Sequence of Operation
RTU Open v5
CARRIER CORPORATION ©2019
Integration Guide
All rights reserved
22
Cooling
The RTU Open's application and configuration determines the specific cooling sequence. The RTU Open can
control up to 2 stages of cooling with an additional output for a reversing valve (heat pump applications). The
number of stages is configurable or is defined by unit type.
The following conditions must be true for the cooling algorithm to operate:
Outdoor Air Temperature
, if valid, is greater than the
Cooling Lockout Temperature
setpoint
The indoor fan is on
The unit has a valid
Supply Air Temperature
input
The unit has a valid
Space Temperature
input
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.
SAV Cooling Mode
- When the space temperature rises above the cooling setpoint and the cooling mode becomes
active, the cooling capacity is calculated by the Cooling PID and the outputs are enabled as required. Initially, the
fan runs at the configured minimum airflow (
IDF Min Speed Voltage / Min VFD Output
) and VFD speed as long as
the SAT remains above the appropriate cooling stage setpoint (
Stage 'x' SAT Stpt
). As the SAT drops below the
configured
Stage ‘x’ SAT Stpt
, the fan speed increases as required up to the configured maximum VFD speed (
IDF
Max Speed Voltage / Max VFD Output
) to provide sufficient airflow across the coil and maintain the desired SAT
setpoint. The number of setpoints displayed and used depends on the unit type and configuration.The specific
setpoint used is based on how many stages of cooling are actively operating.
