Applications (Field Installed)
GPEC Constant Volume Air Handler Unit
Constant 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 Constant Volume Air Handler Program includes the ability for the operator to
select some different sequences. Consult your Carrier representative if the sample sequence
requires alterations.
4.10
Constant Volume Air Handler Unit / Sequence of Operations
A. Description:
Constant Volume AHU w/ Outside Air, Return & Spill Dampers, Heating Coil, Cooling Coil.
B. General:
Each CV-AHU shall be directly controlled and monitored by a EMS compatible dedicated stand-alone
programmable logic General Purpose Electronic Controller (GPEC).
1. Required Algorithms and Schedules:
The following algorithms and schedules are required for typical control of a constant volume air
handler unit.
Algorithms
AO — Cooling CV
AO — Heating CV
AO — Mixed Air Damper CV
DO — Interlock
DO — Time Clock w/ Check
AOSS Schedule
NTFC w Enthalpy Check
Schedules
Time Schedule
Setpoint Schedule
2. Occupancy:
The occupancy status is determined by the AOSS Schedule (Adaptive Optimal Start/Stop)
algorithm, and may be affected by Timed Override.
3. Sequence of Operation:
The DO—Time Clock with Check algorithm will switch the operation from an unoccupied to an
occupied mode whenever the Time Schedule is in an occupied mode.
4. Supply and Return Fans:
The DO—Time Clock w Check algorithm will start the supply fan whenever the mode is occupied.
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 or the supply fan status is false, the return fan will be stopped.
5. Heating and Cooling Coil Valves:
The heating and cooling coil valves will modulate to produce a calculated supply air temperature to
maintain the desired space temperature setpoint.
6. Dampers:
The AO—Mixed Air Damper CV algorithm will modulate the mixed air dampers in conjunction with
the heating and cooling coils to produce a calculated mixed air temperature to maintain space
temperature between the heating and cooling set points. Modulating the mixed air dampers in
conjunction with the heating and cooling coils minimizes the need for mechanical cooling. The
actual damper setpoint may be automatically adjusted based on the current outside air
temperature and the setpoint reset values. The mixed air dampers will be held at their minimum
position when atmospheric conditions are not suitable for cooling.
7. Adaptive Optimal Start/Stop:
The AOSS Schedule algorithm will relax the space temperature set points prior to the unoccupied
time based on present space temperature, set points, K factor, adjustable bias values, Time
Schedule, and past performance. The AOSS Schedule algorithm can also be used to pre-condition
the space prior to occupancy.
C. Sequence of Operation During Unoccupied Hours:
1. Supply Fan and Mixed Air Dampers:
The DO—Time Clock w/ Check algorithm will start the supply fan and its interlocked return fan. It
also allows the heating or cooling valves to modulate, as applicable, until the space temperature
has returned to the required unoccupied space temperature limits. The outside air and exhaust air
dampers will remain closed during unoccupied hours.
2. Heating and Cooling Coil Valves:
The heating and cooling coils will maintain a minimum or maximum allowable supply air
temperature, as applicable, whenever the supply fan is off.
3. Night Time Free Cooling:
The NTFC w/ Enthalpy Check algorithm will start the supply fan and its interlocked return fan and
open the outside air and exhaust air to pre-cool the space prior to occupancy using only outside
air. Once the space is sufficiently cooled the supply fan and its interlocked return fan will stop and
the outside air and exhaust air 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 greater than the enthalpy of the indoor air.
4. Adaptive Optimal Start/Stop: The AOSS algorithm can start the supply and interlocked return
fans to cool or heat the space to the occupied setpoint prior to occupancy. The start time will be
determined by the current space temperature, occupied set points, the building thermal insulation,
and the outside air temperature. The space conditions will be monitored and adjusted to ensure
that the occupied setpoint is achieved at the time of occupancy.
D. Sensor Selection for Base Configuration:
The required sensors for the base configuration are described below.
1. SPT – Space Temperature:
The SPT sensor will be required to serve as the master sensor for the AO—Heating CV, AOCooling CV, and AO—Mixed Air Damper CV algorithms. The space temperature sensor will be
located within the controlled space in a place where it will not be adversely affected by heat
sources.
2. OAT – Outside Air Temperature:
The OAT sensor will be required 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 sub-master sensor for the AO-Heating CV and AO-Cooling CV 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 sub-master sensor for the dampers.
5. RAT – Return Air Temperature:
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.
6. RARH – Return Air Relative Humidity:
The RARH sensor 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.
7. SFS – Supply Fan Status:
The supply fan status will be required to activate the AO-Heating CV, AO-Cooling CV, and AO—
Mixed Air Damper CV algorithms. When the supply fan is on, the air handler is assumed to be
running. A motor current type discrete sensor is used to indicate when the supply fan is on.
8. OARH – Outside Air Relative Humidity:
The OARH sensor may be used by the NTFC w/ Enthalpy Check algorithm to compute the
enthalpy of the outside. A duct mounted RH probe will be located downstream of the return fan.