Air movement plays a critical role in greenhouse climate control. Ventilation helps regulate temperature and humidity, reduces disease pressure, and supports consistent plant growth. While vents allow air to enter and exit a greenhouse, fans are what move that air through the crop zone.
Different fan types serve different purposes. Understanding how exhaust fans and horizontal airflow (HAF) fans work will help you decide how to manage air movement in your growing environment.
Exhaust fans
Exhaust fans, also known as extraction fans, remove warm, humid air from the greenhouse and replace it with cooler outside air. This process helps lower temperatures, reduce plant stress, and limit disease pressure associated with stagnant, humid conditions.
By refreshing the air regularly, exhaust fans also help reduce insect pressure and improve overall air quality inside the greenhouse.
Exhaust fan design and benefits
Exhaust fans use rotating blades to overcome resistance as air moves through louvres or vents. Their simple design makes them versatile and suitable for year-round use.
In winter, exhaust fans are typically used to move small volumes of air without creating drafts. In summer, they must provide enough capacity to exchange air frequently throughout the greenhouse. Without evaporative cooling, an air exchange rate of roughly once per minute can help keep greenhouse temperatures within about 8 °F (≈4.5 °C) of outside conditions.
Where are exhaust fans located?
Exhaust fans are usually installed on the end wall of the greenhouse. Fresh air enters through motorised shutters or vents on the opposite side and is pulled through the growing space toward the fans.
Horizontal airflow fans (HAF)
Horizontal airflow (HAF) fans are designed to circulate air continuously within the greenhouse. Rather than exchanging inside air with outside air, they create a consistent airflow pattern that helps equalise temperature and humidity.
By keeping air moving across the plant canopy, HAF fans help remove excess moisture from leaves. This reduces condensation risk and lowers the likelihood of fungal diseases, particularly in dense canopies.
Monitoring temperature and humidity remains important, as condensation can still occur if leaves reach the dew point.
HAF efficiency and considerations
HAF fans are generally more energy-efficient than exhaust fans because they operate against low static pressure. Since they often run for long periods — sometimes nearly continuously for much of the year — efficiency matters.
When selecting HAF fans, consider:
- energy efficiency ratings
- airflow output (CFM)
- electrical consumption (watts)
Most HAF fans require minimal maintenance and do not need lubrication. However, dust and dirt should be cleaned regularly, as buildup can reduce airflow and increase motor temperature.
Where should HAF fans be located?
HAF fans work best when placed in the centre of the moving air mass. This creates a stable circulation pattern throughout the greenhouse.
Typical placement guidelines include:
- installing the first fan 10–15 ft (3–4.5 m) from one end wall
- spacing remaining fans 30–50 ft (9–15 m) apart
- positioning the last fan at least 50 ft (15 m) from the opposite end wall
Fan height is usually flexible, but mounting fans above head height keeps them out of the way and supports better airflow.
Choosing the right airflow strategy
Exhaust fans and HAF fans are not interchangeable. Exhaust fans are used to exchange air with the outside environment, while HAF fans manage internal circulation. In many commercial greenhouses, both are used together.
The right approach depends on greenhouse size, climate, crop type, and how other systems such as ventilation, heating, and cooling are configured.
Related greenhouse climate topics
You may also find these guides useful:
Air movement directly affects plant health and consistency.
Uneven airflow leads to microclimates, condensation, and inconsistent growth. Bluelab helps growers monitor temperature and humidity across the growing space, so airflow decisions can be evaluated based on real conditions, not assumptions.
Contact Bluelab to discuss monitoring and measurement options for your grow environment.
