22
18-BB41D1-1A-EN
Defrost Control
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The demand defrost control measures heat pump
outdoor ambient temperature with a sensor located
outside the outdoor coil. A second sensor located on
the outdoor coil is used to measure the coil
temperature. The difference between the ambient and
the colder coil temperature is the difference or delta-T
measurement. This delta-T measurement is
representative of the operating state and relative
capacity of the heat pump system. Measuring the
change in delta-T determines the need for defrost. The
coil sensor also serves to sense outdoor coil
temperature for termination of the defrost cycle.
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A fault condition is indicated by the flashing Fault LED
light on the defrost control board located inside the
heat pump control box.
In normal operation, the status LED will flash once each
second when idle or twice each second with a call for
heating or cooling.
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1. TEST_COMMON (Shorting any of the other pins to
this pin causes the function of the other pin to be
executed. Leaving this pin open results in the
normal mode of operation).
2. FRC_DFT = Forced Defrost (Short TEST_COMMON
to this pin speeds up all defrost. Remove the short
after defrost initiates.
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Normal operation requires:
a. Status LED on board flashing 1 time/second in
standby or 2 times/second with a call for heating or
cooling.
b. 24V AC between R & B
c. 24V AC between Y, Y0 & B with unit operating
d. Defrost initiation when FRC_DFT pin is shorted to
TEST_COMMON pin.
D1582
86B
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Using the chart in
, locate (as close as
possible) the actual sensor temperature. The measured
resistance should be relatively close to the resistance
value shown in the chart.
