Instruction manual

90 McQuay OM 920
Operator’s Guide
Water Pump Control
The Pump output is in the on position if:
• The Bypass Valve output is being driven Open
OR
• The Water Regulating Valve output is being driven Open
OR
• A waterside economizer is installed AND
• The unit is in the Economizer state
OR
• The unit is the Mechanical Cooling state
OR
• The Unit is in the Start Initial state AND
• Flush Mode is set to Yes
OR
• The unit has a waterside economizer and a Freeze Fault or Freeze Problem is active
OR
• The unit has a waterside economizer and 10 minutes have not yet passed after a Freeze Fault
or Freeze Problem has disappeared
Cooling: Multistage
Compressor Staging
The staging sequences named Std-1 through Std-6 or Alt-1 through Alt-6 in the tables below
indicate what compressors will be on when cooling stage is set to each stage. A single staging
sequence will be active at any one time. The method for deciding which staging sequence is
active is described for each compressor configuration below.
Two Unequal Sized Compressors, Two Circuits, Three Stages
Only one sequence is provided so the user makes no selections.
•
Total cooling stages is 3
• If neither circuit is disabled, Maximum Cooling stages is set to 3.
• If circuit 2 is disabled, Maximum Cooling stages is set to 1.
• If circuit 1 is disabled, Maximum Cooling stages is set to 2.
• If circuit is disabled, it is not re-enabled until the stage timer clears.
Two Small Comps on Circuit # 1, One Large Comp on Circuit # 2, Four Stages
Only one sequence is provided so the user makes no selections.
• Total cooling stages is set to 4.
• If neither circuit is disabled, the staging sequence is Std-1 if compressor # 1 has fewer hours
than compressor # 3, and the staging sequence is Std-2 if compressor # 1 does not have
fewer hours than compressor # 3
• Maximum cooling stages is set to 4.
Table 53: Standard Staging
Standard Staging
Staging Sequence Stage 1 Compressor Stage 2 Compressors Stage 3 Compressors
Std-1 1 2 1,2
StgdClgCap 33% 67% 100%