Sequence of Operation
RTU Open v5
CARRIER CORPORATION ©2019
Integration Guide
All rights reserved
28
Heating
The specific heating sequence is determined by the controller's application and configuration. The RTU Open
controls up to 2 stages of gas or electric heating with an additional output for a
Reversing Valve
(Heat Pump
applications).
The following conditions must be true for the heating algorithm to operate:
The
Outdoor Air Temperature
is less than the
Heating Lockout Temperature
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
Neither Cool mode nor economizer are active and the time guard between modes has expired
The heating relays are controlled by the Heating Control PID Loop and Heating Stages Capacity algorithm, which
calculate the desired number of stages to satisfy the space by comparing the
Space Temperature
to the:
Effective Occupied Heating Setpoint
when occupied
Effective Unoccupied Heating Setpoint
when unoccupied
When the heating algorithm preconditions have been met, the heating is energized in stages. Anti-recycle timers
are employed to protect the equipment from short-cycling. There are fixed one minute minimum on and off times
for each heating output.
During heating operation, the RTU Open may reduce the number of active stages if the rooftop
Supply Air
Temperature
exceeds the
Maximum Heating SAT
setpoint. A heat stage turned off in this fashion may be started
again after the normal time-guard period has expired, if the
Supply Air Temperature
has decreased below the
Maximum Heating SAT
setpoint.
SAV Heating Mode
— When the space temperature is below the heating setpoint and the heating mode becomes
active, the heating capacity is calculated by the Heating Control PID Loop and the outputs are enabled as
required. Initially, the fan operates at the configured heat airflow (
IDF Heat Speed Voltage/Heating VFD Output
),
whether higher or lower than the minimum VFD speed (
IDF Min Speed Voltage / Min VFD Output
), as long as the
SAT remains below the
Maximum Heating SAT
minus 3°F (1.67°C). As the SAT increases above this value, the
fan speed increases up to the configured maximum VFD speed (
IDF Max Speed Voltage / Max VFD Output
) to
provide sufficient airflow across the coil and maintain the
Maximum Heating SAT
minus 3°F (1.67°C) setpoint. As
the SAT exceeds the
Maximum Heating SAT
, the heat stages will be reduced or disabled.
Supply Air Tempering
The RTU Open can provide supply air tempering to warm the discharge air under conditions where no heating or
cooling is required, the outdoor air is cold, and the volume of outdoor air required for minimum ventilation causes
the supply air temperature to fall below the adjustable
SA Vent / Temper Setpoint
.
To enable the tempering function,
SA Tempering
must be set to
Enable
.
The following conditions must be true for the algorithm to operate:
The unit cannot be a heat pump type (
HP O/B Ctrl or HP Y1/W1 Ctrl
)
The unit has been operating for at least 5 minutes
