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Section 16. Defrost Control
Defrost Control
The demand defrost control measures heat pump out-
door 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 dif-
ference between the ambient and the colder coil tem-
perature 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.
Fault Detection
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.
PIN Identification
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.
Defrost Control Checkout
Normal operation requires:
a. Status LED on board flashing 1 time/second in stand
by 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.
18
Test Sensors
Measure the temperature the subject sensor is ex-
posed to. If the sensor is mounted on a tube, place
the lead on an Annie A-8 (or equiv.) temperature
tester on the same tube near the sensor and insulate
the bulb.
Unplug the sensor and measure the resistance with a
good quality ohmmeter (Simpson 260 or equiv.). Read
the value as quickly as possible to prevent the meter
current from changing the resistance reading.
Using the chart, locate (as close as possible) the
actual sensor temperature. The measured resistance
should be relatively close to the resistance value
shown in the chart.
88-A5HP4001-1A-EN
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