Economizer - Carrier RTU Open v3 Integration Manual

Sequence of Operation
•
Heat mode is not active and the 5-minute time guard between modes has expired
•
Economizer is unavailable, or if the Economizer is active, mechanical cooling is available if the economizer is
open > 90% for at least 7.5 minutes, the SAT > [Minimum Cooling SAT + 5 °F (2.7 °)] and SPT > [Effective
Cooling Setpoint + 0.5 °F (.27 °C)].
The cooling relays are controlled by the Cooling Control PID Loop and Cooling 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 3 minute minimum
on-times, and 5 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 Service Alarm Timer functions are available (1 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 is on
•
The unit has a valid Supply Air Temperature input
•
The unit has a valid Space Temperature input
If the RTU Open is configured for VFD or 2-speed fan, the economizer minimum position is adjusted to provide a
constant amount of outdoor air. If the fan is on high speed or is configured for single-speed fan, the economizer
minimum position will be set to the Vent Dmpr Pos / DCV Min Pos setpoint. If it is configured for VFD or 2-speed
fan, and the fan is on low speed, the economizer minimum position will be set to the Low Fan Econ Min Pos.
If all preceding conditions are true, the economizer PID loop modulates the damper between the minimum
position and 100% open..
During economizer operation, the economizer position is reduced as the SAT falls below the Minimum Cooling
SAT + 5 °F (2.8 °C), but never closes below the applicable minimum position.
The RTU Open provides FDD (Fault Detection and Diagnostics) for economizer operation in compliance with
California Title 24. The FDD logic will detect an economizer that fails to close, fails to open, is stuck fully open, and
fails to fully open. Each condition will cause an Economizer Operation alarm to occur and the specific fault
condition will be displayed.
RTU Open v3
Integration Guide
Carrier Proprietary and Confidential
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