"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.
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 compressors.
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.
Should a high-pressure switch open three times within two hours of operation, the UCB will lock-out the associated com- pressor and flash a code (see Table 38). If the other com- pressor is inactive, the condenser fans will be de-energized.
LOW-PRESSURE LIMIT SWITCH
The low-pressure limit switch is not monitored during the ini- tial 30 seconds of a cooling system's operation. For the fol- lowing 30 seconds, the UCB will monitor the low-pressure switch to ensure it closes. If the low-pressure switch fails to close after the 30-second monitoring phase, the UCB will de- energize the associated compressor, initiate the ASCD, and, if the other compressor is idle, stop the condenser fans.
Once the low-pressure switch has been proven (closed dur- ing the 30-second monitor period described above), the UCB will monitor the low-pressure limit switch for any openings. If the low-pressure switch opens for greater than 5 seconds, the UCB will de-energize the associated compressor, initiate the ASCD, and, if the other compressor is idle, stop the con- denser fans.
If the call for cooling is still present at the conclusion of the ASCD, the UCB will re-energize the halted compressor.
Should a low-pressure switch open three times within one hour of operation, the UCB will lock-out the associated com- pressor and flash a code (Table 38). If the other compressor is inactive, the condenser fans will be de-energized.
FREEZESTAT
During cooling operation, if a freezestat opens, the UCB will de-energize the associated compressor, initiate the ASCD, 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.
Should a freezestat open three times within two hours of operation, the UCB will lock-out the associated compressor and flash a code (Table 38). If the other compressor is inac- tive, the condenser fans will be de-energized.
LOW AMBIENT COOLING
To determine when to operate in low ambient mode, the UCB has a pair of terminals connected to a temperature-activated switch set at 45ºF. When the low ambient switch is closed
