
035-18549-000-B-0402
18 UnitaryProductsGroup
HEATING OPERATION
1. Reversing valve is de-energized and the system
will be in the heating mode.
2. If the fan switch on the thermostat is in the “ON”
position, indoor section blower motor contactor
10M will be energized through terminal G to pro-
vide continuous blower operation. If the switch is in
“AUTO”position, the blower will operate only when
thermostat calls for heating operation.
3. When the TH1 of the thermostat closes for first-
stage heat, a circuit is made from terminal Y
through the defrost control board and safety
switches to energize contactor 1M (on 460 volt
units) and RY1 to start the compressor. Contactor
2M is energized through the NO contacts of RY1-1
(on 460 volt motors) and the NC contacts of K2 on
the defrost control board, which starts the outdoor
fan motors.
4. The thermostat will cycle the unit to satisfy the
heating requirements of the conditioned space.
5. After the unit has shutdown from a heating cycle or
a power interruption, the anti-short cycle feature of
the defrost control board will not permit the unit to
restart for 5 minutes. This feature prevents the unit
from short cycling.
6. If the discharge pressure reaches 430 psig, the HP
control will open and the defrost control board will
lock out the compressor. If the discharge tempera-
ture reaches 255°F, 2TH thermostat will open and
the defrost control board will lock out the compres-
sor. If the suction pressure falls to 7 psig, the low
pressure switch will open and the defrost control
will lock out the compressor.
7. If the control that caused the lockout has automati-
cally reset, the unit can be restarted by one of the
following:
a. Turning the system switch on the thermostat to
the “OFF”position and back to the “HEATING”
position.
b. Decreasing the set point on the thermostat
below the temperature in the conditioned space
and then returning it to its original setting.
c. Opening and closing the power supply main dis-
connect switch.
IN ALL THREE RESET METHODS DESCRIBED
ABOVE,AFIVEMINUTETIMEDELAYWILLTAKE
PLACE AFTER THE RESET BEFORE THE UNIT
WILL RESTART.
8. Supplemental electric heat (if installed) will be con-
trolled by second stage TH2 of the thermostat. The
supplemental electric heat circuit is controlled
through low voltage terminals W, 60 and 66. The
standby portion of electric heat (controlled through
low voltage terminal 53) cannot operate because
relay RY1 is energized, opening the circuit to termi-
nal 53, whenever the compressor is operating.
9. When second stage heating TH2 is satisfied, the
supplemental heaters will be de-energized.
DEFROST CYCLE
When condensate freezes on the outdoor coil during
heating operation, it must be defrosted before it blocks
the flow of air across the coil.
1. A defrost cycle will be initiated by the defrost con-
trol board's timed defrost feature.
2. When the defrost cycle is initiated, the unit oper-
ates as follows:
a. The reversing valve solenoid is energized through
the defrost control board causing the unit to switch
to the cooling cycle.
b. Defrost control board de-energizes 2M contactor,
causing the outdoor section fan(s) to shut down.
c. Supplemental electric heat (if installed) will be ener-
gized through defrost control board. Only that por-
tion of electric heat controlled through low voltage
control terminal 66 will be energized by the defrost
cycle. The operation of supplemental electric heat
during unit defrost will prevent cold drafts in the con-
ditioned space.
3. The defrost cycle will be terminated when:
a. the liquid temperature exceeds 55°F, or
b. 10 minutes have passed since defrost initiation.
The 10 minute cycle time (independent of liquid line
temperature) is controlled by the defrost control board.
4. At defrost termination, the unit returns to the nor-
mal heating operation.