The “Greenhouse Effect” On a sunny day, light easily passes through the greenhouse glazing. Objects inside the greenhouse begin to warm up. The energy absorbed by these objects is re-radiated as heat. This heat is “trapped” inside the greenhouse, raising the greenhouse temperature.
Greenhouse Heat Loss n Both sunlight and greenhouse heaters warm up the inside of the greenhouse. n Eventually this heat is lost and escapes the greenhouse.
Greenhouse Heat Loss n There are 3 different ways that heat can be lost from a greenhouse: –Conduction –Air Leakage –Radiation
Conduction n The movement of heat through solid objects that are touching each other. n For example, roof trusses to glazing.
Air Leakage n The loss of warm air through openings in the greenhouse frame. n For example, heat loss around doors and exhaust fans.
Radiation n Transfer of heat between objects that are not in contact with each other. n For example, the radiation of heat from floor or benches to glazing.
Btu’s n Heat is measured in Btu’s (British Thermal Units). n One Btu is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.
Calculating Heating Requirements n The heating requirements of a greenhouse can be determined using the following formula: n H = (k) (a) (t 1 - t 2 ) n H = heat required in Btu’s per hour. n k = heat transfer coefficient for the glazing. n a = exposed surface area of the glazing in square feet. n t 1 = desired inside temperature in degrees F. n t 2 = outside temperature in degrees F.