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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 completed.
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 minimum
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.
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 energize
compressor #1. If the outdoor air is above the set point, "Y2" will
energize compressor #2. If Y2 brings on compressor #1 and
this condition remains for more than 20 minutes, then
compressor #2 will be energized until the thermostat is
satisfied.
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-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, and the
economizer damper goes to the closed position. If the unit is in
continues fan operation, the economizer damper goes to the
minimum 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 maximum operating
efficiency.
Economizer With Power Exhaust
A unit equipped with an economizer (single or dual enthalpy)
and a power exhaust 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 outdoor
air options with one exception. When the "R" to "G" circuit 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.
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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
compressor and flash a code (see Table 24). If the other
compressor is inactive, the condenser fans will be de-
energized.
Low-Pressure Limit Switch
The low-pressure limit switch is not monitored during the initial
30 seconds of a cooling system's operation. For the following
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. If the LPS is still
open after the ASCD, the compressor will not be energized for
30 seconds. The second and third times that the UCB sees an
open LPS will count towards the three occurrences that will
cause a UCB lock-out.
Once the low-pressure switch has been proven (closed during
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 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 low-pressure switch open three times within one hour
of operation, the UCB will lock-out the associated compressor
and flash a code (Table 24). 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 24). If the other compressor is inactive, the
condenser fans will be de-energized.
178113-BIM-D-0906
Unitary Products Group
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