Why do Greenhouses need Mechanical Ventilation?

Greenhouse ventilation is a key means of reducing humidity and temperature. It also helps to remove indoor pollutants such as airborne microorganisms and dust. Because airflow speeds have a great influence on the climate of the greenhouse, fans are generally needed. It is difficult to control airflow velocity by using only natural ventilation.

Commercial greenhouses commonly use an evaporative cooling system to help keep temperatures down. Some cooling in addition to ventilation must be provided if inside temperatures are to be kept at or below outside temperatures, and evaporative cooling is the most cost-effective method.

These devices require good air exchange rates. Air that is almost saturated with moisture has little potential for further cooling with evaporative systems, so humid air must be exhausted and fresh air introduced for the cooling system to work effectively. In addition, the cooling effect must be transferred to the plants by distributing the cooled air throughout the greenhouse.

An excessive relative humidity (RH) can also cause a number of health problems for plants. An RH of more than 80% is linked to oedema, mineral deficiencies, and plant diseases due to increased bacterial growth. The problem is especially severe when the humidity is high enough to lead to condensation on plants. It tends to affect soft-leafed plants more.

Guidelines for Greenhouse Fan Needs

The ventilation needs of a greenhouse change according to the season and time of day. As a rule, the ventilation system needs to be able to deliver one complete air change per minute in summer.

The requirement is much lower in winter, perhaps as little as one air change per hour, although two or three air changes per hour is more usual. The main role of ventilation in cold weather is to decrease humidity levels. A balance must be found between possible cold-weather heating requirements and the need to exhaust air to keep humidity levels down. Humidistats, thermostats and pressure sensors can be used to control the fan motor and louvers.

Usually, more than one fan is needed to efficiently handle these varying needs. Both exhaust and intake fans are used, often in the same greenhouse.

Fan and pad (wet wall) systems are commonly used for ventilation and evaporative cooling in greenhouses. Wet pads are placed at the inlet and the fan at the exhaust vents. Air is drawn through the pads, causing the water to evaporate and humidifying the air. These systems lead to a temperature gradient, where the coolest areas are near the evaporative pads. They take some of the pressure off the natural transpiration cooling that plants use, enabling optimum growth. While fan and pad systems generally use exhaust ventilation, positive pressure (inlet fans) can be found in some greenhouses.

To ensure that you get the fan capacity you need without overspending, it is advisable to speak to a reputable sales engineer from the manufacturer. The size of the greenhouse, how many plants are kept in it and the general climatic conditions outside will influence your requirements. Fans that are too large may cause draughts that can damage plants.

Greenhouse Fan Efficiency

Two main factors will determine the performance of your fan. The first is its location. Fans need to be placed so as to maximise airflow and prevent dead air zones in the greenhouse. Usually, greenhouse fans are installed at exhaust vents opposite the intake vents, and near the ceiling. This yields good air exchange rates and also helps to circulate air. The fan should be kept out of the way of liquid moisture, saltwater vapour and other chemicals and conditions that could shorten its life. It is also useful to place fans so that they work with the prevailing summer wind. This allows for more efficient ventilation.

The second factor comprises the accessories installed with the fan. Accessories reduce the air movement capacity of the fan somewhat, but are needed to protect the fan, for user safety when the fan is in operation, or to help control airflow. Louvers help to reduce heat losses in winter. An appropriate louver configuration also helps to establish a more even temperature distribution through the greenhouse.