Chapter 2

Greenhouse Heating

Basics of Greenhouse Heat Loss and Gain

The main basis for heating in a greenhouse is the replacement of lost heat. The heat losses and gains in greenhouses occur in four ways: conduction, convection, infiltration, and radiation. All of these heat loss and gain pathways exist in all buildings and greenhouses, regardless of the building type or size. The overall rate of heat loss is expressed as Btu per hour. A Btu (British thermal unit) is the amount of heat required to raise the temperature of 1 lb. of water by 1 degree F.

Conduction

Most of the heat n the greenhouse is lost through conduction, which is the transfer or flow of heat through a material, such as greenhouse glazing. Different materials, such as aluminum sash bars, glass, polyethylene, and cement curtain walls, vary in the rate at which each conducts heat from the warm interior to the colder exterior. The energy flow rate is called the heat transfer coefficient, or U-value, and is measured in British thermal units per square foot per degree Fahrenheit per hour (Btu/ft2-°F-hr).

Convection

Convection heat transfer is the physical movement of a warm gas or liquid to a colder location. As air is heated in a greenhouse, it rises to the roof and loses some of its heat to the roof glazing materials. Then the cooler, heavier air sinks towards the floor until it is warmed by the heater or by warmer floors and benches. Heat losses by convection inside the greenhouse occur through ventilation and infiltration (fans and air leaks). Heat transfer by convection includes not only the movement of air but also the movement of water vapor. When water in the greenhouse evaporates, it absorbs energy. When water vapor condenses back to a liquid, it releases energy. So when water vapor condenses on the surface of an object, it releases energy to the outside environment.

Infiltration

Another mode of heat loss is air infiltration. Heat loss by air infiltration depends on the age, condition and type of greenhouse. Older greenhouses or those in poor condition generally have cracks around doors or holes in covering material through which large amounts of cold air may enter. The greenhouse ventilation system also has a large effect on infiltration.

Radiation

Radiation heat transfer occurs between two bodies without direct contact or the need for a medium such as air. There are two types of radiation that affect greenhouses: solar, or shorter wavelengths and longer wavelengths, or infrared (IR) radiation. Shorter wavelengths from the sun passes through the glazing and heats the greenhouse plants, soil, and structures. As the plants, soil, and greenhouse structures become warmer than the surrounding temperature, they emit heat in the form of IR radiation. The radiated heat travels toward the glazing, and some is reflected back into the greenhouse while some passes through and out to the night sky. The amount of radiant heat loss depends on the type of glazing, ambient temperature, and amount of cloud cover. Radiation cooling is most noticeable on clear winter nights.

Greenhouse Heating Systems

The heating system must provide heat to the greenhouse at the same rate at which it is lost by conduction, convection, infiltration, and radiation. Many types of heating systems are available for use in greenhouses. Selecting the proper heating system is important because of the significant differences between available systems. Some systems cost less to buy or use less expensive fuels. Others may have a higher initial cost, but they are more efficient and cheaper to operate. There are three popular types of heating systems for greenhouses. The most common and least expensive is the unit heater system. In this system, warm air is blown from unit heaters that have self-contained fire­boxes. Heaters are located throughout the greenhouse. A second type of system is central heat, which consists of a central boiler that produces hot water or steam (more commonly hot water), plus a radiating mechanism in the greenhouse to dissipate the heat. The third type of system is radiant heat. In this system, gas is burned within pipes suspended overhead in the greenhouse. The warm pipes radiate heat to the plants.

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