-efficiency replacement motors to reduce electricity costs long term
Fans should be wired with wiring large enough to ensure voltage does not drop below 2% or the fan’s required voltage (overheating may result); when checking voltage, do so at the fan