Q3 & Qht Controls for Direct Expansion Systems High-Pressure Shutdown (HI / PS)
L-2516 ENGLISH 15
FAULT AND ERROR MESSAGES
To protect the equipment, certain fault conditions trigger a shutdown, and the system will not restart until the fault is repaired.
If an operational failure occurs, the display will flash one of the following fault or error code messages. Fault code displays are
canceled by pressing the MODE button (Q3) or POWER button (Qht)..
HIGH-PRESSURE SHUTDOWN (HI / PS)
The high-pressure switch is monitored by the control board. The high-pressure switch opens when the discharge pressure rises
above a preset maximum limit. The high-pressure fault routine operates differently in the Cool and Heat Modes.
IN COOL MODE
In Cool Mode, if the high-pressure switch opens, the control board will attempt three restarts and display a three-minute
countdown between restarts before it shuts down the entire system. The display will alternately flash “HI” and “PS”. This is a
sustained shutdown, and even when the pressure lowers after shutdown, the system will remain off until reset by pressing the
MODE button (Q3) or POWER button (Qht).
IN HEAT MODE
In Heat Mode, if the high-pressure switch opens, the control board will shut down the compressor for three minutes. The display
will alternately flash “HI” and “PS” while the fan continues to run, allowing the heat in the coil to dissipate. This prepares the
system for recycling in the Heat Mode. The compressor will then continue to cycle, based on input from the high-pressure
switch, until the cabin temperature reaches set point, after which compressor cycling is automatically restored to normal
operation.
LOW-PRESSURE SHUTDOWN (LO / PS)
When the optional low-pressure switch is installed, it is monitored by the control board. The low-pressure switch opens when
the suction pressure drops below a preset minimum limit. The low-pressure fault routine operates differently in the Cool and
Heat Modes.
IN COOL MODE
In Cool Mode, if the low-pressure switch opens, the unit will first run for two minutes then shut down for 50 seconds. It will do
this four times. If the switch has not closed, the unit will shut down for 15 minutes, and the display will alternately flash “LO” and
“PS”. After 15 minutes of shut down, the cycle starts again, in which the unit runs for two minutes and then shuts down for 50
seconds.
If, after 8 attempted compressor starts, the low-pressure switch does not stay closed, the unit will go into a sustained shutdown
and the display will alternately flash “LO” and “PS”.
If the low-pressure switch closes at any time before the sustained shutdown, the unit will then operate normally.
IN HEAT MODE
In Heat Mode, if the low-pressure switch opens, the fan will automatically change to low speed to try and raise system pressure.
It will run for 11 minutes in this mode. Note that the fan speed can not be adjusted during this time. Any attempt to raise fan
speed will result the display alternately flashing “LO” and “PS” while the unit continues to run.
After 11 minutes, the unit will run for two minutes, then shut down for 50 seconds. It will do this four times. If the switch has not
closed, the unit will shut down for 15 minutes, and display will alternately flash “LO” and “PS”. After 15 minutes of shut down, the
cycle starts again, in which the unit runs for two minutes and then shuts down for 50 seconds.
If, after 8 attempted compressor starts, the low-pressure switch does not stay closed, the unit will go into a sustained shutdown
and the display will alternately flash “LO” and “PS”.
If the low-pressure switch closes at any time before the sustained shutdown, the unit will then operate normally.
LOW-VOLTAGE SHUTDOWN (LO / AC)
The low-voltage protection feature is always active. If AC line voltage drops and remains below the limit (90 volts for a 100V
system, 100 volts for a 115V system, or 200 volts for a 230V system) for more than three minutes, the control shuts down the
entire system. The display will alternately flash “LO” and “AC”. This is a sustained shutdown, and the system will not resume
operation even if the line voltage returns to normal levels. To reset, press the MODE button (Q3) or POWER button (Qht).
For the low-voltage shutdown function to work properly, the control board internal voltmeter should be calibrated. This should be
done when the system is installed. To check or recalibrate line voltage, see “8: AC Line Voltage Calibration” on page 10.
NOTE
Your system must be equipped with the optional high-pressure switch and low-pressure switch for the high- and low-
pressure shutdown to operate. Check with your dealer to make sure these important protective devices are installed
properly.
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