GSZH5 Service Manual
68
SYSTEM OPERATION
4.5 As the temperature indoors increase, it will reach a
happens, the room thermostat will remove the 24Vac
from “E/W1” of the MBE/AEPF. The contacts on HR1 will
2nd heater elements. If the “Y2” demand is present and
24Vac from “Y/Y2” of the MBE and the blower motor will
change to 60% of the programmed cfm. The VSTB will
remove the 24Vac from “Y/Y2” at the heat pump and the
outdoor fan will change to low speed operation. The heat
pump remains on along with the blower motor because the
room thermostat will remove the 24Vac from “G” and “Ylo/
Y1” of the MBE/AEPF. The VSTB removes the 24Vac from
“Ylo/Y1” at the heat pump and the compressor and outdoor
complete stop based on the time and rate programmed in
the motor control.
5.0 Defrost Operation
On heat pump units, when the room thermostat is set to
the heating mode, the reversing valve is not energized.
As long as the thermostat is set for heating, the reversing
valve will be in the de-energized position for heating except
during a defrost cycle.
5.1 The heat pump will be on and operating in the heating
mode as described the Heating Operation in section 4.
The defrost control in the heat pump unit checks to see
if a defrost is needed every 30, 60 or 90 minutes of heat
pump operation depending on the selectable setting by
monitoring the state of the defrost thermostat attached to
the outdoor coil.
5.3 If the temperature of the outdoor coil is low enough to
cause the defrost thermostat to be closed when the defrost
board checks it, the board will initiate a defrost cycle.
5.4 When a defrost cycle is initiated, the contacts of the
outdoor fan. The contacts of the LVDR relay on the defrost
board closes and supplies 24Vac to “O” and “W2”. The
reversing valve is energized and the contacts on HR1 close
and turns on the electric heater(s). The unit will continue to
run in this mode until the defrost cycle is completed.
5.4a For models with defrost control PCBDM133 or
PCBDM160, a 30 second compressor delay at defrost
initiation/termination is optional. As shipped from the
factory, the control is set for the delay (“DLY”), which will
valve shifts to/from the cooling mode position. To bypass
the delay, which typically reduces sound levels during
defrost mode, change the pin settings from “DLY” to
“NORM”.
5.5 When the temperature of the outdoor coil rises
high enough to causes the defrost thermostat to open,
the defrost cycle will be terminated. If at the end of the
programmed 10 minute override time the defrost thermostat
is still closed, the defrost board will automatically terminate
the defrost cycle.