Product data

the 38HDS microprocessor if the other refrigerant circuit is still operating. The outdoor fan only stops when both
compressors are off. Refer to 38HDS Microprocessor Control of Outdoor Fan section below for more details.
38HDS-microprocessor control of outdoor fan — The microprocessor control has a built-in head pressure control
system that cycles the outdoor-fan motor to maintain a selected discharge pressure. The microprocessor senses
outdoor ambient temperature using a thermistor and refrigerant pressure using a pressure transducer mounted on the
compressor circuit discharge line. The 38HDS024 units have one transducer, and the 048 units have two. The
factory set points for the head pressure control operation are 55 F for outdoor ambient temperature and 250 psig for
discharge pressure. When the outdoor ambient temperature is above the out-door ambient set point and both fan
coils on a compressor circuit are calling for cooling operation, the outdoor fan runs at full speed whenever
compressor operation is permitted.
When only one fan coil is calling for cooling, fan cycling is permitted at all ambients. Fan motor is then cycled on
and off to maintain set point discharge pressure. When the outdoor ambient temperature is below the specified set
point, the fan motor is cycled on and off to maintain a compressor discharge pressure at the specified pressure set
point. On 38HDS048 units with 2 compressors running, the compressor with the lower head pressure controls the
fan operation. If only one compressor is running, that compressor controls the fan operation. If the discharge
pressure on either circuit exceeds 370 psig, the outdoor-fan motor runs continuously, and the fan cycling function is
bypassed until the discharge pressure decreases to 365 psig. Units can be field configured to select head pressure
control at 35 F outdoor ambient and/or 200 psig discharge pressure.
Status indicator lights — The 38HDS control board is equipped with LED indicators to aid in evaluating the status
of the control system, including:
• Time delay function status
• Fan coil unit demand status
• Head pressure control status
• Current unit malfunctions
Also see System Status Red and Green LED’s and Fault Codes in Section #27
Unit malfunction — Each compressor circuit is equipped with a high-pressure switch (HPS), a low-pressure switch
(LPS), and a discharge temperature switch (DTS). These safety devices are located in a Cycle-LOC™ device circuit
that prevents compressor operation if any of these safety devices is activated. The lockout can be reset by turning the
main power to the 38HDS unit off, then on again.
Compressor overcurrent protection is achieved by an internal line break overload, which automatically resets when
the motor temperature cools to a satisfactory level. Manual reset of Cycle-LOC circuit may also be required.
Pressure transducers and the outdoor ambient thermistor are monitored by the 38HDS controller. If a pressure
transducer is found to be out of range, the LED’s flash a code for this fault, but the control continues to cycle the
outdoor fan according to input from the other transducer. If only one transducer is active and is determined to be out
of range, the head pressure control is bypassed and the fan runs continuously. If the thermistor is found to be out of
range, the control allows fan cycling per transducer input, and the LED’s flash a code for this failure. See Trouble
shooting Section #27 for System Status Red and Green LED’s and Fault Codes table. (MORE)