Chapter 5

Greenhouse Heating

Mechanisms of Greenhouse Heat Loss and Gain

The main basis for heating in a greenhouse is the replacement of lost heat. Heat losses and gains in greenhouses occur in four ways: conduction, convection, radiation, and infiltration. The heat demand for a greenhouse is normally calculated by combining all four losses as a coefficient in a heat loss equation. A Btu (British thermal unit) is the amount of heat required to raise the temperature of 1 pound 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

A second mode of heat loss is that of 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).

Radiation

Radiation is heat transfer between two bodies without direct contact or a transport medium. There are two types of radiation that affect greenhouses: solar, or shortwave radiation and longwave, or infrared (IR) radiation. Radiation can result in heat loss or heat gain, depending on conditions. Sunlight is an example of radiation heating: shortwave radiation from the sun passes through the glazing and heats the greenhouse plants, soil, and structures even when it is below freezing outside. Radiation cooling is most noticeable on clear winter nights.

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.

Click on the following topics for more information on greenhouse heating.