689670-BIM-A-0311
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Johnson Controls Unitary Products
Operation
Cooling Sequence Of Operation
For the ZU series of units, the thermostat makes a circuit
between "R" and "Y1" for the first stage of cooling.
The call is passed to the
Unit Control Board (UCB),
which
then determines whether the requested operation is available
and, if so, which components to energize.
For gas heating, the UCB monitors the "W1" call but does not
handle the operation of the gas furnace. An ignition control
board controls the gas heater operation. For electric heat units,
the UCB passes the call to the electric heater. In both cases,
when the "W1" call is sensed, the indoor air blower is energized
following a specified heating delay.
If at any time a call for both heating and cooling are present, the
heating operation will be performed. If operating, the cooling
system is halted as with a completion of a call for cooling.
Heating always takes priority.
Continuous Blower
By setting the room thermostat fan switch to "ON," the supply
air blower will operate continuously.
Intermittent Blower
With the room thermostat fan switch set to "AUTO" and the
system switch set to either the "AUTO" or "HEAT" settings, the
blower is energized whenever a cooling or heating operation is
requested. The blower is energized after any specified delay
associated with the operation.
When energized, the indoor blower has a minimum run time of
30 seconds. Additionally, the indoor blower has a delay of 10
seconds between operations.
No Outdoor Air Options
When the thermostat calls for cooling, the low-voltage control
circuit from “R” to “Y1” and “G” is completed. The compressor and
condenser fan motor are energized. After completing the specified
fan on delay for cooling, the UCB will energize the blower motor.
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 de-energize. Otherwise, the
unit operates the cooling system until the minimum run time 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.
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 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 setpoint 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 setpoint, “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 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 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. 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.
