Mechanical thermostats are still the major heating/cooling system control device in greenhouses. Although electronic thermostats, controllers and computers are readily available, they have not been widely adapted in the industry, probably due to their cost and unfamiliarity to growers. Selecting the right thermostat and locating it to sense plant temperature can save considerable fuel and electricity.
Select a thermostat for greenhouse conditions
When selecting a thermostat for use in the greenhouse, look for one with moisture- and dust-tight boxes. Typical home thermostats have an exposed bi-metalic strip sensing element that can give erroneous signals when covered with dust or moisture. A better choice is a thermostat that has a hydraulic capillary tube filled with a liquid or gas that expands to activate the switch.
The movement of the switch between the “on” signal and the “off” signal is called the differential. This can vary from 1°F to 8°F for mechanical thermostats. If you want to maintain a minimum 60°F (setpoint) in the greenhouse and have a thermostat with a 6°F differential, the furnace will start when the temperature falls to 60°F but doesn’t turn off until it reaches 66°F.
This override overheats the greenhouse and increases both the heat loss and the heating cost. It is important to select a thermostat with a small differential. Some thermostats have a heat anticipator that shuts the furnace off just before the shut off point. This will save some fuel.
Locate the thermostat to sense plant temperature
Proper installation can also have an effect on thermostat operation. The thermostat must feel the air that the plants feel if you want to control the temperature in the plant zone. A good location is at plant height near the center of the growing area. Some growers install the thermostat so that it can be raised or lowered for different crops. A thermostat mounted on an endwall or sidewall of the greenhouse, a location that heating equipment installers usually choose, will not give good results. Also be sure that it is not located directly above heat pipes or in the direct line of a heating duct outlet. Shade the thermostat so that it is out of direct sunlight.
Locating the heating and cooling system thermostats in one location is desirable. Placing them inside a closed aspired box that is painted white will give more accurate sensing. The box has a small muffin fan that draws air past the sensors. Tests at Rutgers University have shown that the temperature spread (difference between high and low) in the greenhouse was reduced from 8°F to 2°F when the heating thermostat was aspirated. Energy is wasted anytime the greenhouse air temperature is above the setpoint.
Check the thermostats for accuracy
All thermostats should be checked for accuracy at least once a year. They can get out of calibration due to dust and dirt on the sensor, being knocked out of level (mercury type) or if the dial is forced too far in one direction. First clean the sensor with a can of compressed air and wipe if there is still dirt on it. Then set the dial to below room temperature for 10 minutes. Slowly raise the dial setting until it just makes contact. Check this against an accurate wall or digital thermometer. If there is a difference, mark it and have the service person make adjustments the next time the furnace is serviced.
For more accurate control, consideration should be given to upgrading to a controller that integrates both the heating and cooling systems. This usually has a payback of less than three years.
John Bartok is a regular contributor to Greenhouse Management and an agricultural engineer and emeritus extension professor at the University of Connecticut. He is an author, consultant and a certified technical service provider doing greenhouse energy audits for USDA grant programs in New England.
Have a question? You can write John at firstname.lastname@example.org.