A. To Shut Off Unit
Set system selector switch at off position. Resetting heating selector lever below room temperature will temporarily shut unit off until space temperature falls below thermostat setting.
XIII. SAFETY RELIEF
A soft-solder joint at the suction service Schrader port provides pressure relief under abnormal temperature and pressure conditions.
XIV. VENTILATION (Continuous Fan)
Set fan and system selector switches at ON and OFF posi- tions, respectively. Evaporator fan operates continuously to provide constant air circulation. When the evaporator-fan selector switch is turned to the OFF position, there is a 30-second delay before the fan turns off.
XV. OPERATING SEQUENCE
A. Cooling, Units Without Economizer
When thermostat calls for cooling, terminals G and Y1 and the compressor contactor (C) are energized. The indoor (evaporator) fan motor (IFM), compressor, and outdoor (con- denser) fan motor (OFM) start. The OFM runs continuously while the unit is in cooling. When the thermostat is satisfied, C is deenergized and the compressor and OFM shut off. After a 30-second delay, the IFM shuts off. If the thermostat fan selector switch is in the ON position, the evaporator motor will run continuously.
B. Heating, Units Without Economizer
When the thermostat calls for heating, terminal W1 is ener- gized. 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. When additional heat is needed, W2 is energized and the high-fire solenoid on the main gas valve (MGV) is energized. When the thermostat is satisfied and W1 is deenergized, the IFM stops after a 45-second time-off delay.
C. Cooling, Units With EconoMi$er IV
When free cooling is not available, the compressors will be controlled by the zone thermostat. When free cooling is avail- able, the outdoor-air damper is modulated by the EconoMi$er IV control to provide a 50 to 55 F supply-air temperature into the zone. As the supply-air temperature fluctuates above 55 or below 50 F, the dampers will be modu- lated (open or close) to bring the supply-air temperature back within the set points.
Integrated EconoMi$er IV operation on single-stage units requires a 2-stage thermostat (Y1 and Y2).
For EconoMi$er IV operation, there must be a thermostat call for the fan (G). This will move the damper to its mini- mum position during the occupied mode.
If the increase in cooling capacity causes the supply-air tem- perature to drop below 45 F, then the outdoor-air damper position will be fully closed. If the supply-air temperature continues to fall, the outdoor-air damper will close. Control returns to normal once the supply-air temperature rises above 48 F.
If optional power exhaust is installed, as the outdoor-air damper opens and closes, the power exhaust fans will be energized and deenergized.
If field-installed accessory CO2 sensors are connected to the EconoMi$er IV control, a demand controlled ventilation strategy will begin to operate. As the CO2 level in the zone increases above the CO2 set point, the minimum position of the damper will be increased proportionally. As the CO2 level
decreases because of the increase in fresh air, the outdoor-air damper will be proportionally closed. Damper position will follow the higher demand condition from DCV mode or free cooling mode.
Damper movement from full closed to full open (or vice versa) will take between 11/2 and 21/2 minutes.
If free cooling can be used as determined from the appropri- ate changeover command (switch, dry bulb, enthalpy curve, differential dry bulb, or differential enthalpy), a call for cool- ing (Y1 closes at the thermostat) will cause the control to modulate the dampers open to maintain the supply air tem- perature set point at 50 to 55 F.
As the supply-air temperature drops below the set point range of 50 to 55 F, the control will modulate the outdoor-air damp- ers closed to maintain the proper supply-air temperature.
D. Heating, Units With EconoMi$er IV
When the room temperature calls for heat, the heating con- trols are energized as described in the Heating, Units With- out Economizer section. When the thermostat is satisfied, the economizer damper moves to the minimum position.
E.Units With Perfect Humidity™ Dehumidification System
Normal Design Cooling Operation
When the rooftop operates under the normal sequence of operation, the compressors will cycle to maintain indoor con- ditions. See Fig. 41.
The Perfect Humidity adaptive dehumidification system includes a factory-installed Motormaster® low ambient con- trol to keep the head and suction pressure high, allowing normal design cooling mode operation down to 0° F.
Subcooling Mode
When subcooling mode is initiated, this will energize (close) the liquid line solenoid valve (LLSV) forcing the hot liquid refrigerant to enter the subcooling coil (see Fig. 42).
As the hot liquid refrigerant passes through the subcooling/ reheat dehumidification coil, it is exposed to the cold supply airflow coming through the evaporator coil. The liquid is fur- ther subcooled to a temperature approaching the evaporator leaving-air temperature. The liquid then enters a thermostatic expansion valve (TXV) where the liquid drops to a lower pres- sure. The TXV does not have a pressure drop great enough to change the liquid to a 2-phase fluid, so the liquid then enters the Acutrol™ device at the evaporator coil.
The liquid enters the evaporator coil at a temperature lower than in standard cooling operation. This lower temperature increases the latent capacity of the rooftop unit. The refriger- ant passes through the evaporator and is turned into a vapor. The air passing over the evaporator coil will become colder than during normal operation. However, as this same air passes over the subcooling coil, it will be slightly warmed, partially reheating the air.
Subcooling mode operates only when the outside-air temper- ature is warmer than 40 F. A factory-installed temperature switch located in the condenser section will lock out subcool- ing mode when the outside temperature is cooler than 40 F.
The scroll compressors are equipped with crankcase heaters to provide protection for the compressors due to the addi- tional refrigerant charge required by the subcooling/reheat coil.
When in subcooling mode, there is a slight decrease in sys- tem total gross capacity (5% less), a lower gross sensible capacity (20% less), and a greatly increased latent capacity (up to 40% more).
