There are several ways greenhouse structures are heated. These include unit heaters, central hot air or water systems, and infrared radiant heaters.
Unit heaters are a good choice for individual Quonset or hoop greenhouses. Heat is moved around the house by air circulation. Oil or gas heaters must be vented to the outside because they can produce combustible products that are toxic to people and plants.
Large greenhouse ranges are usually heated by centralized systems. Boilers produce steam or hot water that is moved through the greenhouse range by a series of pipes.
Heating pipes within the greenhouse may have fins to increase the surface are for heat exchange making the system more efficient. Specialized greenhouse and flood floors often have hot water heat under the floor.
Infrared radiant heaters work by heating the plants directly rather than the large volume of air in the greenhouse. Infrared tubing runs the length of the greenhouse 6 to 12 feet above the crop.
The main advantage is a reduction in heat costs associated with heating only the crop. Radiant heaters are not as common as other heating systems, but they can be very effective.