Chapter 5

Greenhouse Heating

Greenhouse Infrared Heating Systems

Greenhouses can be heated using an infrared heating system that supplies radiant energy (See Figure 4.10). Infrared heaters emit infrared radiation, which travels in a straight path at the speed of light. An infrared heating system transfers heat to objects (e.g., plants, the floor, soil, benches, trays, etc.) first by the infrared rays; the heat “charge” in the object allows the secondary heat transfer processes from those objects to take place by radiation, conduction, or convection in all directions. Spreading the heat to multiple surfaces elevates the mean infrared temperature of the entire indoor environment. The heated objects will also transfer heat to the air by convection and raise the air temperature. In the conventional system, the air is heated first; the air then heats the plants. Greenhouses heated with infrared heat often feel cold, as the air is not heated. This type of system is used primarily on uniformly low-growing crops such as bedding plants and some potted crops.

Benefits of Infrared Heating

Infrared heating can be more economical. Heating plants directly is inherently more efficient than convection systems which must heat the air so that the air can heat the plants. When you run an infrared heating system you won’t be paying extra to generate heat to make up for heat lost in the circulation of air. The air temperature remains nearly the same anywhere in the greenhouse from the floor to the peak with infrared heating sytems. In a conventional air-heated greenhouse, the air temperature may be as much as 1degree F (0.5°C) warmer for each foot higher of elevation.

Types of Infrared Heaters

Infrared heaters are usually classified into two simple groups, high-intensity infrared heaters and low-intensity infrared heaters. High-intensity infrared heaters mix gas and air behind a porous ceramic grid. The surface temperatures of high intensity infrared heaters are above 1500 degrees F (815°C), generally between 1600 degrees F (870°C) and 1800 degrees F (980°C). Since the physical size of the heater is small, the heat felt from the unit is localized in a small area and has higher intensity. Due to many factors, including open flame, localized intense heat and the red/orange glow, high-intensity infrared heaters are not well-suited for greenhouse heating.

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