Applications (Field Installed)
GPEC Variable Air Volume Air Handler Unit
Variable Air Volume Air Handler Unit - Sequence of Operations
Note: The following is a sample of a typical sequence of operation that may be adapted for a
particular project. This is provided as a suggestion on how to write a sequence of operation for a
specific project. The VAV Air Handler Program includes the ability for the operator to select some
different sequences. Consult your Carrier representative if the sample sequence requires
alterations.
4.9 Variable Air Volume Air Handler Unit / Sequence of Operations
A. Description: VAV-AHU w/ OA, Return & Spill Dampers, HC, CC, & Fan Tracking
1. General:
Each VAV-AHU shall be directly controlled and monitored by a EMS compatible dedicated standalone programmable logic General Purpose Electronic Controller (GPEC).
The VAV algorithms listed use Linkage to interface to electronically controlled air terminals.
Linkage provides:
A. A dynamic exchange of zone information between the air handler and the terminals.
B. Zone information is necessary to coordinate and optimize the operation of the air handler.
Examples of zone information are: Average weighted zone temperature, average weighted
heat/cool setpoints, and next occupied time.
C. Dynamic air handler operating information to the terminals. Examples of operating information
are: heat/cool mode, morning warm-up, and nighttime free cooling.
A return air temperature (RAT) sensor shall be wired to the GPEC Controller for backup
control. Whenever Linkage is active, it will replace the RAT sensor and configured setpoint
schedule. If Linkage is not active, the algorithms will automatically revert to the return air
sensor and configured setpoint.
2. Required Algorithms and Schedules:
The following algorithms and schedules are required for typical control of a variable volume air
handler unit.
Algorithms
AO — Cooling VAV
AO — Fan Tracking
AO — Heating VAV
AO — Static Pressure
AO — Mixed Air VAV w/ IAQ
DO — Interlock
DO — Timeclock with Check
NTFC w Enthalpy Check
Linkage/AOSS Schedule
Schedules
Time Schedule
Setpoint Schedules (two)
3. Occupancy:
The occupancy status is determined by the Linkage function, and may be affected by the Adaptive
Optimal Start and Timed Override.
4. Sequence of Operation during Occupied Hours:
The DO—Time Clock with Check algorithm will enable the supply fan whenever the Linkage
function indicates that the system is in an occupied mode.
5. Return Fan:
The DO—Interlock algorithm will start and stop the return fan based on the supply fan status.
Whenever the supply fan is started and the supply fan status is true, the return fan will be started.
Whenever the supply fan is stopped and the supply fan status is false, the return fan will be
stopped.
6. Heating Coil Valve:
The heating coil valve will be controlled by the AO—Heating VAV algorithm. If heating is required
and enabled for occupied heat, the AO—Heating VAV algorithm will provide heat only if the
discharge temperature is below its setpoint and the weighted average zone temperature is below
the weighted average occupied heating setpoints as supplied by Linkage.
If required, the AO—Heating VAV algorithm can provide heat for morning warm-up only. During
morning warm-up, the heating valve will be modulated open based on the lowest space sensor
temperature as supplied by Linkage and compared to the weighted average occupied heating
setpoint. If Linkage is not active, the return air temperature sensor will be used. The discharge
sensor will be used as the submaster sensor and the weighted average space temperature will be
used as the master sensor.
When the Return Air Relative Humidity (RARH) value exceeds the configured high humidity
setpoint while the air handler is providing cooling for dehumidification, the AO—Heating VAV
algorithm will add a user-adjustable offset value to the weighted average space temperature to
force a controlled reheat. When the air handler is no longer providing dehumidification, the heating
coil valve will be controlled as described previously.
7. Cooling Coil Valve:
The cooling coil valve will be controlled by the AO—Cooling VAV algorithm. This algorithm will
modulate the cooling coil valve to maintain its user-adjustable supply air temperature setpoint. The
algorithm is capable of resetting the supply air temperature if the weighted average occupied
space temperature is below the weighted average occupied cooling setpoint.
When the Return Air Relative Humidity (RARH) sensor exceeds the configured high humidity
setpoint, the air handler will provide cooling for dehumidification, and the PID Master Loop will be
forced to its maximum output value (thus, forcing cooling) until the RARH sensor no longer
indicates a need for dehumidification.
8. Dampers:
The AO—Mixed Air VAV w IAQ algorithm will modulate the outside, return, and exhaust air
dampers. Whenever the supply fan is off, the outside and exhaust air dampers will be fully closed
and the return air damper will be fully opened. When the supply fan is enabled and the enthalpy
calculation determines that outside air conditions are unsuitable for using atmospheric cooling, the
dampers will be held at their adjustable minimum position. When the enthalpy calculation
determines that outside air conditions are suitable for using atmospheric cooling, the mixed air
dampers will be modulated to maintain a mixed air temperature equal to the supply air temperature
setpoint (plus any reset value) minus 30°F.
During morning warm-up, the dampers will be held closed.
9. Duct Static Pressure:
The AO—Static Pressure algorithm will modulate the supply fan's inlet guide vanes (or variable
frequency drive) to maintain a user adjustable duct static pressure setpoint. Whenever the supply
fan is off, both the supply and return fans will be unloaded.
10. Fan Tracking: The AO—Fan Tracking algorithm will modulate the return fan speed or fan capacity
(RFVC) position, as applicable, to maintain a constant delta CFM between the supply and return
fans. (The setpoint calculation can also be compensated for different barometric pressures, if
required.)
11. Linkage with Air Terminals: The linkage function will provide the following information, as a
minimum, to the air terminals:
A. Supply air temperature
B. Mode (Heating, Cooling, Night Time Free Cooling, Morning Warm-up, and Off)
C. Air handler fan status
D. The air handler linkage function will be capable of receiving the following dynamic space
information, as a minimum, for use in the Airside Linkage algorithm:
1. Occupied weighted average heating and cooling setpoints
2. Unoccupied weighted average heating and cooling setpoints
3. Weighted average zone temperature
4. Weighted average occupied zone temperature
5. Pre-set occupancy status and next unoccupied and occupied times
Linkage supports AOSS. If Linkage is not active, the algorithm will revert to the redundant
local time schedule, setpoint schedule and RAT sensor.
B. Sequence of Operation during Unoccupied Hours:
1. Heating and Cooling Coil Valves:
The heating and cooling coils will maintain a minimum or maximum allowable duct temperature, as
applicable, whenever the supply fan is off. If the lowest or highest space temperature value (as
supplied from Linkage) exceeds the weighted unoccupied heating or cooling setpoints, the DO—
Timeclock with Check algorithm will start the supply fan and modulate the heating or cooling
valves, as applicable, until the space temperature has returned to the required unoccupied space
temperature limits.
2. Dampers:
The outside and exhaust air dampers will be fully closed. The return air damper will be fully open,
unless NTFC is being performed.
3. Night Time Free Cooling: The NTFC w/Enthalpy algorithm will start the supply fan and
interlocked return fan, and open the outside air damper to pre-cool the space up to seven hours
prior to occupancy using only outside air. Once the space is sufficiently cooled, the supply fan will
stop and the mixed air dampers will close.
NTFC will not operate if:
A. The outside air temperature is below a configured value.
B. Space conditions do not warrant it.
C. The enthalpy of the outside air is too high.
The cycle will end whenever the:
D. Weighted average space temperature is less than 1°F than the weighted average cooling
setpoint.
E. Space temperature is less than the outside air temperature plus 3°F.
F. The occupied cycle has begun.
4. Linkage/AOSS Schedule Algorithm:
AOSS can start the supply and interlocked return fans to cool or heat the space to the occupied
setpoint prior to occupancy. Linkage will supply the values of the highest space sensor and the
weighted average cooling setpoint for use in its calculations.
Adaptive Optimal Start will monitor the space temperature and adjust setpoints and start/stop
times to ensure that the weighted average occupied setpoint is achieved at the time of occupancy.
C. Sensor Selection for Base Configuration:
The sensors required for the base configuration are described below.
1. SPT – Space Temperature
2. OAT – Outside Air Temperature:
The OAT sensor reading will be used by the NTFC w/ Enthalpy Check algorithm to determine
whether outside air conditions are suitable for cooling.
3. SAT – Supply Air Temperature:
A rigid duct probe (SAT) will be located downstream of the supply fan. A single SAT can serve as
the submaster sensor for the AO-Heating VAV during morning warm-up and the master sensor for
AO-Cooling VAV algorithms.
4. MAT – Mixed Air Temperature:
A duct-averaging element (MAT) will be located in the mixed air plenum before the heating coil
and will be used as the master sensor for the dampers.
5. RAT – Return Air Temperature:
The RAT sensor will be required to provide backup (redundant) control when Linkage is not active.
When Linkage is not active, RAT will be used as the master sensor for the AO—Heating VAV
algorithm. The RAT is also used as the reset sensor for AO-Cooling VAV.
A RAT sensor will be required by the NTFC w/ Enthalpy Check algorithm to determine if outside air
conditions are suitable for cooling. The return air enthalpy calculation requires a return air sensor.
A rigid duct probe (RAT) will be located downstream of the return fan to provide a good air mix
reading.
6. OARH – Outside Air Relative Humidity:
The OARH sensor can be used by the NTFC w/ Enthalpy Check algorithm to determine if the
outside air enthalpy is suitable for cooling. Locate the OARH sensor next to the OAT sensor.
7. RARH – Return Air Relative Humidity:
The RARH sensor value may be used by the NTFC w/ Enthalpy Check algorithm to compute the
enthalpy of the return air. A duct mounted RH probe will be located downstream of the return fan.
8. SP – Static Pressure:
Select a location in the ductwork where the static pressure pickup will be representative of the
static pressure that will be monitored and maintained (2/3 of the distance down the duct from the
supply fan). The static pressure sensor will typically have a 0 to 5" range.
9. SFS – Supply Fan Status:
The supply fan status will be required to activate the A0—Heating VAV, A0—Cooling VAV, and
A0—Mixed Air Damper VAV algorithms. When the supply fan is on, the air handler is assumed to
be running. The motor current type discrete sensor is used to indicate when the supply fan is on.
10. SVP1 and RVP2 – Supply and Return Air Velocity Transducers:
For best results, the supply and return air velocity sensors should be 25 to 75% of the optimum
pressure range and 10 to 90% of the theoretical pressure range. As a rule, the maximum air
velocity range should be about 75% of the transducer's maximum value.