# 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 but 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. In other words, the lower the U value of the glazing system, the easier it is to heat the greenhouse. All of the common glazing materials on the market carry a U value rating. In a well constructed and maintained structure as much as 10 percent of heat loss can still be due to leakage. In greenhouses with poorly fitting doors, partially opened vents, other gaps or broken covering materials, significantly more heat can be lost this way. Infiltration losses are often relatively low in plastic-film covered greenhouses, but may be very significant in older glass houses, particularly if the joints between glass panes are not sealed.

### 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) ascertaing the glazing heat coefficients (U), and (3) knowing the temperature requirements for the crops to be grown.

#### 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. 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-layer polyethylene, square feet of acrylic-twin wall, etc.).

#### Calculating Temperature Difference (Δ)

The temperature difference (Δ), 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. This may be a compromise temperature to reduce heating costs. For example, while you may want a minimum night temperature of 64 degrees F (17.8°C), to reduce your heating costs, you might decide to use a set point of 60 degrees F (15.6°C) instead. If the lowest outside temperature is -2 degrees F (-16.7°C) and your set point is 60 degrees F (15.6°C), the ΔT is 60 degrees (DT = 58?F - -2?F).

#### 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 Bt per hour for the greenhouse can now be calculated 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: (a) transmission heat loss, and (b) infiltration. For greenhouse design, infiltration is generally analyzed via the air change method. This method is based upon the number of times per hour (ACH) that the air in the greenhouse is replaced by cold air leaking in from outside. The number of air changes which occur is a function of wind speed, greenhouse construction, and inside and outside temperatures. Infiltration heat loss can be significant and should be calculated and added to conduction heat losses. The equation for infiltration heat transfer is:

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# Topics Within This Chapter:

- Introduction to Greenhouse Heating
- Mechanisms of Greenhouse Heat Loss and Gain
- Greenhouse Heating Requirements
- Greenhouse Unit Heaters
- Greenhouse Central Heating Systems
- Greenhouse Infrared Heating Systems
- Air Distribution in Greenhouses
- Types of Fuels for Greenhouses
- Renewable Energy for Greenhouses