Lennox XP21 Series Installation Instructions Manual page 35

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.
NOTE No system alert codes can
be active for defrost calibration to be
achieved.
100
E47
DEGREE
TARGET
*Heat Pump Control (A175)
Defrost Termination (E47)
Pins (Factory Default is 50ºF)
90
DEGREE
TARGET
70
DEGREE
TARGET
*50
DEGREE
TARGET
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.
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.
Accumulate compressor run−time while outdoor
coil temperature sensor input (RT21) is below
35°F (2°C)
Initiate a defrost and monitor coil temperature
sensor input (RT21) and accumulate defrost time.
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
YES
Figure 27. Defrost Calibration Sequence of Operations
Defrost Calibration
Coil sensor
below 35°F {2°C}
YES
30−35 minutes
compressor
run−time
YES
Defrost Mode
How did defrost
terminate?
Defrosted for 14 minutes with-
out the coil temperature sensor
input (RT21) reaching defrost
termination temperature as set
on heat pump control E47
pins*.
NO
Page 35
No attempt
NO
to calibrate.
Continue to
accumulate
NO
run time.
XP21 SERIES