998659-YIM-C-0614
Johnson Controls Unitary Products
43
Operation
Cooling Sequence Of Operation
For the XP 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 heating, the thermostat makes a circuit between “R” and
“W1” for the first stage heating. The UCB energizes the
compressors #1 and #2 and their condenser fans. The “W1” call
also energizes a separate relay (RY1), de-energizing the
reversing valve allowing the unit to run in the heating mode. A
time/temperature control operates the defrost cycle.
The thermostat makes a circuit between “R” and “W2” for the
second stage of heating. The UCB passes the “W2” signal on to
the electric heaters if available. In both cases, when the “W1”
call is sensed, the indoor blower is energized.
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 the first stage of cooling, the low-
voltage control circuit from “R” to “Y1” and “G” is completed.
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, compressor
#1 is energized. If compressor #1 is active for first stage cooling
or the first compressor is locked-out, compressor #2 is
energized. In free-cooling mode, if the call for the second stage
of cooling continues for 20 minutes, compressor #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, 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.
* 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 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.
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.
