5188238-YIM-D-0518
Johnson Controls Ducted Systems
43
Operation
Sequence Of Operation
For the XN 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”. The UCB energizes the compressor and condenser fan
allowing the unit to run in heating mode. A demand defrost
control operates the defrost cycle on all 3 thru 5 ton units.
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 minimum off
delay of 10 seconds.
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 Evaporator Low Limit sensor
(EC1)
detecting a temperature below 26° F 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 continuous fan operation the
economizer damper goes to the min. position.
Table 17: Additional Static Resistance
Size
(Tons)
CFM
Economizer
1
2
1. Deduct these values from the available external static pressure shown in the respective Blower
Performance Tables.
2. The pressure drop through the economizer is greater for 100% outdoor air than for 100% return air. If the
resistance of the return air duct is less than 0.25 IWG, the unit will deliver less CFM during full economizer
operation.
Electric Heat kW
1
7 - 15
20 - 30
036 (3.0)
048 (4.0)
060 (5.0)
1000
0.07
0.04
0.06
1200
0.08
0.05
0.07
1400
0.09
0.06
0.08
1600
0.11
0.07
0.09
1800
0.13
0.08
0.11
2000
0.15
0.10
0.13
2200
0.17
0.12
0.15
2400
0.20
0.14
0.17
2600
0.23
0.16
0.20
2800
0.23
0.19
0.23
3000
0.30
0.22
0.26
