NO OUTDOOR AIR OPTIONS
When the thermostat calls for the first stage of cooling, the low-voltage control circuit from “R” to “Y1” and “G” is com- pleted. The UCB energizes the economizer (if installed and free cooling is available) or the first available compressor* and the condenser fans. For first stage cooling, compressor #1 is energized. If compressor #1 is unavailable, compressor #2 is energized. After completing the specified fan on delay for cooling, the UCB will energize the blower motor.
When the thermostat calls for the second stage of cooling, the low-voltage control circuit from “R” to “Y2” is completed. The control board energizes the first available compressor. If free cooling is being used for the first stage of cooling, com- pressor #1 is energized. If compressor #1 is active for first stage cooling or the first compressor is locked-out, compres- sor #2 is energized. In free-cooling mode, if the call for the second stage of cooling continues for 20 minutes, compres- sor #2 is energized, provided it has not been locked-out.
If there is an initial call for both stages of cooling, the UCB will delay energizing compressor #2 by 30 seconds in order to avoid a power rush.
Once the thermostat has been satisfied, the it will de-ener- gize Y1 and Y2. If the compressors have satisfied their mini- mum run times, the compressors and condenser fans are de- energized. Otherwise, the unit operates each cooling system until the minimum run times for the compressors have been completed. Upon the final compressor de-energizing, the blower is stopped following the elapse of the fan off delay for cooling.
*To be available, a compressor must not be locked-out due to a high or low-pressure switch or freezestat trip and the anti-short cycle delay (ASCD) must have elapsed.
ECONOMIZER WITH SINGLE ENTHALPY SENSOR -
When the room thermostat calls for "first-stage" cooling, the low voltage control circuit from "R" to "G" and "Y1" is com- pleted. The UCB energizes the blower motor (if the fan switch on the room thermostat is set in the "AUTO" position) and drives the economizer dampers from fully closed to their min- imum position. If the enthalpy of the outdoor air is below the set point of the enthalpy controller (previously determined), "Y1" energizes the economizer. The dampers will modulate to maintain a constant supply air temperature as monitored by the discharge air sensor. If the outdoor air enthalpy is above the set point, "Y1" energizes compressor #1, condenser fan motor #1, and condenser fan motor #2 (if the ambient tem- perature is above 60°F).
When the thermostat calls for "second-stage" cooling, the low voltage control circuit from "R" to "Y2" is completed. The UCB energizes the first available compressor If the enthalpy of the outdoor air is below the set point of the enthalpy controller (i.e. first stage has energized the economizer), "Y2" will ener-
gize compressor #1. If the outdoor air is above the set point, "Y2" will energize compressor #2.
Once the thermostat has been satisfied, it will de-energize Y1 and Y2. If the compressors have satisfied their minimum run times, the compressors and condenser fans are de-ener- gized. Otherwise, the unit operates each cooling system until the minimum run times for the compressors have been com- pleted. Upon the final compressor de-energizing, the blower is stopped following the elapse of the fan off delay for cooling, and the economizer damper goes to the closed position. If the unit is in continues fan operation the economizer damper goes to the min. position.
ECONOMIZER WITH DUAL ENTHALPY SENSORS -
The operation with the dual enthalpy sensors is identical to the single sensor except that a second enthalpy sensor is mounted in the return air. This return air sensor allows the economizer to choose between outdoor air and return air, whichever has the lowest enthalpy value, to provide maxi- mum operating efficiency.
ECONOMIZER (SINGLE OR DUAL) WITH POWER EXHAUST -
This system operates as specified above with one addition. The power exhaust motor is energized 45 seconds after the actuator position exceeds the exhaust fan set point on the economizer control. When the power exhaust is operating, the second stage of mechanical cooling will not operate. As always, the "R" to "G" connection provides minimum position but does not provide power exhaust operation.
MOTORIZED OUTDOOR AIR DAMPERS -
This system operation is the same as the units with no out- door air options with one exception. When the "R" to "G" cir- cuit is complete, the motorized damper drives open to a position set by the thumbwheel on the damper motor. When the "R" to "G" circuit is opened, the damper spring returns fully closed.
COOLING OPERATION ERRORS
Each cooling system is monitored for operation outside of the intended parameters. Errors are handled as described below. All system errors override minimum run times for compres- sors.
HIGH-PRESSURE LIMIT SWITCH
During cooling operation, if a high-pressure limit switch opens, the UCB will de-energize the associated compressor, initiate the ASCD (Anti-short cycle delay), and, if the other compressor is idle, stop the condenser fans. If the call for cooling is still present at the conclusion of the ASCD, the UCB will re-energize the halted compressor.
