Unit installation

IOMM ACZ1-2 ACZ 010A through 039A 35
Low Ambient Lockout
If the OAT drops below the low ambient lockout setpoint, the unit will do a normal stop.
Once the lockout has been triggered, no compressors will start until the OAT rises to the
lockout setpoint plus 5°F.
Compressor Control
Compressor Available
A compressor is available to start when the following are true:
• Unit state = auto
• Evap state = run
• Low OAT lockout is not active
• Power start delay is expired
• No limit events active
• No cycle timers active for the compressor
• Compressor enable setpoint = On
Compressor Start/Stop Timing
This section determines when to start or stop a compressor. There are two separate
functions used, one for staging up and one for staging down.
Stage Up Now
The Stage Up Now flag is set based on the following tests:
IF Stage Request > Stages On AND
Stage up timer expired THEN
Stage Up Now = True
Stage Down Now
The Stage Down Now flag is set based on the following tests:
IF Stage Request < Stages On AND
Stage down timer expired THEN
Stage Down Now = True
Compressor Sequencing
Compressor staging is primarily based on compressor run-hours and starts. Compressors
that have fewer starts will normally start before those with more starts. Compressors that
have more run hours will normally shut off before those with fewer run hours. In the event
of a tie on number of starts, the lower numbered compressor will start first. In the event of
a tie on run-hours, the lower numbered compressor will shut off first. Run-hours will be
compared in terms of tens of hours.
Next On = 1 if compressor 1 starts <= compressor 2 starts, or compressor 2 not available
Next On = 2 if compressor 1 starts > compressor 2 starts, or compressor 1 not available
Next Off = 1 if compressor 1 run-hours > compressor 2 run-hours
Next Off = 2 if compressor 1 run-hours <= compressor 2 run-hours