10
4TWL6036A-SF-1F-EN
Defrost Control
DDeeffrroosstt CCoonnttrrooll
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.
FFaauulltt DDeetteeccttiioonn
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.
PPIINN IIddeennttiiffiiccaattiioonn
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.
DDeeffrroosstt CCoonnttrrooll CChheecckkoouutt
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.
TTeesstt SSeennssoorrss
Measure the temperature the subject sensor is exposed
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.
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