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%.
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.
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:
|Unit Heater 2||Unit Heater 1||HAF||Intake Shutters||Fan 1 Low Speed||Fan 1 Hi Speed||Fan 2 Hi Speed||Pad Pump|
|High heat (60°)||On||On||On||Closed||Off||Off||Off||Off|
|Low heat (62°)||Off||On||On||Closed||Off||Off||Off||Off|
|Minimum Venting (70°)||Off||Off||On||Open||On||Off||Off||Off|
|Intermediate venting (73°)||Off||Off||Off||Open||Off||On||Off||Off|
|Maximum venting (76°)||Off||Off||Off||Open||Off||On||On||Off|
|Evaporative Cooling (79°)||Off||Off||Off||Open||Off||On||On||On|
Many step controllers have additional features or functions that improves their versatility.
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.