2. Turn the minimum outside air adjuster to the 0% outside air
position. All outside air damper blades should be fully
closed. The linkage connecting the outside air and the
return air dampers must move freely.
3. Return the minimum outside air adjuster to the 100%
outside air position and check for complete freedom of
linkage movement as the return air dampers close.
4. Set the minimum outside air adjuster for the minimum
ventilation requirement of the job.
HEATING SYSTEM
Two different types of heat are offered as factory-mounted
options. Refer to the appropriate instruction for information on
the heating system.
•
530.25-N1.1 - Gas-Fired Heating Option
•
530.25-N1.9 - Electric Heating Option
COOLING SYSTEM
These units have nominal cooling capacities of 30 and 40 tons.
The DSC360 units have one semi-hermetic compressor. Two
cooling stages are accomplished by unloading the compressor.
The DSC480 unit has two semi-hermetic compressors. Two
cooling stages are accomplished by cycling the individual
compressors.
Normal sequence of operation is as follows:
1. Power is supplied to the unit through the disconnect switch.
The switch can be field installed or factory-installed as an
option.
2. As soon as power is supplied to the unit, the compressor
crankcase heaters 1TH (and 2TH) will be energized.
CAUTION: Do not attempt to start the compressors without
at least 8 hours of crankcase heat or compres-
sor damage will occur.
3. The system switch on the room thermostat or the status
panel must be in the AUTO or COOL position to complete
the circuit between the terminals 119 and 103/RC.
4. On demand of the room thermostat for cooling, contact TC1
closes and energizes 1R coil.
5. An internal connection in the thermostat or status panel
energizes 7R coil. 7R contact closes energizing 3M coil. 3M
contacts close, starting the supply air fan motor. After an
adequate air flow is established, 5 LP closes to energize
6R coil. 6R contact closes in the compressor control circuit.
NOTE: The above operation is for intermittent fan operation.
For continuous operation, the fan switch on the room
thermostat or the status panel should be manually
closed to energize 7R.
6. Relay contact 1R closes and energizes 1M coil, which
starts the No. 1 compressor. At the same time, 4M coil is
energized to start the condenser fans.
7. If the room temperature continues to rise, contact TC2
closes and energizes 2R coil.
8. On DSC480 units the 2R contacts close, energizing the 2M
coil which starts the No. 2 compressor or on DSC360 units
the 2R contacts close de-energizing the compressor
unloading solenoid which loads the compressor.
9. The compressor short cycle time and low voltage protection
is part of the solid state compressor protection module. It
prevents a compressor from starting unless it has been off
for five minutes. It also monitors the voltage of the 120 volt
control circuit and will shut down the compressor if the 120
volt control voltage drops below 85 æ4.5 volts.
10. To maintain sufficient head pressure during low ambient
operation, condenser fans No. 1 and No. 2 will be
de-energized by 1TH. Condenser fans No. 3 and 4 will be
de-energized by 2TH (see Table 6).
EXHAUST AIR SYSTEM
The exhaust air fans are energized by manually closing a circuit
between terminals 60 and 64 of terminal block 2TB or
automatically by an end switch on the economizer damper
motor. As the economizer damper opens, it closes the end
switch on the damper motor and turns on the exhaust air fans.
The degree of blade opening at which the exhaust fans are
energized can be adjusted by the following procedure.
1. Use the black scale plate as an indicator of motor position
for switch operation.
2. Loosen the thumb nut and the cam locking screws.
3. Set the operational and differential cams.
4. Check switch adjustment by moving the adjuster 2RH to
move the motor. Switch should click when desired make
and break points are lined up with index mark and the
exhaust fan should start and stop.
WARNING: 120 volt power is connected to the switch.
5. Tighten thumb nut.
530.25-N3Y
14
Unitary Products Group
