OM 780-2 Page 113
Decoupler Flow Meter Fail
If the conditioned analog signal from the flow rate sensor used to determine flow through the decoupler line falls below 1
Vdc (or the current signal from the flow meter falls below 2 mA), the decoupler flow meter fail alarm occurs. As a result,
the system continues to operate, but the decoupled sequencing logic is modified to eliminate excess primary water flow as a
stage-down precondition. Thus stage-down control is based on the Active Capacity of the Next-OFF chiller and time only.
This may affect the system in the following way:
• When the secondary loop demand for flow exceeds the primary loop flow, the decoupler line temperature sensor will
detect flow going backwards through the decoupler line and the CSM will enable another chiller to increase the
primary water flow. If the capacity of this newly enabled chiller is not required the CSM may quickly disable this
chiller, which turns off the evaporator pump. This again allows flow to travel backwards in the decoupler line raising
the supply water temperature.
• The flow meter is used to make sure that the primary water flow through the decoupler line is greater that the primary
water flow that will be lost when the Next-OFF chiller is disabled. If this flow sensor fails, one of the chiller
evaporator pumps may short-cycle. A properly operating flow meter will keep this chiller running (so that its
evaporator pumps will run) until the secondary loop demand for flow decreases.
When the alarm condition is gone, the alarm automatically clears.
If a flow meter is not connected to the CSM, leave the Flow Meter Present Flag = No (I/O Config screen) so that the alarm
is inhibited from changing.
Cooling Load Pump #X Fail (x could be pump 1, 2, 3, 4, 5, or 6)
If the CSM enables pump x and pump
x
fails, the cooling load pump #
x
fail alarm occurs, where
x
is the pump number. If
there are two or more cooling load pumps the CSM attempts to start another pump, and the system continues to operate. A
pump is considered “failed” if its status digital input is open while its digital output is closed for any continuous period of
time equal to the Pump Status Check Delay Time variable (Load Flow Control screen). The alarm must be manually
cleared at the user interface. If the Pump Control Option = “Sequencing” and a pump that failed returns to operation, the
alarm will clear automatically.
Outdoor Air Temperature Sensor Fail
If an outdoor air temperature sensor is connected to the CSM and it fails, the outdoor air temp sensor fail alarm occurs. As
a result, the system continues to operate, but the following features are affected:
• Low ambient lockout
The low ambient lockout feature acts as though the OAT is extremely high (120 F). Therefore, low ambient lockout
never occurs.
• Optimal start
The optimal start feature acts as though the OAT is extremely high (100°F, 38°C) which likely results in an earlier
start-up time.
• Chilled water reset based on OAT
The Outdoor Air reset function acts as though the OAT is extremely high. Therefore, it sets the System Setpoint equal
to the Minimum System Setpoint.
• 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.
• Cooling tower bypass valve initial position
The initial bypass valve position function acts as though the OAT is extremely high. Therefore, it sets the initial valve
position equal to the Maximum Tower Valve Start-Up Position variable.
When the alarm condition is gone, the alarm automatically clears.
If an OAT value is not supplied to the CSM, the operator should disable this alarm by setting the Outdoor Air Temp Source
= None (I/O Config screen).
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