Carrier RTU Open v3 - Economizer

Sequence of Operation
RTU Open v3 CARRIER CORPORATION ©2019
Installation and Start-up Guide All rights reserved
46
• 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 and OAT > [
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.
The economizer will modulate to maintain the configured
Minimum Cooling SAT
limit when the unit is in an
economizer only mode and will modulate closed only when the SAT drops below the
Minimum
Cooling SAT
limit -
5
°F (-2.8 °C) when mechanical cooling is also operating.