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XP21 SERIES
Continue to
accumulate
run time.
Heat pump control monitors mode of
operation through outdoor ambient
temperature (RT13), outdoor coil
temperature (RT21) and compressor
run−time to determine when a defrost
cycle is required.
Defrost Calibration
Defrost Mode
Coil temperature sensor input
(RT21) reached specified de-
frost termination temperature
as set on heat pump control
E47 pins*.
Calibration Attempted
(Unit Running in Heat Mode)
Calibration
Demand Mode
After the initial calibration, the heat pump control
disables the defrost mode for 30−34 minutes in
order to avoid unnecessary defrosts. Beyond
this timing period, both the ambient (RT13) and
coil (RT21) sensors are continuously moni-
tored. If the coil temperature is below 35°F (2°C)
and the calibration delta is reached or accumu-
lated compressor run−time is more than six (6)
hours, a defrost is to be initiated.
100
90
DEGREE
TARGET
70
DEGREE
TARGET
*50
DEGREE
TARGET
100 90 70 50
DEGREE
TARGET
100 90 70 50
100 90 70 50
100 90 70 50
E47
*Heat Pump Control
(A175)
Defrost Termination (E47)
Pins (Factory Default is 50ºF)
Coil sensor
below 35°F {2°C}
No attempt
to calibrate.
30−35 minutes
compressor
run−time
Accumulate compressor run−time while outdoor
coil temperature sensor input (RT21) is below
35°F (2°C)
Defrosted for 14 minutes with-
out the coil temperature sensor
input (RT21) reaching defrost
termination temperature as set
on heat pump control E47
pins*.
How did defrost
terminate?
NO
NO
YES
YES
NO
Coil sensor (RT21) detects temperature below 35°F {2°C}
during either initial power−up, after loss of power or after
cooling mode, a sacrificial defrost will be used to ensure there
is a clear coil before attempting calibration.
Initiate a defrost and monitor coil temperature
sensor input (RT21) and accumulate defrost time.
YES
NOTE No system alert codes can
be active for defrost calibration to be
achieved.
Figure 27. Defrost Calibration Sequence of Operations
