Lennox Merit 14HPX Installation Instructions Manual page 30

NOTE Within a single room thermostat demand, if
5−strikes occur, the defrost control will lockout the unit.
Demand defrost control's 24 volt power R must be cycled
OFF or the TEST pins on defrost control must be shorted
between 1 to 2 seconds to reset the defrost control.
Defrost Temperature Termination Shunt (Jumper)
Pins (P1)
The demand defrost control selections are: 50, 70, 90, and
100°F (10, 21, 32 and 38°C). The shunt termination pin is
factory set at 50°F (10°C). If the temperature shunt is not
installed, the default termination temperature is 90°F
(32°C).
Compressor Delay Mode (P5)
The defrost control has a field−selectable function to
reduce occasional sounds that may occur while the unit is
cycling in and out of the defrost mode. When a jumper is
installed on the DELAY pins, the compressor will be cycled
off for 30 seconds going in and out of the defrost mode.
Units are shipped with jumper installed on DELAY pins.
NOTE The 30 second off cycle is NOT functional when
jumpering the TEST pins.
OPERATIONAL DESCRIPTION
The defrost control has three basic operational modes:
normal, calibration, and defrost.
1. Normal Mode
monitors the O line, to determine the system operating
mode (heat/cool), outdoor ambient temperature, coil
temperature (outdoor coil) and compressor run time to
determine when a defrost cycle is required.
2. Calibration Mode
considered uncalibrated when power is applied to the
defrost control, after cool mode operation, or if the coil
temperature exceeds the termination temperature
when it is in heat mode.
Calibration of the defrost control occurs after a defrost
cycle to ensure that there is no ice on the coil. During
calibration, the temperature of both the coil and the
ambient sensor are measured to establish the
temperature differential which is required to allow a
defrost cycle.
3. Defrost Mode The following paragraphs provide a
detailed description of the defrost system operation.
506649−01
The demand defrost control
The defrost control is
DETAILED DEFROST SYSTEM OPERATION
The demand defrost control initiates a defrost cycle based
on either frost detection or time.
1. Frost Detection
30 minutes and the actual difference between the clear
coil and frosted coil temperatures exceeds the
maximum difference allowed by the control, a defrost
cycle will be initiated.
IMPORTANT
a greater accumulation of frost and will initiate fewer
defrost cycles than a time/temperature defrost
system.
2. Time If 6 hours of heating mode compressor run
time has elapsed since the last defrost cycle while the
coil temperature remains below 35°F (2°C), the
demand defrost control will initiate a defrost cycle.
When the reversing valve is de−energized, the Y1 circuit is
energized, and the coil temperature is below 35°F (2°C),
the defrost control logs the compressor run time. If the
defrost control is not calibrated, a defrost cycle will be
initiated after 30 minutes of heating mode compressor run
time. The control will attempt to self−calibrate after this
(and all other) defrost cycle(s).
Calibration success
temperatures during the 20−minute calibration period. If
the defrost control fails to calibrate, another defrost cycle
will be initiated after 45 minutes of heating mode
compressor run time. Once the defrost control is
calibrated, it initiates a demand defrost cycle when the
difference between the clear coil and frosted coil
temperatures exceeds the maximum difference allowed
by the control OR after 6 hours of heating mode
compressor run time has been logged since the last
defrost cycle.
NOTE − If ambient or coil fault is detected, the defrost
control will not execute the TEST mode.
The defrost cycle ends when the coil temperature exceeds
the termination temperature or after 14 minutes of defrost
operation. If the defrost is terminated by the 14−minute
timer, another defrost cycle will be initiated after 30
minutes of run time.
TEST MODE
A TEST option is provided for troubleshooting. See figure
23 for this function.
Page 30
Defrost Cycles
If the compressor runs longer than
The demand defrost control will allow
Actuation
depends on stable
Termination
system