Carrier 39L - Manual (Page 99)

MIXED-AIR DAMPER CONTROL — The mixed-air damper
control adjusts modulating outside-air, return-air, and exhaust-
air dampers. When outside air conditions are unsuitable for
atmospheric cooling, the dampers are held to an adjustable
minimum outside air position. When outside air conditions
are suitable for atmospheric cooling, the mixed-air dampers
are modulated to maintain a space temperature that is be-
tween the heating and cooling set points in an effort to mini-
mize the need for heating or mechanical cooling. The damper
set point is automatically adjusted as a function of outdoor-
air temperature or can be set to a fixed value by the user.
If the supply fan is OFF, the mixed-air dampers are kept
closed to outside air and open to return air.
If the fan is ON, the system checks to see if the system is
in the HEAT mode. If system is in the HEAT mode, the mixed-
air dampers are held in the minimum position.
If the system is not in the HEAT mode, it determines if the
outside conditions are suitable for atmospheric cooling. The
control compares the outdoor-air temperature to the space
temperature. If the outdoor-air temperature is less than the
space temperature, the system does either an enthalpy check
(using an outside-air enthalpy switch) or a differential en-
thalpy check. (Return-air temperature is compared to return-
air relative humidity for return-air differential enthalpy. Outside-
air temperature is compared to outside-air relative humidity
for outside-air differential enthalpy.)
The control then determines if the outdoor-air tempera-
ture is below the Nighttime Free Cool Lockout (NTLO). If
so, the damper set point is changed to 1.0 degree less than
the Occupied Cooling set point, allowing the system to use
return air during the heating mode.
If the outdoor-air temperature is higher than 68 F, the con-
trol sets the damper set point to 1.0 degree higher than the
Occupied Heating set point to provide cooling, allowing the
system to use outside air.
If the outdoor-air temperature is higher than NTLO but
less than 68 F, the system sets the damper set point halfway
between the Occupied Heating and Occupied Cooling set points
to provide ventilation.
If outside conditions are suitable, the control compares the
space temperature to the damper set point and computes the
supply-air temperature required to satisfy conditions.
Once the required supply-air temperature has been cal-
culated, it is compared to the mixed-air temperature sensor
value (if installed), otherwise the supply-air temperature sen-
sor determines the damper position. The damper adjustment
rate is limited to 5% per minute if the outside-air tempera-
ture is less than 40 F. This rate limit prevents nuisance low
temperature thermostat tripping.
ELECTRIC HEATER CONTROL — Electric heater stag-
ing regulates the electric heater. The heater is staged to pre-
vent the space temperature from falling below the desired
set point. (The number of heat stages is factory set for each
unit’s heater.)
If the supply fan is OFF, all stages of electric heat are
turned off.
If the fan is ON, the control reads the space temperature
sensor and calculates the supply-air temperature required to
satisfy conditions.
Once the required supply-air temperature has been cal-
culated, it is compared to the actual supply-air temperature
to determine the number of heat stages required to satisfy
conditions. The required stages are energized one at a time,
with 2-second intervals between stages.
ADAPTIVE OPTIMAL STOP —Optimal stop allows the space
temperature to drift to an expanded occupied set point dur-
ing the last portion of an occupied period. The control cal-
culates a stop time offset, which is the time in minutes prior
to the scheduled unoccupied time, during which expanded
set points can be used. Adaptive optimal stop utilizes space
temperature, an expanded Occupied set point, and a K factor
to calculate stop time offset. The amount to expand the Oc-
cupied set point is user configurable. Like adaptive optimal
start, the control corrects itself for optimal operation by ad-
justing the K factor.
TWO-STEP DEMAND LIMITING (Available on Units
Connected to the Carrier Comfort Network Only) — If the
Demand Limit option is enabled, the control receives and
accepts redline alert and loadshed commands from the
Network Loadshed option. See the CCN Loadshed manual
for additional information.
When a redline alert is received, the Control decreases the
Occupied Heating set point by 2° F and increases the Oc-
cupied cooling set point by 2° F.
When a loadshed command is received, the supply fan turns
OFF.
The maximum loadshed timer prevents the system from
remaining in loadshed, should the control lose communica-
tions with the Loadshed option. If the timer expires be-
fore the loadshed command is cleared by the Loadshed op-
tion, the control clears the loadshed command itself and
returns to normal control. The loadshed timer is factory set
at 60 minutes.
Variable Air Volume Units Only
CONSTANT OUTSIDE AIR (OAC) — During Occupied
periods, this feature provides a continuous flow of outside
air into the building. The OAC control modulates the mixed-
air damper to a configurable minimum open position to en-
sure outside air is admitted. During Unoccupied periods, the
control signals the dampers to fully close.
To monitor the outside airflow during Occupied periods,
the OAC control reads the outside-air velocity pressure (OAVP)
sensor, compares its value to the OAVP set point, and ad-
justs the mixed-air dampers to the position required to achieve
the configured velocity pressure.
If a control routine (other than OAC) or the configured
minimum damper position require the dampers to be more
fully open than the OAC setting, that routine or position over-
rides the OAC control. To protect against coil freeze-up in
colder climates, the PIC provides selectable low tempera-
ture protection to limit the minimum mixed-air temperature
to approximately 45 F.
See Fig. 79 for OAC control flowchart.
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