Views: 0 Author: Site Editor Publish Time: 2026-05-25 Origin: Site
Properly sizing a greenhouse exhaust system helps protect crops from heat stress, high humidity, and poor air circulation. Many growers choose fans only by size or price. This often leads to weak cooling in summer or unstable airflow inside the greenhouse.
In most greenhouse projects, the common solution is simple: use exhaust fans only, or use exhaust fans together with cooling pads. This guide explains how to calculate greenhouse exhaust fan sizing, how to estimate fan CFM, and how to match fans with cooling pads for better summer cooling.
The Baseline Rule: A standard summer cooling setup usually needs one complete air exchange per minute.
Two Common Systems: Greenhouses usually use exhaust fans only, or exhaust fans with cooling pads.
Cooling Pad Matching: Cooling pad area should be matched with total fan CFM and target pad air velocity.
Layout Matters: Fans and cooling pads should be installed on opposite sides to pull air through the crop area.
An exhaust fan’s capacity is measured in CFM, or Cubic Feet per Minute. Before choosing equipment, you need to know how much air your greenhouse holds. This is the starting point of your greenhouse airflow calculation.
For small and medium greenhouses, use the internal volume to calculate fan capacity.
Measure the length and width of the greenhouse.
Calculate the average height by adding the roof peak height and sidewall height, then dividing by two.
Multiply Length × Width × Average Height to get the total air volume.
The standard summer baseline is one complete air exchange per minute. So, the greenhouse volume is also the basic target CFM.
Length × Width × Average Height = Required Fan CFM
For example, if a greenhouse is 30 ft long, 20 ft wide, and 10 ft high on average, the air volume is 6,000 cubic feet. The target fan capacity should be about 6,000 CFM.
Large agricultural greenhouses often use a simplified floor-area method.
Freestanding Greenhouses: Multiply total floor area by 7 to estimate the required CFM.
Gutter-Connected Greenhouses: Multiply total floor area by the gutter height, then add a safety allowance for trapped upper heat.
Basic CFM calculation is only the first step. Hot regions, strong sunlight, high altitude, dense crops, and long-span greenhouse structures may require more fan capacity.
Greenhouse Condition | Suggested CFM Adjustment | System Impact |
|---|---|---|
Mild climate or partial shade | Baseline CFM | Standard fan sizing is usually enough. |
Full sun or warm climate | Baseline CFM × 1.15 | More airflow is needed to remove solar heat. |
Hot region or dense crops | Baseline CFM × 1.25 | Fan and cooling pad systems are often recommended. |
Very hot or dry region | Baseline CFM × 1.30–1.40 | A complete fan and cooling pad design is usually required. |
A greenhouse cooling system should match the local climate and crop needs. For mild areas, exhaust fans may be enough. For hot areas, fans are often combined with evaporative cooling pads. This is a common part of greenhouse cooling design.
Exhaust fans remove hot, humid, and stagnant air from the greenhouse. They are usually installed on one end wall. A good exhaust fan for greenhouse use should have a moisture-resistant motor and stable airflow performance.
Exhaust fans are mainly used for summer cooling, humidity removal, and air exchange.
Cooling pads lower the temperature of incoming air through water evaporation. When exhaust fans run, outside air passes through the wet pad surface. The air becomes cooler before entering the greenhouse.
This system is useful for hot and dry climates, commercial greenhouses, vegetables, flowers, and high-value crops.
HAF fans do not remove air from the greenhouse. They circulate air inside the structure. They reduce temperature differences, prevent stagnant corners, and help keep plant leaves dry.
A complete greenhouse airflow plan may include exhaust fans for air exchange and HAF fans for internal air movement.
The cooling pad should match the total capacity of the greenhouse ventilation fan system. If the cooling pad area is too small, air moves through the pad too fast. This reduces cooling efficiency and increases system resistance.
Use total fan CFM and target pad air velocity to calculate the required cooling pad area.
Total Fan CFM ÷ Target Pad Air Velocity = Required Cooling Pad Area
In many greenhouse projects, the target pad air velocity is about 200–250 FPM. The final value should depend on pad type, local climate, and greenhouse design.
For general greenhouse cooling, divide total greenhouse fan CFM by 250 FPM.
For higher cooling efficiency, use a larger pad area and lower air velocity.
Total Fan CFM | Target Pad Velocity | Required Cooling Pad Area | Application |
|---|---|---|---|
3,000 CFM | 250 FPM | 12 sq ft | Small greenhouse |
5,000 CFM | 250 FPM | 20 sq ft | Medium greenhouse |
10,000 CFM | 250 FPM | 40 sq ft | Commercial greenhouse |
20,000 CFM | 250 FPM | 80 sq ft | Large growing area |
A fan and cooling pad system should be designed as one complete system. The fans decide how much air moves through the greenhouse. The cooling pads decide how effectively the incoming air is cooled.
For better results, make sure the cooling pad surface is large enough, the water distribution is even, and the airflow path is clear.
Correct layout is important for greenhouse cooling. Even well-sized equipment may perform poorly if the installation position is wrong.
Place Fans and Cooling Pads Opposite Each Other: Install exhaust fans on one end of the greenhouse and cooling pads on the opposite end. This helps air move across the full crop area.
Keep Airflow at Crop Level: Air should pass through the growing area, not only the roof space. This helps remove heat and humidity around the plants.
Avoid Short Air Paths: Do not install fans and cooling pads too close together. Otherwise, air may enter and leave without cooling the middle area.
Reduce Obstructions: Benches, dense crops, partitions, and equipment can block airflow. Keep the main airflow path open.
A single large fan may create sudden temperature changes. Commercial greenhouses often use staged control to match different seasons and cooling needs.
In cold seasons or at night, the main goal is humidity control. A small fan or low-speed fan can run for gentle air exchange. This helps reduce condensation without losing too much heat.
Mild seasons need moderate ventilation. One group of fans can remove daytime solar heat and keep the greenhouse stable.
In hot summer, the full exhaust fan system may run. If the greenhouse uses cooling pads, the water pump should also start. This provides stronger evaporative cooling and faster heat removal.
Fans, cooling pad pumps, thermostats, humidity sensors, and controllers should work together. When temperature rises, fans start first. If the temperature keeps rising, the cooling pad pump starts. This staged control saves energy and protects crops from sudden climate changes.
Greenhouse exhaust fan sizing should not rely on guesswork. First, calculate the greenhouse volume and target fan CFM. Then, decide whether exhaust fans alone are enough, or whether a fan and cooling pad system is needed.
For mild climates, exhaust fans may provide enough ventilation. For hot climates and commercial production, exhaust fans combined with cooling pads can provide stronger and more stable cooling.
Contact Yueneng to choose suitable greenhouse fans, cooling pads, and ventilation solutions for your growing project.
A: It depends on the climate. In mild areas, exhaust fans may be enough. In hot regions, exhaust fans are often used with cooling pads for better temperature control.
A: Cooling pads are recommended when fan ventilation alone cannot keep the greenhouse temperature within the target range. They are especially useful in hot and dry climates.
A: Divide total fan CFM by the target pad air velocity. For example, 10,000 CFM divided by 250 FPM requires about 40 sq ft of cooling pad area.
A: No. In most greenhouse systems, exhaust fans and cooling pads should be installed on opposite sides. This allows cooled air to move through the full greenhouse.
A: Yes. Exhaust fans remove air from the greenhouse, while circulation fans mix air inside the greenhouse. Both can improve the growing environment.
