Refrigeration Maintenance
134
2. Set the sepoints in all zones well below the
zone temperatures so that all zones will be in
High Speed Cool.
3. Raise the discharge pressure of the
compressor by blocking the condenser coil air
flow by covering the roadside condenser grille
with a piece of cardboard.
NOTE: The discharge pressure should never
be allowed to exceed a pressure of 470 psig
(3241 kPa) on R-404A systems.
4. Failure of the HPCO system to stop
compressor operation should be investigated
first by checking the control circuit operation
and secondly by HPCO switch replacement.
Low Side Pump Down
NOTE: Operate the unit with all zones in COOL
for 2 to 5 minutes before performing the low side
pump down.
1. Unplug the hot gas bypass solenoid.
2. Install a gauge manifold to the suction service
valve and the discharge service port. A low
loss fitting must be used on the hose
connected to the discharge service port.
3. Operate the unit with all zones in low speed
cool.
4. Front seat the receiver tank outlet valve and
allow the low side to pump down to 0 to 5 in.
Hg vacuum (0 to -17 kPa).
5. Turn the unit Off.
6. Prepare to perform service on the low side by
equalizing the high side and low side
pressures through the gauge manifold.
7. Equalize low side pressure to 1 to 2 psig (7 to
14 kPa).
NOTE: Repeat the pump down procedure if
the pressures equalize above 20 psig
(138 kPa). If acceptable low pressures
cannot be achieved after the third pump
down, the refrigerant must be recovered to
perform service on the low side.
NOTE: Valve stem MUST be back seated
during normal unit operation.
8. Plug in the hot gas bypass solenoid.
9. Open the receiver tank return outlet valve,
remove the gauges and return the unit to
normal operation.
Discharge Pressure Regulator
Valve
This unit is equipped with a discharge pressure
regulator valve, which is located in the hot gas
line between the hot gas valves and the discharge
line. The discharge pressure regulator valve
maintains the compressor discharge pressure at
340 to 400 psig (2344 to 2758 kPa) during heat
and defrost operation. These higher discharge
pressures create higher discharge temperatures,
resulting in enhanced heating capabilities.
Figure 161: Discharge Pressure Regulator
The compressor discharge pressure is regulated by
a spring and bellows. This valve is pre-set at the
factory to maintain the compressor discharge
pressure at 340 to 400 psig (2344 to 2758 kPa)
during heat and defrost operation. This valve
should not require any adjustment.
CAUTION: Do not pull scroll compressor
into a deep vacuum to perform routine
maintenance procedures.
CAUTION: Do not run scroll compressor
in a vacuum for more than 1 minute.
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