Instruction manual

McQuay OM 920 91
Operator’s Guide
• If Circuit #1 is disabled, the staging sequence is set to Alt-1 after the stage timer, unless
Comp #2 is already on. Then the switch to Alt-1 happens immediately. Maximum cooling
stages is set to 1.
• If Circuit #2 is disabled, the staging sequence is set to Std-1 if comp #1 has fewer hours than
comp #3, and the staging sequence is set to Std-2 if comp #1 does not have fewer hours than
comp #3, after the stage time. Maximum cooling stages is set to 2.
• Any comp on a disabled circuit stays off if it is already off and is turned off if it is on.
• The staging sequence is changed based on Run Time only when the number of stages stage
is zero or Maximum cooling stages. This means that disabling and re-enabling the
compressors or turning the unit off and on may be necessary to cause sequence changes.
• Alt-1 is used if circuit # 1 is disabled.
• If a circuit is disabled, Maximum cooling stages is set to 2.
Two Equal Sized Compressors, Two Circuits
The user selects the following:
• Lead Circuit = Auto, # 1 or # 2
• A separate Alternate circuit is not provided for this configuration
•
Total cooling stages = 2
• If Lead Circuit is set to Auto and neither circuit is disabled, the compressor
sequence is Std-1 if compressor # 1 has fewer hours than compressor # 2,
and the staging sequence is Std-2 if compressor # 1 does not have fewer
hours than compressor # 2. Maximum cooling stages is set to 2.
• If Lead Circuit is set to # 1 or Circuit # 2 is disabled, sequence Std-1 is used.
• If Lead Circuit is set to # 2 or Circuit # 1 is disabled, sequence Std-2 is used.
• The Compressor Sequence changes only when the compressor stage is
Maximum cooling stages or zero. This means that disabling and re-enabling
the compressors or turning the unit off and on may be necessary to cause
sequence changes.
• If a circuit is disabled, Maximum cooling is set to one. The compressor on
the disabled circuit stays off if it is already off and is turned off if it is on.
Table 54: Standard Staging
Standard Staging
Staging Sequence Stage 1
Compressor
Stage 2
Compressors
Stage 3
Compressors
Stage 4
Compressors
Std-1 1 1,3 2,3 1,2,3
Std-2 3 3,1 1,2 1,2,3
StgdClgCap 25% 50% 75% 100%
Table 55: Alternate Staging
Standard Staging
Staging Sequence Stage 1
Compressor
Stage 2
Compressors
Stage 3
Compressors
Stage 4
Compressors
Alt-1 0 2 1,2 1,2,3
StgdClgCap 0% 50% 75% 100%
Table 56: Standard Staging
Standard Staging
Staging Sequence Stage 1 Compressor Stage 2 Compressors
Std-1 1 1,2
Std-2 2 1,2
StgdClgCap 50% 100%