Carrier RTU OPEN 11-808-427-01 - Manual (Page 51)

RTU Open 45
The cooling relays are controlled by the Cooling Control PID Loop and Cooling Stages Capacity algorithm. They
calculate the desired number of stages needed to satisfy the space by comparing the Space Temperature to
the:
•
Effective Occupied Cooling Setpoint when occupied
• Effective Unoccupied Cooling Setpoint when unoccupied
When the cooling algorithm preconditions have been met, the compressors are energized in stages, as
applicable. Anti-recycle timers are employed to protect the equipment from short-cycling. There are fixed
three-minute minimum on-times, and five-minute off-times for each compressor output.
During compressor operation, the RTU Open may reduce the number of active stages if the rooftop supply air
temperature falls below the Minimum Cooling SAT Setpoint. A compressor staged off in this fashion may be
started again after the normal time-guard period has expired, if the Supply Air Temperature has increased
above the Minimum Cooling SAT Setpoint.
Compressor 2 Service Alarm Timer
functions are available (one for each stage of compression). This function
tracks the number of compressor run hours and generates an alarm when the accumulated runtime exceeds
the threshold set by the adjustable compressor service alarm timers.
Economizer
The RTU Open provides an analog economizer output for rooftop units with economizer dampers. Economizer
dampers may be used to provide indoor air quality control and free cooling when outside air conditions are
suitable.
The following conditions must be true for economizer operation:
•
The Outdoor Air Temperature is less than the Space Temperature and less than the Economizer High
OAT Lockout Temp setpoint
• The indoor fan has been on for at least 30 seconds
• The unit has a valid Supply Air Temperature input
• The unit has a valid Space Temperature input
If any of the preceding conditions are not true, the economizer will be set to the Vent Dmpr Pos / DCV Min
Pos setpoint.
If all preceding conditions are true, the economizer PID loop will modulate the damper from the Vent Dmpr
Pos / DCV Min Pos setpoint.
The economizer moves to the Vent Dmpr Pos / DCV Min Pos setpoint if the SAT falls below the Minimum
Cooling SAT (+ 5°F).