Chapter 2

Greenhouse Heating

Calculating Heat Greenhouse Heating Requirements

Calculation of the design heat load of the greenhouse is required for determining the size and type of the heater and heat distribution systems within the greenhouse. The heat load is a function of the:

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. A large surface area can lose more heat than a smaller surface area over the same period of time. To evaluate transmission loss, the first step is to calculate the surface area of the greenhouse. This surface area should be subdivided into the various materials employed, i.e. square feet of double plastic, square feet of fiberglass, etc.

Calculate Temperature Difference

The temperature difference 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. This may be a compromise temperature to reduce heating costs. For example, while you may want a minimum night temperature of 66 degrees F (19°C), to reduce heating costs, but decide to use a set point of 62 degrees F (17°C) instead.

Calculate the Total Transmission of Heat Loss

After determining the total surface area (A) of the various construction materials and the temperature difference (DT) for the inside and outside temperatures these values are then combined with a heat loss factor (U) for each component material, to calculate the total transmission heat loss (q):

Allowances for Wind Speed

The U factor is also influenced by wind speed. The above values in Table 12.1 are based upon a wind speed of 15 mph. If other wind speeds are expected to occur at the design outside condition, then allowances should be made for this by adjusting the U factor are shown in Table 12.2. The stronger the wind is, the greater the heat loss.

Calculating Greenhouse Air Volume

As the number of air changes is related to the volume of the greenhouse, after selecting the appropriate figure from above, it is necessary to calculate the volume of the structure. For the example greenhouse, this is most easily accomplished in two steps. These figures do not include ventilation.

Calculate Heat Loss by Convection

From the Table 2.3, the number of air changes/h (ACH) would be 1.0 to 1.5--use 1.0 (double poly with high fiberglass sides).

Calculate Total Greenhouse Heating Requirement

Total greenhouse heating (qT) requirement:

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