Automatic Temperature Control For A Glenn Dale Box
D. C. Purdy, New York
A problem which must be wide spread among raisers of rhododendron seedlings is temperature control in Glenn Dale boxes. Most basements in modern suburban homes are too warm to maintain the ideal temperature of 65° F inside a Glenn Dale box. To obtain this temperature, it is necessary to provide cooling. At first, I did this by opening a window in the box in varying degrees, depending on the weather. This proved only moderately successful, because of the eternal vigilance necessary to combat daily and longer term weather variations. Last winter I installed an automatic system for this purpose, which does a much better job of maintaining constant temperature, and obviates the need for constant attention.
Fig. 57. Inside view of fan set in Masonite
board in window.
Fig. 58. Thermostat in box with small "plastic
Fig. 59. Showing use of flapper and re-circulation chamber in
connection with thermostatically controlled fan.
The system has two major components - a Rotron model RE 147 four inch, 100 CFM "Muffin" fan, and a Honeywell type T451B thermostat. The fan is mounted on a Masonite hoard in the window, as shown on the accompanying sketch and photograph. A recirculation hole and mixing chamber are provided so that cold air from the outside does not impinge directly on the plants. A flapper on the outside prevents drafts when the fan is not operating.
The thermostat must have contacts which close on rise of temperature to turn the fan on when the temperature in the box gets too high. Originally we tried to make do with a free war surplus thermostat. This proved to generate disturbances in the power lines which rendered every television set in the neighborhood inoperative. The neighbors were irate. The Honeywell thermostat will cause interference for an instant when it turns the fan on, and again when it turns the fan off. The neighbors have not found this objectionable. I am told that any good grade thermostat with snap action will give equal performance.
Last winter this system kept temperature in the box constant at 65° F whenever the outside temperature was between 58° F and 7° F. Below 7° F the fan shut off completely. When the outside temperature was 0° F. the low for the winter in our vicinity, the temperature in the box was 58° F. This low temperature performance might possibly be improved by using something heavier than Masonite for the mounting board. When the outside temperature was above 58° F, the fan ran continuously and maintained the temperature in the box 7° F above outside temperature. When such high temperatures occur it is usually spring, and time to move seedlings out of doors.
Use of a larger fan is not advisable, since it might cause too rapid a change in temperature. The temperature in the box can obviously be changed by changing the set point of the thermostat. This is useful when starting seeds.