Cooling Operation, Heating Operation

035-15410-002-B-0404

OPERATION

!8GENERAL'?!

CAUTION: Reversing valves and check valves are precise mechanical devices and will not tolerate any me- chanical abuse such as hammering. If a refrigerant system isn't properly cleaned after a compressor burn-out, scale may build up at these devices and prevent them from operating properly.

SYSTEM(SEQUENCEOFOPERATION

COOLING OPERATION

The following controls will be energized through terminal O on the thermostat to put the system in the cooling mode.

• The reversing valve solenoid (7-1/2 Ton units)

"• Relay 2R and the reversing valve solenoid (10 Ton units)

If the fan switch on the thermostat is in the “ON” position, in- door section blower motor contactor 10M will be energized through terminal G to provide continuous blower operation. If the switch is in the “AUTO” position, the blower will oper- ate only when the thermostat calls for cooling operation.

3.When TC1 of the thermostat closes on a demand for cool- ing, a circuit is made from terminal Y through the defrost control board and safety switches to energize contactor 1M and relay 1R, starting the compressor. Contactor 2M is en- ergized through the NO contacts of 1R-2 and the NC con- tacts of K2 on the defrost control board in order to start the outdoor fan motor(s).

NOTE: On 10 Ton units, both outdoor fan motors will oper- ate if the ambient temperature is above 65 F. If the ambient temperature falls below 65 F, control 3TH will shut down the No. 1 fan motor.

4.Relay 1R also prevents the 10KW portion of accessory electric heat referenced as standby electric heat from being utilized whenever the compressor is in operation. This part of the circuit is covered under HEATING OPERATION.

5.The thermostat will cycle the unit to satisfy the cooling re- quirements of the conditioned space.

6.After the unit has shutdown from a cooling cycle or a power interruption, the anti-short cycle feature of the defrost con- trol board will not permit the unit to restart for 5 minutes. This feature prevents the unit from short cycling.

7.If the discharge pressure reaches 430 psig, the HP control will open and the defrost control board will lock out the com- pressor. If the discharge temperature reaches 255°F, 2TH thermostat will open and the defrost control board will lock out the compressor. If the suction pressure falls to 7 psig, the low pressure switch will open and the defrost control will lock out the compressor.

8.If the control that caused the lockout has automatically re- set, 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 “COOLING” position.

b. Increasing the set point on the thermostat above the temperature in the conditioned space and then return- ing it to its original setting.

c.Opening and closing the power supply main disconnect switch.

IN ALL THREE RESET METHODS DESCRIBED ABOVE, A FIVE MINUTE TIME DELAY WILL TAKE PLACE AFTER THE RESET BEFORE THE UNIT WILL RESTART.

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, in- door section blower motor contactor 10M will be energized through terminal G to provide continuous blower operation. If the switch is in “AUTO” position, the blower will operate only when thermostat calls for heating operation.

3.When 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 and start the compressor. Contactor 2M is energized through the NO contacts 1R-2 and the NC contacts of K2 on the de- frost control board which will start the outdoor fan motor(s).

NOTE: On 10 Ton units, if 3TH thermostat opens due to outdoor ambient temperature being below 65 F, the No. 1 fan motor will continue to operate through the NC contacts of relay 2R.

4.The thermostat will cycle the unit to satisfy the heating re- quirements 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 con- trol 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 com- pressor. If the discharge temperature reaches 255°F, 2TH thermostat will open and the defrost control board will lock out the compressor. 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 automatically re- set, 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 return- ing it to its original setting.

c.Opening and closing the power supply main disconnect switch.

Unitary Products Group