York International ZQ Series Installation Manual page 63

Economizer With Single Enthalpy Sensor for ETC,
and Economizer With Dry Bulb Sensor for Smart
Equipment™
When the room thermostat calls for 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 sensor for the ETC, or Dry bulb
temperature sensor for Smart Equipment™ reads
outdoor air is below the setpoint for their respective
controller, "Y1" will energize the economizer.
The dampers will then modulate to maintain a
constant supply air temperature as monitored
by the discharge air sensor. If the outdoor air is
above the enthalpy setpoint for ETC or dry bulb
temperature setpoint for Smart Equipment™, "Y1"
energizes the compressor and condenser fan motor
only.
Once the thermostat has been satisfied, it will de-
energize "Y1". If the compressor has satisfied its
minimum run time, the compressor and condenser
fan are de-energized. Otherwise, the unit operates
the cooling system until the minimum run times
for the compressor has been completed. After the
compressor de- energizes, 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 continuous 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 maximum
operating efficiency.
Economizer With Power Exhaust
A unit equipped with an economizer (dry bulb 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.
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
compressor, initiate the ASCD (Anti-short cycle
delay), and stop the condenser fan. If the call for
cooling is still present at the conclusion of the ASCD,
the UCB will re-energize the 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 on
the ETC or send error message to the LCD Smart
Equipment™ Control.
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 compressor, initiate the ASCD, and stop
the condenser fan.
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 compressor, initiate the
ASCD, and stop the condenser fan.
INSTALLATION MANUAL ZQ SERIES 3 - 5 ton 60 Hertz R-410A
Johnson Controls Ducted Systems
63