Liebert 50 & 60Hz User Manual

The humidity control for the MM2 is based on a calculated percent requirement for dehumidification 

or humidification. The percent requirement is calculated from the difference between the sensor read-

ing and the humidity setpoint, divided by the sensitivity. The control method is selectable between 

relative and absolute. Relative humidity control is the default.

For the 8-ton unit operation, 1 or 2 stage dehumidification is selected through DIP switch #7. For 2 

stages, the 5-ton compressor is the lead compressor when dehumidifying. Dehumidification will be 

staged on at 50 and 100% call. The electric reheats are turned on at 25% cooling (reheat 1) and 25% 

heating (reheat 2) for improved temperature control. The reheats are deactivated at 0% and 50% cool-

ing requirement, respectively. If overcooling occurs, the first stage of dehumidification is disabled at 

125% call for heating. If 1 stage or 2 stage is selected, dehumidification is disabled at 200% call for 

heating. Dehumidification is re-enabled at 66% call for 2 stage and 33% call for 1 stage. For optional 

hot water heating, the valve is energized at its normal point, 100% heating requirement.

The humidifier is activated when the humidity control calculates a requirement for 100% humidifica-

tion, and is deactivated when the requirement falls below 50%.

Dehumidification is locked out if overcooling occurs. Dehumidification on the 8-ton unit is disabled at 

125% (first stage) and 200% (all stages) heating requirement. Dehumidification is re-enabled at 66% 

and 33% heating requirement.

The control system monitors the compressor and prevents it from turning on within a 3-minute period 

of being off. If this on-off cycle occurs to often (e.g. 10 times within a one hour period) a Short Cycle 

Alarm will occur.

The control system uses a two-wire, RS-422 channel to communicate with Liebert Site Products via a 

proprietary protocol. A converter board (ECA2) is available to allow communications with a “dumb” 

terminal or a computer using RS-232 channel. More details are provided in the Site Products and 

ECA2 User Manual.
The communications channel provides both monitoring and control options, including:

• TEMPERATURE/HUMIDITY: Current temperature and humidity readings.
• STATUS (%), Cooling/heating and humidify/dehumidify operating status. 
• PRESENT ALARMS: Alarms currently activated.
• SETPOINTS: 

• Temperature Setpoint
• Temperature Sensitivity
• Humidity  Setpoint
• Humidity Sensitivity
• High Temperature Alarm
• Low Temperature Alarm
• High Humidity Alarm
• Low Humidity Alarm

• ON/OFF STATUS and CONTROL
• SILENCE ALARM