Trane Tracer ZN.520

CNT-SVX04A-EN
Binary inputs
30
Defrost
For direct expansion (DX) compressor cooling applications, a defrost-sensing
device is placed in the coil to indicate and help prevent possible frost
conditions. The defrost-sensing device disables the Tracer ZN.520 zone
controller compressor operation when a frost condition exists by opening the
controller’s triac compressor output circuit. The compressor cannot run
because the open circuit prevents the compressor relay from energizing. The
Tracer ZN.520 zone controller control senses the open triac circuit and
commands the triac output off. Fan and economizer damper operation continue
to operate, defrosting the coil. The control reports a Defrosting—Cmpr Lockout
informational diagnostic when the compressor is disabled by the defrost
condition.
The defrost-sensing device automatically resets when the coil temperature
returns to a normal value, closing the compressor triac output circuit. The
Tracer ZN.520 zone controller control senses the defrost device reset and
immediately returns to normal cooling control, clearing the defrosting and
compressor lockout status, and sending out a Normal diagnostic message. The
minimum off time for DX compressors is obeyed, so the compressor may not
turn on immediately when the defrost condition ends.