OM 780-2 Page 115
Sequencing pump logic acts as though the loop differential pressure is extremely high (1035). Therefore, stage-down
control is based on time only, and thus the pump set quickly stages down to stage 1.
• Variable speed pump control
The variable speed pump control process acts as though the loop differential pressure is extremely high (1035).
Therefore, it quickly reduces the pump speed to 0%.
• Chilled water loop bypass valve control
The loop bypass valve control process acts as though the loop differential pressure is extremely high (1035).
Therefore, it quickly increases the valve position to 100% (full bypass).
If the Spare Sensor Type = Second Loop Diff Pressure Sensor and two loop differential pressure sensors are used for
modulation control of the pressure across the cooling loads, the sensor with the greatest deviation from setpoint is used as
the controlled variable. When the spare sensor input is used for the second loop differential pressure input and the first
loop differential pressure sensor is in alarm, the second sensor is used exclusively as the controlled variable and all above
features are not affected. If both sensors are in alarm all above features are affected as stated.
When the alarm condition is gone, the alarm automatically clears
If a loop differential pressure sensor is not connected to the CSM, leave the Loop Differential Pressure Sensor Present Flag
= “No” (I/O Config screen) so that the alarm is inhibited from changing.
Relative Humidity Sensor Fail
If the conditioned analog signal from the relative humidity sensor falls below 1 Vdc (< 2 mA) or above 10 Vdc (> 20 mA),
the Relative Humidity Sensor Fail alarm occurs. As a result, the system continues to operate, but the following features are
affected:
• Cooling tower Stage 1 Setpoint Reset based on Constant Approach to Wet Bulb
The constant approach reset function is disabled and the Operator Defined (or BAS reset input) Stage 1 Setpoint are
used.
• Dew Point, Wet Bulb and Enthalpy display values are invalid
When the alarm condition is gone, the alarm automatically clears.
If a Relative Humidity sensor is not connected to the CSM, leave the Relative Humidity Sensor Present Flag = No (I/O
Config screen) so that the alarm is inhibited from changing.
Spare Sensor Fail
If the conditioned analog signal from the spare sensor falls below 1 Vdc (< 2 mA), the Spare Sensor Fail alarm occurs. As a
result, the system continues to operate, but the following features are affected:
• If the Spare Sensor Type = Second Loop Diff Pressure Sensor
When two loop differential pressure sensors are used for modulation control of the pressure across the cooling loads,
the sensor with the greatest deviation from setpoint is used as the controlled variable. When the spare sensor input is
used for the second loop differential pressure input and this input is in alarm, the first loop differential pressure sensor
is used exclusively as the controlled variable.
If both sensors fail, all features listed in the Loop Differential Pressure Sensor Fail alarm are affected.
When the alarm condition is gone, the alarm automatically clears.
If a Spare sensor is not connected to the CSM, leave the Spare Sensor Type = None (I/O Config screen) so that the alarm is
inhibited from changing.
Chiller Availability Greater Than One
The maximum possible value for any chiller’s availability (Misc screen) is “1”, which means the chiller is capable of
providing 100% of its cooling capacity. The Chiller Availability Greater Than One alarm occurs if the CSM calculates the
chiller availability to be greater than one for any chiller. Since chiller availability is calculated by dividing a signal from
each compressor on a chiller by the chiller’s Number of Compressors variable, a value greater than one typically means the
Number of Compressors variable has been entered incorrectly for a chiller. Go to the Chiller Setup screen and check the
Number of Compressors variable matches the physical chiller’s number of compressors.
If any chiller availability value is greater than zero, the CSM will overestimate that chiller’s cooling capacity and
sequencing logic will be affected.
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