To improve greenhouse ventilation, first establish what needs to change: heat removal, moisture exchange or the distribution of air through the crop. Record the existing operation before changing controls. An open vent or a running fan does not, by itself, show that the required exchange reaches the crop.
A tomato crop inside a greenhouse.
Separate ventilation from circulation
Ventilation exchanges greenhouse air with outside air. Circulation moves air within the greenhouse. Both belong in an airflow review, but an internal fan is not an outside-air outlet.
University of Massachusetts Extension explains how wind and buoyancy drive natural ventilation, while exhaust fans draw replacement air through inlets. Wind direction, openings, the crop and other restrictions affect the route. The practical starting point is a plan showing where air enters, crosses the growing area and leaves.
| Arrangement | Identify on the plan | Check in the operating record |
| Natural ventilation | Roof and side openings | Actual opening positions and outdoor conditions |
| Mechanical exhaust | Intake openings and exhaust fans | Fan state and whether the intended inlet is open |
| Combined arrangement | Both routes and their control sequence | Which mode was active during the problem |
| Internal circulation | Recirculating air path | Whether the observation concerns mixing or outside-air exchange |
Avoid choosing a “best” arrangement from greenhouse floor area alone. Supply the layout, crop arrangement, opening details and operating requirement when requesting a design review.
Compare outdoor and indoor moisture conditions
Outdoor RH alone cannot show whether incoming air contains less water vapour than greenhouse air. RH depends on temperature. Compare a suitable moisture quantity, such as vapour pressure or humidity ratio, using simultaneous indoor and outdoor measurements; account for air pressure when calculating humidity ratio. FAO's humidity definitions explain why equal percentages at different temperatures are not equal moisture conditions.
Consider the intended exchange together with temperature and CO₂ requirements. Record why a vent was opened and what changed afterward. For the equipment comparison, see ventilation versus dehumidification.
Trace the actual air route
Walk through the layout with the responsible operator and mark the intended intake, exhaust and internal circulation paths. Record the positions of screens, doors, shade or thermal curtains and the current crop arrangement. Then compare that plan with the conditions present during the reported problem.
UMass identifies blocked or restricted openings and dirty fan shutters as factors affecting airflow. It also recommends representative, shielded control sensing. These points support inspection; they do not establish a universal fan size, sensor spacing or maintenance interval.
A useful fault record identifies an observation precisely: which opening failed to move, which fan was commanded on, which zone differed from the control sensor and at what time. “Poor airflow” is too broad to tell the service team what happened. Follow the equipment's access and isolation instructions for any physical inspection.
High tunnel closed periods
For a high tunnel, record the state of the roll-up sides, end doors and any internal row cover beside the humidity observations. The air around a covered crop can differ from the main tunnel. University of Nevada, Reno's winter guidance discusses row covers trapping humidity and the need to monitor at crop height; its cold-season observations do not prescribe one setting for every crop or climate.
| Operating state | Additional check | Practical decision |
|---|---|---|
| Sides or end doors open | Record actual openings, weather and the route through the crop | Confirm that useful air exchange reaches the affected zone before adding equipment |
| Row cover in place | Compare conditions beneath the cover with the main tunnel, and record wet leaves or cover drips | Review cover opening and air movement with the crop adviser while retaining required frost protection |
| Cold night with sides closed | Record closure times, indoor/outdoor air conditions, local cold-surface observations and irrigation | Evaluate feasible ventilation, heating and circulation; only a defined remaining moisture-removal duty proceeds to equipment review |
Circulation redistributes air; it does not itself export water vapour. Use the existing ventilation versus dehumidification guide and night-time dew-point guide to interpret the record.
If a repeatable closed period leaves a measurable air-side load, send its duration, opening states, air and surface observations, power and drain constraints for a greenhouse dehumidifier review. An open-tunnel ventilation problem does not establish a mechanical dehumidifier duty.
Check measurements before changing offsets
Preserve the original readings and instrument identities. Compare sensors only after accounting for location, temperature, response time and the reference method being used. Follow the instrument manufacturer's procedure for stabilisation and any adjustment.
Changing a controller setpoint changes the requested operating condition. Changing a calibration adjustment changes the measurement system. Do not alter either merely to make disagreeing displays match. Keep the before-and-after readings and the reason for the change. The sensor selection and calibration guide develops that distinction.
Test a defined change
Choose a specific question, such as whether a particular vent sequence serves a previously unobserved zone. Write down the configuration before and after the change. Keep the test within the crop team's agreed operating limits.
| Comparison record | Why it belongs in the test |
| Crop stage, density and location | Defines what occupied the greenhouse |
| Indoor and outdoor climate | Describes the conditions during comparison |
| Vent positions and fan states | Identifies the action actually taken |
| Sensor locations and time stamps | Makes observations comparable |
| Electricity, heating and other recorded inputs | Defines the cost boundary |
| Exceptions and alarms | Shows when the intended operation was interrupted |
A lower fan-energy total alone is not a whole-greenhouse saving. Compare the same service requirement and include the other inputs within the chosen cost boundary. If weather or crop conditions changed, retain that limitation rather than attributing the entire difference to the control change.
Turn findings into an operating plan
Keep a short list of unresolved faults, their locations, the responsible person and the evidence needed to close them. Use the equipment documentation and site conditions to define maintenance, rather than borrowing a generic weekly or monthly schedule.
For automatic control, document the intended sequence, available sensor inputs, alarms and operator response. Test the recorded sequence before describing it as successful optimisation. The acceptance record should show what changed and whether the agreed condition was achieved; it should not infer a yield or disease benefit that was never measured.
If the review identifies a moisture-removal requirement beyond the current ventilation arrangement, assemble the operating record for an agriculture equipment discussion or project enquiry.
