Step Controllers Optimize Greenhouse Energy Use

This is a four-step controller with a solar sensor shield and shielded shielded cable, suitable for greenhouse use.

A step controller is an advanced electronic device that controls conditions in a greenhouse better than a mechanical thermostat and can save money in the long run. It uses a solid-state integrated circuit to monitor environmental data in the greenhouse and create output signals that activate equipment based on a set of internal programmed instructions. The microprocessor is a simple, low-cost device that is reliable, accurate, and works well in the greenhouse environment. It can replace several thermostats and may include other control functions for other equipment such as vents, alarms, lighting and irrigation. Step controllers are not as sophisticated as computer control system, but they are also less expensive to install.

Due to the smaller differential between the on and off points (+/- ½ to 1 degree F. for a controller vs. 4 to 8 degrees F. for a mechanical thermostat), the energy savings pays for the installation of the more accurate controller. For each 1 degree F. that the greenhouse is heated above the setpoint, energy usage increases 3%.

Controller advantages

• The heating and cooling functions of the greenhouse are divided into stages; the controller steps between stages as conditions in the greenhouse change. The sequence of equipment operation at each stage is programmed by the grower.
• Most controllers have day/night/DIF set points.
• All components are located in one waterproof enclosure, reducing moisture, dust problems, and maintenance.
• Installation time is reduced as relays, switches, and controls are prewired.
• The sensor is remote up to 100 feet and can be located among the plants, while the control box can be placed on an end wall or in the headhouse.
• Energy usage is reduced due to more accurate sensing and control.

How controllers work

The simplest step controller is designed to provide heating and cooling system integration for one greenhouse or zone. It has two heating stages, a setpoint and two or four cooling stages. Operation is simple. For example, when heat is needed, one furnace or heater may be turned on. If the outside temperature continues to fall and the inside temperature drops, a second unit is activated. Once the temperature gets up to the setpoint, the heaters turn off. The setpoint is the temperature that you desire for the greenhouse and is a stage between heating and cooling. The air circulation system may be the only equipment operating at this stage.

When cooling is needed, a fan may turn on the “first stage” – low speed. If it can’t keep up with the ventilation needs, the next stage kicks in and turns the fan to high speed. If this isn’t enough, a second fan may be activated. Motorized shutters or vents are opened as ventilation demand increases.

Most 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 require motor starters or contactors that are activated by the relay. Some manufacturers can supply boxes with the starters and contactors prewired. This reduces installation time.

Programming Example

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.

Here is a typical step controller sequence that might be used for two stages of heating and four stages of cooling:

Optional features

Many step controllers have additional features or functions that improves 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 – desirable to allow manual control for special conditions, calibration or servicing equipment.
• Ramping – controls the rate of change between day/night.
• Temperature sensitivity – accuracy should be between 1/2 to 1 degree F. for best control.
• Temperature difference between stages – some controllers come with a fixed differential, others provide a variable setting.
• Day-night temperature setting – most manufacturers use a light sensor to detect the graying of the sky at sunrise or sunset. This can usually be adjusted for light level.
• Time delay between stages – a desirable feature to prevent 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 – a unit that will provide variable vent opening is necessary for good cooling if your greenhouse has vents.
• Tracking – data storage of high/low temperature for each setpoint period. Storage of data up to 7 days. Some units have software to allow connection to a personal computer that allows monitoring of greenhouse conditions from an office area.

Summary

Growers that are using mechanical thermostats for their greenhouse should consider upgrading to an electronic step controller. The improvement in growing conditions and reduction in energy use make it a cost effective choice for almost all commercial greenhouse operations.

Additional Resources for Greenhouse Energy Conservation and Efficiency

Introduction to Greenhouse Efficiency and Energy Conservation

Contributors to this Article

Authors

• John Bartok, Jr., Agricultural Engineer,  University of Connecticut Extension
• Vern Grubinger, Professor,  University of Vermont Extension

Peer Reviewer

• Daniel Ciolkosz, Extension Associate, Penn State
• A.J. Both, Bioresource Engineering Specialist, Rutgers University