Chapter 5

Greenhouse Heating

Greenhouse Heating Requirements

In order to select a heating system for a greenhouse, the first step is to determine the peak heating requirement for the structure. Heat loss for a greenhouse is composed of two components: (1) transmission loss through the walls and roof and (2) infiltration and ventilation losses caused by the heating of cold outside air. The heat transfer coefficient for the glazing material, U-value, depends upon many variables. Still, for normal design practice, most heating system designers use the values shown in the table below. The higher the number, the more heat that can be transmitted through the glazing

Calculating Greenhouse Heat Load

To evaluate transmission loss, the first step is to calculate the surface area of the structure. This surface area should be subdivided into the various materials employed (i.e., square feet of double plastic, square feet of fiberglass, etc.). Heat loss for a greenhouse is composed of two components: (a) transmission loss through the walls and roof and (b) infiltration and ventilation losses caused by the heating of cold outside air. To evaluate transmission loss, the first step is to determine the size of the heating unit required in Btu/hr., which requires the following: (1) calculating the surface area of the greenhouse (walls and roof), (2) ascertaining the glazing heat coefficients (U), and (3) knowing the temperature requirements for the crops to be grown. The equation for heat loss is:

Calculating Surface Area of Greenhouse

The amount of heat that can be transferred out of the greenhouse is dependent on the surface area of the structure.

Calculating Temperature Difference (Δ)

The temperature difference (∆), called delta T, is the difference between the minimum required temperature in the greenhouse and the lowest outside temperature. The outside temperature is the lowest average expected temperature during the heating season for the local area. The required temperature in the greenhouse is the set point temperature or the desired/required air temperature for plants that are to be grown.

Ascertain Heat Loss Coeffcient for Glazing Material

The final value in the transmission heat loss equation is the heat transfer coefficients (U). Acceptable values for various materials are shown in Table 4.1.

Calculating Total Greenhouse Heat Loss

The size of the heating unit required in Btu per hour for the greenhouse can now be calculated by knowing the surface area of the greenhouse, the greenhouse heat loss rate of the glazing material, and the maximum possible temperature difference, the Delta T.

Calculating Greenhouse Infiltration Rate

As mentioned previously, total heat loss is a function of two components: (1) transmission heat loss and (2) infiltration. For greenhouse design, infiltration is generally analyzed via the air change method. This method is based upon the number of air exchanges per hour (NC) that the air in the greenhouse is replaced by cold air leaking in from outside.

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