Instruction manual

McQuay OM 920 89
Operator’s Guide
The Water Regulating Valve is driven closed in all states except the Fan Only, Economizer,
and Mechanical Cooling states, and during Dehumidification.
Before the unit will transition from the Fan Only to Cooling operating state when mechanical
cooling is required, the water regulating valve is opened for an adjustable minimum to allow
time for water to flow through the condenser before a compressor starts. This occurs when
ALL of the following conditions are true:
• Cooling status is enabled or off ambient
• Airside economizer operation is disabled or not installed
• The Control Temperature is calling for cooling operation. This occurs when the Control
Temperature is above the Zone Cooling Setpoint by more than the Zone Cooling Deadband.
When the unit is equipped with and airside economizer and before the unit will transition from
the Econo to Cooling operating state when mechanical cooling is required, the water
regulating valve start up start up sequence is initiated to allow time for water to flow through
the condenser before a compressor starts. This occurs when the following conditions are true:
• Airside economizer position is equal to or greater than 95% open for longer than the cooling
stage timer
• The discharge air temperature is warm is above the Discharge Cooling Setpoint by more
than half the Discharge Cooling Deadband for longer than the cooling stage timer
OR
• Cooling is enabled AND
• Economizer is disabled
Special Procedures for Units with WRV and More Than Two Cir-
cuits.
• When the unit enters Cooling, either Compressor # 1 or # 2 must start first if one or both of
them are enabled because the pressure sensors are on circuits # 1 and # 2
• If both compressor # 1 and #2 are disabled due to High Pressure or Low Pressure alarms, the
cooling stage is set to stage 2 when the unit enters Cooling and, the appropriate compressor
(# 3 or higher) is turned on
• If both Compressor # 1 and Compressor # 2 are disabled, The WRV will be controlled based
on the EWT and the InitPos formula above
• Control of the WRV reverts to normal PI control if either Compressor # 1 or Compressor # 2
is enabled and turned on
• All compressors are disabled if EWT drops below the MinWRV@ value by more than the
OATClgLkDiff. Compressors are re-enabled if EWT rises above the MinWRV@Tmp value