Carrier 48TC*D08 - Manual (Page 75)

75
OPERATING SEQUENCES
Base Unit Controls
Cooling, Units Without Economizer
When thermostat calls for Stage 1 cooling, terminals G
and Y1 are energized. The indoor--fan contactor (IFC),
outdoor fan contactor (OFC) and Compressor 1 contactor
(C1) are energized and indoor-fan motor, outdoor fan and
Compressor 1 start. The outdoor fan motor runs
continuously while unit is in Stage 1 or Stage 2 cooling.
(D08 and D12 units have two outdoor fans; both run while
unit is in Stage 1 or Stage 2 cooling.)
If Stage 1 cooling does not satisfy the space load , the
space temperature will rise until thermostat calls for Stage
2 cooling (Y2 closes). Compressor 2 contactor (C2) is
energized; Compressor 2 starts and runs.
Heating, Units Without Economizer
When the thermostat calls for heating, terminal W1 is
energized. To prevent thermostat short-cycling, the unit is
locked into the Heating mode for at least 1 minute when
W1 is energized. The induced-draft motor is energized
and the burner ignition sequence begins. The indoor
(evaporator) fan motor (IFM) is energized 45 seconds
after a flame is ignited.
If Stage 1 heating does not satisfy the space load, the
space temperature will fall until thermostat calls for Stage
2 heating (W2 closes). Terminal W2 is energized and the
high-fire solenoid on the main gas valve (MGV) is
energized. Firing rate increases to high-fire. When space
load is partially satisfied, terminal W2 is deenergized; the
high-fire solenoid is deenergized and heating operation
continues on low--fire.
When the space heating load is fully satisfied, thermostat
terminal W1 is also deenergized. All heating operations
cease. The IFM stops after a 45-second time off delay.
Cooling, Unit With EconoMi$er IV
For Occupied mode operation of EconoMi$er IV, there
must be a 24-v signal at terminals TR and N (provided
through PL6-3 from the unit’s IFC coil). Removing the
signal at N places the EconoMi$er IV control in
Unoccupied mode.
During Occupied mode operation, indoor fan operation
will be accompanied by economizer dampers moving to
Minimum Position setpoint for ventilation. If indoor fan is
off, dampers will close. During Unoccupied mode
operation, dampers will remain closed unless a Cooling
(by free cooling) or DCV demand is received.
When free cooling using outside air is not available, the
unit cooling sequence will be controlled directly by the
space thermostat as described above as Cooling, Unit
Without Economizer. Outside air damper position will be
closed or Minimum Position as determined by occupancy
mode and fan signal.
When free cooling is available as determined by the
appropriate changeover command (dry bulb, outdoor
enthalpy, differential dry bulb or differential enthalpy), a
call for cooling (Y1 closes at the thermostat) will cause
the economizer control to modulate the dampers open and
closed to maintain the unit supply air temperature at 50 to
55_F. Compressor will not run.
During free cooling operation, a supply air temperature
(SAT) above 50_F will cause the dampers to modulate
between Minimum Position setpoint and 100% open. With
SAT from 50_F to 45_F, the dampers will maintain at the
Minimum Position setting. With SAT below 45_F, the
outside air dampers will be closed. When SAT rises to
48_F, the dampers will re-open to Minimum Position
setting.
Should 100% outside air not be capable of satisfying the
space temperature, space temperature will rise until Y2 is
closed. The economizer control will call for compressor
operation. Dampers will modulate to maintain SAT at 50
to 55_F concurrent with Compressor 1 operation. The
Low Ambient Lockout Thermostat will block compressor
operation with economizer operation below 42_F outside
air temperature.
When space temperature demand is satisfied (thermostat
Y1 opens), the dampers will return to Minimum Damper
position if indoor fan is running or fully closed if fan is
off.
If accessory power exhaust is installed, the power exhaust
fan motors will be energized by the economizer control as
the dampers open above the PE-On setpoint and will be
de----energized as the dampers close below the PE-On
setpoint.
Damper movement from full closed to full open (or vice
versa) will take between 1-1/2 and 2-1/2 minutes.
Heating With EconoMi$er IV
During Occupied mode operation, indoor fan operation
will be accompanied by economizer dampers moving to
Minimum Position setpoint for ventilation. If indoor fan is
off, dampers will close. During Unoccupied mode
operation, dampers will remain closed unless a DCV
demand is received.
When the room temperature calls for heat (W1 closes), the
heating controls are energized as described in Heating,
Unit Without Economizer above.
Demand Controlled Ventilation
If a field-installed CO2 sensor is connected to the
Economize IV control, a Demand Controlled Ventilation
strategy will operate automatically. As the CO2 level in
the space increases above the setpoint (on the EconoMi$er
IV controller), the minimum position of the dampers will
be increased proportionally, until the Maximum
Ventilation setting is reached. As the space CO2 level
decreases because of the increase in fresh air, the
outdoor-damper will follow the higher demand condition
from the DCV mode or from the free-cooling mode.
48TC