There are different types of localized systems, distinguished by the type of fuel they require. Most are unable to produce an amount of heat comparable to the central heating systems, but that should not be expected of them.
Unit heaters or forced-air heaters as they are also known produce their heat within a fuel box and a fan blows it into the greenhouse. The heated air may be allowed to mix with the greenhouse air in the immediate vicinity of the heater (Figure 19-10), or it may be forced into
figure 19-10. Fan jet ventilation and heating unit blows hot air toward the central circulation fan (Delmar/Cengage Learning. Photo by Steven Newman.)
a polyethylene tube that is connected to the heater and runs the length of the greenhouse. The latter method allows a more even distribution of the heat throughout the greenhouse. Propane, natural gas, or kerosene are the usual fuels for unit heaters.
Radiant heaters comprise a linear system that runs the length of the greenhouse and is especially suited for houses that are narrow and long. The system uses a long tube covered by an aluminum reflector. Propane or natural gas is burned within the tube, causing it to reach a temperature of approximately 900° F. The hot gas is then pulled through the tube by a pump at the exhaust end of the system. As it passes along the tube, the heat of the gas causes the tube to emit infrared radiation, and it does so at a 90-degree angle to the surface of the plants, benches, and floor. Upon striking the flat surfaces, the infrared radiation is converted to the desired heat.