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Replace your thermostats with an electronic controller?

Mechanical thermostats are rapidly becoming obsolete. When you consider that replacing a thermostat with a 4 - 6°F differential between on and off with an electronic  controller having a 2°F differential, the savings in energy are significant. The heater shuts off much sooner, which results in a lower heat loss from the greenhouse surface and an energy savings.

For exampIe, if we assume a 30’ x 100’ double covered greenhouse in a northern climate (Hartford CT) heated to a night temperature of 60°F during the winter, replacing the mechanical thermostat having a differential of 6°F with a controller having a 2°F differential will result in a savings of about 750 gallons of fuel oil. Instead of heating the greenhouse air to 66°F before the heater shuts off, the controller will shut the heater off at 62°F. For each degree that the temperature in the greenhouse can be lowered, there is an approximate 3% savings in fuel. At today’s energy prices, the payback for installing the controller is less than one year.

A programmable controller is a control device that has a logic potential but is not powerful enough to be called a computer. The simplest ones replace several thermostats and usually have 5 to 6 stages of control (two heat stages and 3 or 4 cooling stages plus a setpoint).

Step controllers use 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 a greenhouse environment. Cost for basic step controllers usually runs from $600 to $1500 depending on the number of steps and relays or contactors that are needed to control the equipment.

Controller advantages include:

Controllers operate with a setpoint, the air temperature that you would like in the greenhouse. If the temperature falls below the setpoint, the heating system will be activated. If the temperature exceeds the setpoint, the vents will open or the fans will activate, providing cool air. With multiple cooling stages, a different setting on the vents or additional fans will start. A final stage could include the activation of an evaporative cooling system. An override is provided at each stage for manual operation.

The following features or functions may be standard or optional depending on the manufacturer and model:

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.

The above describes the typical temperature controller. There have been many specialized controllers developed during the past few years that control other greenhouse equipment.

USDA NRCS and some state and utility energy programs have grants that pay up to 75% of the cost of more efficient control systems. A good website to view for grant programs is www.dsireusa.org.

Controllers are the next level of control above thermostats and time clocks. They have proven to provide better environment control while at the same time saving energy.

John W. Bartok, Jr., Extension Professor Emeritus & Agricultural Engineer, Department of Natural Resources and the Environment, University of Connecticut, Storrs CT – Updated 2013.

Environment Controller – Sequence of Operation*


Maximum heat

Minimum heat


Stage 1

Stage 2

Stage 3

Unit Heater #2







Unit Heater #1














Exhaust Fan #1
Low speed







Exhaust Fan #1
High speed







Exhaust Fan #2
High speed







*Typical sequence of operation for a six stage greenhouse controller with two unit heaters, HAF fans and two exhaust fans.

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