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Chapter 4: System Operation
Sequence of Operation
Cooling and Heating
All units in the COM4T product line are equipped with variable speed compressors that can vary its
frequency to meet capacity demand. The compressor speed will continuously modulate depending on
return temperature and cooling/heating stage. When stage 1 command is received on Y1, the system
will start the compressor at medium speed and slowly modulate between low to medium speed until a
stable return temperature is found. When stage 2 command is received on Y2, the system will increase
its speed and modulate between medium and high speed until a stable return temperature is found.
Compressor protection
After supplying power to the unit and after compressor has stopped, the compressor will remain off
for 3 minutes to prevent short cycling. Low and high pressure alarms, if triggered 3 or more times in
an hour, will prevent the compressor from running for 12 hours or until power is reset. A supply fan
alarm, if triggered 5 or more times in an hour, will prevent the compressor from running for 12 hours
or until power reset.
Aux/Emergency Heating
If auxiliary heaters are installed, they are triggered by W2 command.
Supply Fan (Indoor Blower)
The system will engage indoor blower when G command is received. If cool/heat command is received
without G, the supply fan will also engage.
Defrost
Frost can build up on outdoor coil when the heat pump is operating in low temperatures and can
adversely affect rate of heat transfer. If conditions are met, the automatic defrost function will engage
to prevent frost buildup.
When the heating mode has been running for at least 4 minutes AND coil temperature is below 23°F
for at least 10 minutes AND coil temperature is lower than outdoor air temperature by at least 15°F,
defrost will engage. When defrost mode starts, the reversing valve is disengaged, and compressor
runs at high speed to heat the outdoor coil. Defrost will disengage immediately after coil temperature
reaches 64°F OR coil temperature stays above 50°F for 2 minutes OR continuous defrost for 10
minutes.
Dehumidification (optional)
When dehumidification is triggered on the
dehum command, the EEV and compressor will automatic
adjust to maximum latent (moisture removal) capacity. The system can seamlessly transition between
cooling/heating and dehumidification without restarting the system.