McQuay OM 920-1
105
Operator’s Guide
Cooling: Modulating
Modulating Cooling Control: Chilled Water
When the unit's cooling type is set to chilled water and is in the Cooling operating state, or in
the dehumidification operating state the chilled water valve is modulated to maintain the
discharge air temperature at the Discharge Cooling Set Point (or Leaving Coil Temperature if
in the dehumidification operating state).
Modulating Cooling Control: Face & Bypass
When the unit's cooling type is set to Face & Bypass and is in the Cooling operating state, or
in the dehumidification operating state, the chilled water valve is driven fully open and the
face and bypass dampers are modulated to maintain the discharge air temperature at the
Discharge Cooling Set Point (or Leaving Coil Temperature if in the dehumidification
operating state).
Discharge Air Temperature Setpoint Reset - Cooling
The Cooling DAT Setpoint may be reset for units with DAT Cooling Control. The Discharge
Air Temperature Setpoint will never be set below the Minimum DAT Cooling Setpoint or
above the Maximum DAT Cooling Setpoint on the Standard Menus/Cooling/Cooling Reset
menu. The reset type may be set to one of the following:
•
None Discharge Cooling Spt = User Adjustable
•
Network Discharge Cooling Spt = Network DAT Clg Setpoint when it is valid.
•
Space Discharge Cooling Spt is based on the Space Sensor
•
Return Discharge Cooling Spt is based on the Return Air Sensor
•
OAT Discharge Cooling Spt is based on the Outdoor Air Temperature
•
Ext mA Discharge Cooling Spt is determined by a 0-20 mA signal
•
Ext V Discharge Cooling Spt is determined by a 0-10 VDC signal
•
Airflow Discharge Cooling Spt is based on the airflow as indicated by the variable
frequency drive speed
Reset reverts from Return to None when a Return Air Sensor opens or shorts. Reset reverts
from Space to None when a Space Sensor opens or shorts. Reset reverts from OAT to None
when an Outdoor Air Sensor opens or shorts.
When Space, Return, OAT, Airflow, Ext mA, or Ext V is selected, the Discharge Cooling Spt
equals the Max Clg Spt when the selected value equals the Max Clg Spt @ value. Similarly,
the Discharge Cooling Spt equals the Min Clg Spt when the selected value equals the Min Clg
Spt @ value.
When Space, Return, OAT, or Airflow is selected, it only makes sense for the DAT Cooling
setpoint to decrease as the selected temperature increases as shown in the graph below.
Figure 12: Cooling Setpoint
Max Clg Spt
Min Clg Spt
Max Clg Spt @
Min Clg Spt @