Johnson Controls
5586996-JSG-A-0120 | Start-Up & Operation Guide
48
The suction pressure on any compressor system
is ignored for the first 60 seconds of operation.
After the 60 second time delay expires, the unit
controller monitors the suction line pressure.
When the suction line pressure falls below the
suction line pressure low limit (80 psig) for 10 sec-
onds, the unit controller:
• Logs a low suction line pressure alarm and
disables the smallest HP compressor of that
circuit
• Logs the current OAT at the time of the suc-
tion line pressure alarm
• Recalculates the compressor staging and at-
tempts to continue supply DAT at the active
DAT cooling setpoint
This process is repeated every 60 seconds until
the suction line pressure rises above the suction
line low pressure limit or all compressors in the
circuit have been disabled.
When the final compressor running is a variable
speed drive compressor and the suction line pres-
sure is still less than the suction line low pressure
limit, the variable speed drive compressor speed
is reduced to the minimum speed for an additional
minute.
When the pressure is still less than the suction line
low pressure limit after the 60 second time delay,
the variable speed drive compressor is disabled.
To clear the low suction line pressure alarm:
• The suction line pressure must be 10 psig
greater than the suction line low pressure
limit for 10 minutes
OR
• The current OAT must be 5.0°F greater than
the OAT logged at the time of the initial low
suction line pressure alarm
When any compressor is disabled 3 times within
a 120 minute window, that compressor enters a
safety lockout and a manual rest is required.
Condenser Systems and Fans
The rooftop units use microchannel condenser coils.
The condenser coils are used to remove heat from the
refrigerant and condense the high pressure/high tem-
perature gas to a high pressure/high temperature liquid.
Ensuring that all the refrigerant is condensed to a liquid
is critical for the proper operation of the overall com-
pressor system.
The unit uses different combinations of factory installed
condenser fans, both single speed and variable speed,
to accomplish the task of condensing the refrigerant to
a liquid. The unit controller regulates single speed and
VFD controlled condenser fans based on the saturated
liquid line temperature.
Saturated Liquid Line Temperature Control
The unit controller continuously monitors the saturated
liquid line temperature of any operating compressor
system.
Upon initial start-up of a compressor system, the satu-
rated liquid line temperature is ignored for 60 seconds,
and one condenser fan is started for every compressor
that is started.
After the stabilization period expires, the unit controller
regulates the condenser fans in the following sequence:
Table 21: Saturated Liquid Line Temperature Set-
points and Values
Setpoint/Value
°F
Stage Up
110
Stage Down
80
High Critical
135
Low Critical
60
Staging Up
• When the current liquid line temperature re-
mains between the stage up and stage down
setpoints, the number of operating condens-
er fans remains the same
• When the current liquid line temperature
rises above the stage up setpoint for more
than 2 seconds, an additional condenser fan
is started
• When the current liquid line temperature
does not drop below the stage up setpoint
within 60 seconds, another fan is started
• This process repeats every 60 seconds until
all condenser fans in a circuit are operating
• When the liquid line temperature rises to the
high critical value, all condenser fans are im-
mediately energized
Staging Down
• When the current liquid line temperature re-
mains between the stage up and stage down
setpoints, the number of operating condens-
er fans remains the same
