Instruction manual

McQuay OM 920 105
Operator’s Guide
When Airflow is selected, the values “Min Clg Spt @” and “Max Clg Spt @” are entered as
percentage values. When Ext mA is selected, the values “Min Clg Spt @” and “Max Clg Spt
@” are entered as mA values. When Ext VDC is selected, the values “Min Clg Spt @” and
“Max Clg Spt @” are entered as VDC values.
If Ext mA or Ext V is selected as the type of reset, the Min Clg Spt @ value may be set above
the Max Clg Spt @ value to cause a decrease in the DAT setpoint as the external signal or the
Min Clg Spt @ value may be set below the Max Clg Spt @ value to cause an increase in the
DAT setpoint as the external signal increase.
Actually, the Min Clg Spt @ value can be set below the Max Clg Spt @ value for all types of
reset, but it only makes sense for external reset.
When ever the Clg Reset Type or Engineering Units is changed, the Min Clg Spt @ and Max
Clg Spt @ values revert to default values as follows:
• None: Min Clg Spt @=0NA, Max Clg Spt @=100NA
• Network: Min Clg Spt @=0NA, Max Clg Spt @=100NA
• Space, Return: Min Clg Spt @=73.0F, Max Clg Spt @=71.0F
• OAT: Min Clg Spt @=90.0F, Max Clg Spt @=70.0F
• ExtmA: Min Clg Spt @=4.0mA, Max Clg Spt @=20.0mA
• ExtVDC: Min Clg Spt @=0.0V, Max Clg Spt @=10.0V
• Airflow: Min Clg Spt @=0%, Max Clg Spt @=100%
Condenser Fan Control
There are up to four condenser fans per circuit. The first fan on each circuit is always turned
on when any compressor on the circuit is turned on through auxiliary switches on the
compressor contactors. The second two fans on each circuit (if applicable) are controlled by
outputs from the Main Controller. These are controlled via Condenser Fan Output A (MCB-
DO7) and Condenser Fan Output B (MCB -DO8). The last fan on each circuit (if applicable)
is controller by an external refrigerant pressure switch.
Cond Fan Output A is turned on when any compressor on is on and the OAT rises above the
Condenser Fan A Set Point. Cond Fan Output A is turned off when all compressors are off or
the OAT drops below the Condenser Fan A Set Point by more than the Condenser Fan
Differential (Default = 5°F). External relays allow operation of condenser fans associated with
a specific circuit only when a compressor on that circuit is on.
Cond Fan Output B is turned on when any compressor on is on and the OAT rises above the
Condenser Fan B Set Point. Cond Fan Output B is turned off when all compressors are off or
the OAT drops below the Condenser Fan B Set Point by more than the Condenser Fan
Differential (Default = 5°F). External relays allow operation of condenser fans associated with
a specific circuit only when a compressor on that circuit is on.
Note – During the phase in/out period of R22 to R410A there will be a time period in which a third
condenser fan output will be required on R22 units. This output will be designated Cond
Fan Output C (EMA-DO3).
Cond Fan Output C is turned on when any compressor on is on and the OAT rises above the
Condenser Fan C Set Point. Cond Fan Output C is turned off when all compressors are off or
the OAT drops below the Condenser Fan C Set Point by more than the Condenser Fan
Differential (Default = 5°F). External relays allow operation of condenser fans associated with
a specific circuit only when a compressor on that circuit is on.