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

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.