22 PB
Defrost Mode Operation
When the unit goes into heat pump mode for the rst time and the outdoor coil temp drops below 30 ˚F it will run a defrost to
calibrate the control even though there is no ice build up.
Normally the unit will go into defrost when the outdoor coil temp sensor provides signal to the board reads below 30 ˚F and the
defrost control board detects a frost condition based on the the calibration point and the ambient temp sensor. At this point the unit
will set a timer for 38 minutes and then begin a defrost. The defrost control board sends 24 vac from pin d of the control board to
energize the defrost heat relay and close contacts to one strip of electric heat. Basepan heat is also provided at this time. The defrost
control board also removes L2 power to fan out, shutting off the outdoor condenser fan motor.
After the outdoor coil reaches 70 ˚F or after 14 minutes the defrost cycle will be terminated. The outdoor coil temp defrost nish
point can be changed by moving the jumper on the defrost control board to 50, 60, 70, or 80 ˚F. Factory default is 70 ˚F.
degrees.
Defrost Calibration Mode
The control is considered un-calibrated when power is applied to the control, after cool mode operation. Calibration of the controller
occurs after a defrost cycle to ensure that there is no ice on the coil. During calibration, the temperature of both the coil and ambient
sensors are measured to establish a Dry Coil Delta T.
When the controller is in an un-calibrated state, the controller should initiate a sacricial defrost after 34 minutes of accumulated
compressor runtime with coil temperature below 35°F (Enable Temperature). The defrost will terminate if the coil sensor
reaches the selected termination temperature or after a 14 minute defrost. If the above defrost was concluded due to termination
temperature, or if terminated by time with a coil temperature of greater than the enable Temperature for more than two back-to-
back 2-minute transient delay periods, a clear coil (non-iced condition) can be assumed. Otherwise, the control remains in an un-
calibrated state.
A stabilized coil would be one that is determined stable after (4) minutes following the defrost cycle allowing the system pressures
and temperatures to stabilize. If the coil temperature is not considered stable and the accumulated runtime is ≥ 14-minutes, the
control will operate in time-temperature mode for 90 minutes. Upon the completion of the 90-minute operation the control will
initiate a sacricial defrost. At that point a Dry Coil Delta T at the particular outdoor ambient temperature can be determined. After
initial calibration has been completed, the controller prevents a defrost occurrence for 34 minutes of accumulated runtime in order
to avoid unnecessary defrost operation due to system transient conditions.
Demand Defrost Operation
The need for a defrost cycle while in Demand Defrost operation is determined by one of two factors: Time or Frost Detection.
Should six hours of compressor run time elapse without a defrost cycle and the coil temperature is below the frost accumulation
temperature, a defrost will be initiated. If this defrost is terminated on time rather than temperature, the controller shall establish
a new Dry Coil Delta T. If the defrost is terminated on temperature the unit will continue with demand mode operations. The
compressor run time will be reset when the defrost cycle is complete. The control shall be capable of detecting frost accumulation
on the outdoor coil and initiate a defrost when the Dry Coil Delta T + the Coil Temperature Dependant Variable is sensed. As the
ambient temperature changes, a slope of 1˚F Defrost Delta T change for every 8˚F ambient change will be used to adjust the
detection of frost accumulation.
Defrost Mode Activation
To activate a defrost sequence, the “B” thermostat input must be active, and the coil temperature must be below 35˚F. When these
conditions are met, the defrost enable timer tracks the compressor output, and accumulates compressor run time in the heating
mode. If the “B” thermostat input is inactive, the defrost enable timer is cleared. If the coil temperature is above 35˚F, the defrost
enable timer is not cleared, but does not accumulate compressor run time. If the coil temperature is above the selected defrost
termination temperature, the defrost enable timer will be cleared. When the defrost enable timer reaches 34 minutes, the defrost
mode is enabled. If the control is in the time/temperature defrost operation mode, defrost operation will be initiated immediately. If
the control is in demand mode, defrost operation will be initiated as described in defrost demand operation.
Defrost Mode Operation
When operating in the defrost mode, the control will have the compressor contactor, and auxiliary heat outputs activated. The
condenser fan relay contacts will be open, de-energizing the fan motor. The accumulated defrost time is monitored while in the
defrost mode and compressor is energized. The Low Pressure Switch is ignored during Defrost. When a defrost cycle has been
initiated, if the Y thermostat input is removed, the current defrost cycle will be suspended, but the accumulated defrost time is
frozen, and the control will resume defrost operation at the start of the next heating cycle (Y active, B active and coil temperature is
below 35˚F) with a minimum of the ASCD between compressor activations.
The accumulated defrost time resumes when the compressor output is re-energized. The defrost relay will de-energize when the Y
thermostat input is removed and energize when the Defrost is resumed.
OPERATION
Sequence of Operation
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