Greenhouse Ventilation: Managing Heat, Humidity and Energy Use

Draw the air path before choosing equipment

Mark the outdoor inlet, route through the crop and exhaust or roof outlet. Natural ventilation relies on wind and temperature differences; fan ventilation adds a driven flow through specified openings. Circulation mainly redistributes air already inside. UMass explains the distinction and the importance of inlet and fan sizing. Ventilation guidance. Record the greenhouse geometry, neighbouring structures, insect screens and normal crop height. A fan’s catalogue airflow does not describe installed performance against screens, shutters and other resistance.

Check what happens when the weather changes

An opening that works with one wind direction may perform differently in still air or a storm. Review wind, rain and low-temperature limits with the vent supplier and crop manager. In Hamfy’s proposed control sequence, weather protection limits the permitted opening, while crop temperature and humidity determine the demand inside that boundary. Log requested and actual vent positions where feedback exists. A command without movement should generate an equipment check. Define how the crop is protected when weather constraints prevent sufficient ventilation.

Treat sunny-day overheating as a heat-balance problem

Record radiation, outdoor temperature, shading state and temperatures at several crop positions before increasing fan speed. Ordinary air exchange has limited cooling potential when outdoor air is already hot. Compare shading, ventilation and, where appropriate, an engineered cooling system as interacting measures. UMass discusses this limit and the dependence of evaporative cooling on outdoor conditions. Cooling context. Do not infer delivered airflow from motor power alone. Confirm inlet opening, maintenance condition and airflow distribution, then assess how quickly the crop zone recovers after a controlled adjustment.

Use moisture content and dew point for winter decisions

Relative humidity changes with temperature. Cool outdoor air at high RH may still contain less water vapour than warmer greenhouse air. Heating that air lowers its RH but does not itself remove water. Condensation risk also depends on the temperature of leaves and other surfaces. Humidity principles. Compare humidity ratio at the same pressure and observe representative leaf or surface temperatures. Coordinate heating and ventilation so that moisture removal does not push the crop below its permitted temperature.

Worked psychrometric example at 101.325 kPa: indoor air at 20 °C and 85% RH has a humidity ratio of about 12.4 g/kg of dry air. Outdoor air at 5 °C and 90% RH is about 4.9 g/kg. The difference indicates drying potential despite the higher outdoor RH. This is not a ventilation-rate calculation: crop transpiration, infiltration, heat loss and equipment limits still determine the required action.

Put the measurement where the decision matters

Use a representative crop-height air sensor protected from direct radiation and irrigation, with appropriate aspiration where required. Compare additional points near inlets, the far end and dense canopy zones during commissioning. Record time, height and position, not just a sensor number. A central average can hide a cold edge or a warm stagnant bay. UMass describes how air circulation reduces spatial temperature differences. Circulation guidance. Pair local observations with our VPD analysis and inspect short episodes as well as daily summaries.

Coordinate vents, fans, screens and heating

Write a sequence the operator can explain: which demand acts first, what limits it, and when another device may intervene. Agree deadbands, minimum run times and recovery periods with the designer so equipment does not repeatedly reverse or fight itself. State how screens alter the air route, how CO₂ enrichment interacts with vent opening, and who can override a setting. Include high-temperature and power-failure alarms with an assigned response. Verify actual actuator behaviour before adding predictive or AI control to the sequence.

Commission the greenhouse with a crop and a record

Choose representative sunny, cool-humid and weather-limited periods. Record outdoor conditions, crop-stage information, temperature, RH, vent position, fan state and heating demand on a common time base. Compare excursions, recovery time, spatial spread and measured energy against agreed acceptance criteria. Change one planned part of the sequence and repeat under comparable conditions. Hamfy proposes this evidence as the basis for an upgrade: improved uniformity or fewer wet-leaf episodes should be demonstrated separately from any claimed yield benefit. The sensor-data analyser can help inspect exported logs.