Liebert 50 & 60Hz - Manual (Page 46)

Microprocessor Control
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3.10.2 C/F Degrees
The control may be selected to show readings and setpoints in either degrees Fahrenheit (°F) or in
degrees Celsius (°C). To change the value use ENTER to select this function, then use the UP/DOWN
arrow to change the value. Press ENTER to store the value.
3.10.3 Humidity Control Method
The operator may select either relative (direct) or absolute (predictive) humidity control. If “relative”
is selected, the RH control is taken directly from the RH sensor. If “absolute” is selected, the RH con-
trol is automatically adjusted whenever return air temperature deviates from the desired tempera-
ture setpoint (i.e., predictive humidity control). The LCD display will indicate percentage relative
humidity for both methods of control. If the “absolute” feature is selected, the adjusted humidity read-
ing will also be shown. When utilizing the predictive humidity control feature, the humidity level is
automatically adjusted ~2% RH for each degree difference between the return air temperature and
the temperature setpoint.
Unnecessary dehumidification can result when overcooling occurs during a dehumidification cycle.
This is due to a higher than normal RH reading caused by overcooling the room (about 2% RH for
each degree of overcooling). This drop in temperature extends the dehumidification cycle. Later, when
the dehumidification ends and the temperature rises to the setpoint, the RH reading falls. The final
RH reading will then be lower than actually desired. If the temperature drop was significant enough,
the percentage RH could be low enough to activate the humidifier.
If the absolute humidity control is selected, over-dehumidification may be avoided. When overcooling
occurs (i.e., causing an increase in the RH reading) the humidity control program estimates what the
RH will be when the dehumidification cycle ends and temperature returns to the setpoint. This allows
the dehumidification cycle to end at the proper time. Predictive humidity control can greatly reduce
energy consumption by minimizing both compressor/reheat operation. Use the UP/DOWN ARROW
key to select the desired humidity control method.
3.10.4 Lead Compressor
This function allows the user to select which compressor is the lead: Compressor 1 (3 ton), Compres-
sor 2 (5 ton), or Auto. The factory default is Auto. If Auto is selected, the control will determine which
compressor is to be the lead compressor based on the average room load over the previous one hour of
operation.
3.10.5 Show DIP Switch
This function shows the position of the DIP switches which are located on the control board in the
unit. 1 = Switch is “ON” and 0 = Switch is “OFF”. For more information on the DIP switches and their
functions, see Table 17 Equipment Switch Settings.
3.10.6 Valve Time (For Systems With a Modulating Chilled-Water Valve)
This function shows the full valve travel time of the modulating valve on a chilled-water system. This
is the time it takes for the valve to travel from full closed to full open. It is programmable from 50 to
250 seconds; factory default time is 165 seconds and should not be changed unless actual valve travel
time is not correct. The full valve travel time is used by the control to determine the appropriate valve
position. For example, if the valve travel time is 165 seconds and 50% cooling is being called for, the
valve will open for 83 seconds to achieve 50% open.