Dehumidification / Humidification Percent Required; Staged Dehumidification, Compressorized Direct Expansion (Dx) Systems; Humidification Operation; Dehumidification Lockout - Liebert 8 Tons User Manual

4.4

Dehumidification / Humidification Percent Required

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.
4.4.1

Staged Dehumidification, Compressorized Direct Expansion (DX) Systems

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.
4.4.2

Humidification Operation

System Activation
The humidifier is activated when the humidity control calculates a requirement for 100% humidifica-
tion, and is deactivated when the requirement falls below 50%.
4.4.3

Dehumidification Lockout

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.
4.5

Load Control Features

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.
4.6

Communications

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
System Performance Microprocessor Controls
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