Demand Defrost Control; Defrost Initiation; Defrost Termination; Temperature Sensors - Heat Controller HRE1318/1360-1A Installation And Operation Manual

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8.0 DEMAND DEFROST CONTROL

The demand defrost control is a printed circuit board assembly consisting of solid
state control devices with electro-mechanical outputs. The demand defrost control
monitors the outdoor ambient temperature, outdoor coil temperature, and the com-
pressor run-time to determine when a defrost cycle is required.

8.1 DEFROST INITIATION

A defrost will be initiated when the three conditions below are satisfied:
1) The outdoor coil temperature is below 35°F.
2) The compressor has operated for at least 34 minutes with the outdoor coil tem-
perature below 35°F.
3) The measured difference between the ambient temperature and the outdoor
coil temperature is greater than the calculated delta T.
Additionally, a defrost will be initiated if six hours of accumulated compressor run-
time has elapsed without a defrost with the outdoor coil temperature below 35°F.
8.2 DEFROST TERMINATION (See Figure 4)
Once a defrost is initiated, the defrost will continue until fourteen minutes has
elapsed or the coil temperature has reached the terminate temperature. The termi-
nate temperature is factory set at 70°F, although the temperature can be changed
to 50°F, 60°F, 70°F or 80°F by relocating a jumper on the board.
FIGURE 4
DEFROST TERMINATION SETTINGS

8.3 TEMPERATURE SENSORS

The coil sensor is clipped to the top tube on the outdoor coil at the point feed by the
distribution tubes from the expansion device (TEV) (short 3/8" dia. tube). The air
sensor is located on the defrost control board.
If the ambient sensor fails the defrost control will initiate a defrost every 34 minutes
with the coil temperature below 35°F.
If the coil sensor fails the defrost control will not initiate a defrost.