Greenhouse Ventilation Control: When to Open and How Wide

A ventilation system is only as good as the rules it is given: when the openings move, how far they travel, and whether the pad set joins in are control decisions. This article covers what triggers it, how a movement is staged, how the two cooling routes are kept apart, and what happens when a signal fails.

The layer sits between the crop and the hardware in three parts: the mechanical side that moves the openings, the control side that reads the house, and the wireless network joining them in a bus-communication arrangement. The control and mechanical systems collect the greenhouse conditions, and the lower machine at the opening drives the vent opening and closing. The host, or upper computer, holds the setpoints and the day plan, and a GPRS module ties the arrangement to the mobile network.

Once the working period begins, the host scans the addresses of the units on the bus and each one that answers uploads its data; the reading is compared with the limits the grower set, the opening position follows, and the unit at the vent turns it into motor travel through the reducer. Nothing decides on its own: the house runs to a rule rather than to attention, narrowing the temperature difference and releasing the labour that walked the ridge.

The openings answer to three signals, and the three do not always agree. Temperature is the first: ventilation begins as the house passes the upper limit in force for that crop and stage, and stops when the reading returns to the lower one. Both are host parameters, changed with the crop and stage.

Humidity is the second. In a house shut for hours the crop transpires into still air, and that moisture must be exchanged before it settles on leaves and fruit. A humidity trigger usually asks for a short exchange rather than a wide opening, since heat leaves with the moisture.

Carbon dioxide is the third and least visible. A closed house draws the level below what outside air holds and growth slows quietly. Where enrichment is fitted, an open vent is the moment the gas stops being worth supplying, so this line limits enrichment as much as it starts ventilation.

When two lines pull against each other the order is settled in advance: the crop's temperature ceiling sets the limit and the others borrow time inside it. The values belong to the crop, the stage and the local climate.

An opening that jumps from shut to wide is a shock to the crop and hard on the drive, so movement is staged: a first step to begin exchange, further steps only while the reading keeps climbing, and a return in the same steps. The number of steps and their width are host settings, not properties of the frame.

Two timers sit underneath. A minimum interval keeps the drive from reversing before it finishes the previous move, and a minimum dwell holds a step long enough to be judged, so the house does not hunt between open and shut all afternoon.

How long an opening stays open follows in reverse: late in the day the steps close in the opposite order and the house is sealed before the night layer goes down, since an opening left wide after dark is a hole in the night. A wind or rain signal outranks temperature while it is active.

Both routes move air, which is why they must not run together: pad cooling needs outside air drawn through a wet pad into a closed house, while ventilation is exchange, and exchange stops when the house is shut. With the vents open the cooled air leaves as fast as it was made, and water and fan power are wasted.

The interlock is a rule, not a preference: pad cooling is enabled only with the openings shut, and starting it suspends ventilation. The changeover is never instant: the openings travel to a neutral position and the pad set waits, so one part of the house does not undo another.

A summer day follows that order: exchange carries the morning, when the house is barely above the outside air; as the day builds the pad set takes over with the house closed; in the evening it stops and the openings clear the moisture; at night neither runs. Fogging and CO2 enrichment assume a closed house too, so an opening left open cancels them. The device side has its own articles: the natural ventilation system and the fan and wet curtain set; this layer decides which is asked.

Sample where the crop lives, because the ridge is warmest and a probe up there can read comfortable while the fruit zone is not. Average several points rather than trusting one, since a sensor that disagrees with its neighbours for long should be suspected. Keep the probe dry and in moving air: a wet or shaded one drifts silently.

The house reads itself in a second way too: images are collected automatically, features are extracted from them, and a database of crop growth models can take a hand in holding the temperature. Temperature and humidity return to the screen and are kept as a curve, so the host's link to the trend shows whether a setting was right. That link also reaches past the screen: a signal goes over the network to the address of the host, or from a phone to the platform and on to the host, so the grower can follow the house from elsewhere, and where several solar greenhouses run together the curves show which holds its temperature. The routine is seasonal: setpoints reviewed at every crop change, and the interlock tested before the first hot week.

The question that decides whether any of it is safe is what the house does when the reading stops. The safe response is neither to freeze nor to shut everything: go to a known position, alarm the operator, fall back on a plain time band and keep the openings reachable by hand. Manual mode is a real mode rather than an emergency: the wireless receiver module takes the coded signal from the handset and passes the key value to the vent unit.

Our factory has built greenhouses for above 35 years. Send the house type, the covering, the crop and the climate to be held, and we will advise on the control arrangement, the sensor points and the interlock called for; the planting technology notes in this section carry the same conditions into the crop, and the heat preservation routine covers the hours after dark. Call +86 191 5068 3942 or write to sales@cngreenhouses.com. Two independent references help when a routine is written for a site: the USDA publishes protected cultivation material, and HortiDaily reports on how commercial houses are run.

Q: What decides the moment the vents open?
A comparison in the controller, not a clock alone: the unit at the vent uploads its reading, the host compares it with the upper limit in force, and the opening position follows.

Q: Why must pad cooling wait for the vents to close?
The two do opposite jobs. Pad cooling needs outside air drawn through the wetted pad into a closed house; with the vents open the cooled air leaves and the water and fan power are wasted.

Q: Where should the temperature probe sit?
At crop level rather than only under the ridge, in moving air, out of direct sun and away from the pad face, and preferably several points averaged.

Q: What should the house do if a sensor fails?
Go to a known position rather than freeze or shut down, alarm the operator, fall back on a plain time band where one is set, and keep manual operation available.

Q: How long should an opening stay at each step?
Long enough for the reading to be judged, and never so short that the drive reverses before finishing the previous move; interval and dwell are controller settings.

Q: Can one house's setpoints be copied to another?
No. The limits belong to the crop, the stage and the climate the house stands in, and two houses on one site can carry different pairs.