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Limit controls
There is a high temperature limit control located on the
furnace vestibule panel. This is an automatic reset control
that provides over temperature protection due to reduced
airflow. This may be caused by the following:
•
A dirty filter
•
Indoor fan motor failure
•
Too many supply or return registers closed or blocked
off
•
Incorrectly sized duct system
The control module locks out if the limit trips five
consecutive times. If this occurs, the control resets and
tries ignition again after 1 h.
Sequence of operation
The following describes the sequence of operation of
the furnace. Refer to the User's Manual for component
location.
Continuous blower
Cooling or heating thermostats have a fan switch that
has ON and AUTO positions. In the ON position, the
thermostat circuit is completed between terminals R and
G. The motor operates continuously on the speed that is
selected on the G terminal on the main control board.
Intermittent blower - cooling
On cooling/heating thermostats with a fan switch,
when the fan switch is set in the auto position and
the thermostat calls for cooling, a circuit is completed
between the R, Y, and G terminals. The motor is energized
through the Y1 cool terminal and runs on the speed
selected on the COOL tap of the control board. The fan-
off setting is fixed at 60 s for SEER enhancement. The
control board can accommodate two-stage cooling. When
a two-stage cool thermostat is connected to the Y1 and
Y2 terminals on the board, the blower operates on LOW
COOL speed when Y1 is energized and on HI COOL speed
when Y1 and Y/Y2 are energized.
Heating cycle
When the thermostat switch is set on HEAT and the fan
is set on AUTO, and there is a call for heat, a circuit is
completed between terminals R and W of the thermostat.
When the correct amount of combustion air is being
provided, the pressure switch closes and the ignition
control provides a 17 s ignitor warm-up period. The gas
valve then opens, the gas starts to flow, ignition occurs,
and the flame sensor begins its sensing function. The
blower motor energizes 30 s after the gas valve opens, if
a flame is detected. Normal furnace operation continues
until the thermostat circuit between R and W is opened,
which causes the ignition system and gas valve to de-
energize and the burner flames to be extinguished.
The vent motor operates for 15 s and the blower motor
operates for the amount of time set by the fan-off delay
dip switch located on the control board. See Figure 29.
The heating cycle is now complete and ready for the start
of the next heating cycle.
Installation Manual: Z8ES Single-Stage Standard ECM Residential Gas Furnaces (Non-condensing Multi-position Standard
If the flame is not detected within 7 s of the gas valve
opening or if the flame is lost for 2 s during the 10 s
stabilization period, the gas valve is shut off and a retry
operation begins. During a retry operation, the vent
motor starts a 15 s inter-purge and the ignitor warm-up
time is extended to 27 s. If the flame is established for
more than 10 s after ignition during a retry, the control
clears the ignition attempt (retry) counter. If three retries
occur during a call for heat, the furnace shuts down for 1
h. If at the end of the 1 h shut down there is a call for heat,
the furnace initiates a normal start cycle. If the problem
has not been corrected, the furnace locks out after three
retries.
A momentary loss of gas supply, flame blowout, or a
faulty flame probe circuit results in a disruption in the
flame and is sensed within 1 s. The gas valve de-energizes
and the control begins a recycle operation. A normal
ignition sequence begins after a 15 s inter-purge. If during
the five recycles the gas supply does not return, or the
fault condition is not corrected, the ignition control locks
out for 60 min.
During burner operation, a momentary loss of power for
50 ms or longer de-energizes the gas valve. When the
power is restored, the gas valve remains de-energized
and the ignition sequence immediately restarts.
Hot surface ignition system
Do not attempt to light this furnace by hand (with
a match or any other means). There may be a
potential shock hazard from the components of the
hot surface ignition system. The furnace can only be
lit automatically by its hot surface ignition system.
Troubleshooting
Make the following visual checks before troubleshooting:
1.
Check to see that the power to the furnace and the
ignition control module is on.
2.
Check the manual shut-off valves in the gas line to
the furnace are open.
3.
Make sure all wiring connections are secure.
4.
Review the sequence of operation. Start the
system by setting the thermostat above the room
temperature. Observe the system's response. Then
use the troubleshooting section in this manual to
check the system's operation.
Johnson Controls Ducted Systems
WARNING
and Low NOx)
35
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