Regulating greenhouse temperatures with integrated computer controllers

Thermostats can work well for a startup or seasonal greenhouse with limited heating and cooling equipment, but most greenhouses today have additional equipment that needs control. Installing an accurate integrated controller for all the equipment can achieve more consistent plant growth and save labor and energy, helping to offset the cost.

A controller in this context is a device that uses a solid-state integrated circuit to monitor environmental conditions in the greenhouse and create output signals that activate equipment based on a set of internal programmed instructions. The microprocessors used are simple, low-cost devices that are reliable, accurate and work well in a greenhouse environment.

Controllers operate with a setpoint, such as the air temperature you would like in the greenhouse. If the temperature falls below the setpoint, the heating system will be activated. The differential between when the equipment will turn on and when it will turn off can usually be varied. An override is provided at each stage for manual operation. If the temperature exceeds the setpoint, the vents or the fans can activate, providing cooling.

Today, there are many companies that make greenhouse controllers. Some have limited devices particular to a certain environmental factor, such as irrigation or carbon dioxide. Others manufacture a full line of controls for all equipment that might be used.

When selecting a controller, consider the following parameters:

• Zones: Is it needed for single or multiple climate zones?
• Equipment type and diversity: What equipment will be controlled?
• Access: Where will it be located?
• Purpose: What features (i.e., DIF, alarms, data collection) are desired?
• Future use: Is the controller expandable for additional greenhouses or zones?
• Customer care: How much technical support is available?
• Recordkeeping: Does the system store historical data?
• Off-site control: Can access and setting changes be made remotely?
• Outdoor climate: Does the system include a weather station?

Controller advantages

The heating and cooling functions of the greenhouse are divided into stages, and the controller steps between stages as conditions in the greenhouse change. Multiple pieces of equipment can be assigned to each stage, such as two heaters, HAF fans and an energy screen. The sequence of equipment operation and the temperature at each stage is programmed by the grower.

• Most components are in one waterproof enclosure, reducing moisture, dust and maintenance.
• Installation time is reduced, as relays, switches and controls are pre-wired.
• The greenhouse environment sensors are usually remote and connect to the control box with small low-voltage wire. Sensors can be located among the plants, while the control box is in an accessible location out of the plant zone.
• Controllers eliminate the potential for both heating and cooling equipment operating at the same time, which can happen with individual thermostats.
• Energy use is reduced due to more accurate sensing and control.
• Many controllers come with switching relays wired for each stage. These can be direct-connected to low-power equipment, such as motorized shutters, vent operators and steam or solenoid valves. Equipment with motors that draw large amounts of power when starting will require motor starters or contactors that can be activated by the relay. Some manufacturers can supply boxes with the starters and contactors pre-wired. This reduces installation time.
• The number of programmable equipment terminals varies widely between models. Some have as many as 40 outputs. This allows you to control more than one zone or to change the stage in which equipment operates without any wiring changes.

Many controllers have additional features or functions that improve their versatility:

• LCD screen: Displays current temperature, time, date and other information.
• Battery backup: Stores settings in case of power failure.
• Alarm activation: Can be connected to a trouble alarm.
• Override switches: Allow manual control for special conditions, calibration or servicing equipment.
• Ramping: Controls rate of change between day/night.
• Temperature sensitivity: Accuracy should be between 1/2 °F to 1 °F for best control.
• Temperature difference between stages: Some come with a fixed differential; others have a variable setting.
• Day-night temperature setting: Most manufacturers use a light sensor to detect the graying of the sky at sunrise or sunset. Can usually be adjusted for light level.
• Time delay between stages: Prevents rapid cycling of equipment.
• Indicator lights: Show which stage is activated and what equipment is operating.
• Programmable night cooling lockout: Can disable one or more ventilation stages after dark.
• Aspirated sensor box: Includes a small fan that draws air over the sensors to obtain an average temperature in the greenhouse.
• Cycle timer: Operates irrigation, misting or lights.
• Vent control: Provides variable vent opening to get proper air velocity through the vent for good cooling.

Advanced controllers offer many of the above features plus additional stages of control, input from a weather station and the ability to control multiple zones or greenhouses.

The above describes the typical temperature controller. There have been many specialized controllers developed in the past few years that control other greenhouse equipment, ventilation, irrigation and misting, energy and shade screen systems, and electricity demand shedding.