5750156-UIM-B-0919
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Johnson Controls Ducted Systems
Post Purge
The control board runs the induced draft motor for a 15-second post-
purge period, and then de-energizes the inducer. If a call for heat
occurs during post-purge, the control finishes the postpurge, and imme-
diately begins the next ignition sequence.
Heat Blower Off Delay
After a heating sequence the control board de-energizes the indoor
blower motor after a delay time as selected by a movable shunt (60, 90,
120, or 180 seconds). Blower off timing begins when the thermostat is
satisfied and removes (W) the call for heat. The control returns to
standby when the blower off delay is complete.
If the thermostat call for heat returns before the blower off delay is com-
plete, the control begins an ignition sequence with prepurge while the
blower off delay continues.
Lockout
While in lockout, the control board keeps the main gas valve and
induced draft motor de-energized.
Lockouts due to failed ignition or flame losses may be reset by remov-
ing the call for heat (W) for more than 1 second, or by removing power
from the control for over 0.25 seconds. The control automatically resets
lockout after 60 minutes.
Lockouts due to detected internal control faults reset after 60 minutes or
power interruption. Cooling operations are available during a heating
lockout.
High Temperature Limit Switch
Any time the high temperature limit switch is open less than 5 minutes,
the control board runs the indoor blower motor on heat speed, runs the
inducer, de-energizes the gas valve, and flashes “4” on the LED. When
the high temperature switch closes, the control restarts the ignition
sequence beginning with pre-purge.
If the limit switch has been open more than 5 minutes the control de-
energizes the inducer, continues to operate the indoor blower motor on
heat speed, and flashes “11” on the LED.
Rollout Switch
If the limit circuit is open for more than 15 minutes, the control board
flashes “5” on the LED. The blower output is energized during an open
rollout condition.
If the rollout switch is reset, the control remains locked out until power is
removed or a call for heat (W) is removed.
Rollout switch lockout does not reset automatically.
Power Interruptions
Power interruptions of any duration shall not cause lockout or any oper-
ation requiring manual intervention.
Flame Present with Gas Off
If flame is sensed for longer than 4.0 seconds during a period when the
gas valve should be closed, the control enters lockout, flashing “1” on
the LED. The control energizes the inducer blower while the flame is
sensed.
GAS VALVE FAULT
If the main valve output is sensed to be energized for more than 1 sec-
ond when commanded to be off, the control de-energizes the induced
draft motor (if flame is not present) to attempt to open the pressure
switch to de-energize the gas valve. If the main gas valve is still sensed
as energized after the inducer has been off for 10 seconds, the control
re-energizes the inducer to vent the unburned gas. The control enters a
hard lockout, flashing “10” on the LED.
The only way to recover from a hard lockout is to remove and reapply
24 VAC power to the control board.
Safety Controls
The control circuit includes the following safety controls:
• Limit Switch (LS) - This control is located inside the blower com-
partment and protrudes into the heat exchanger compartment.
The limit switch is set to open at a temperature selected to prevent
the outlet air temperature from exceeding the maximum shown on
the unit rating plate. It resets automatically. The limit switch oper-
ates when a high temperature condition occurs. The limit switch
shuts down the ignition control, closes the main gas valve, and
energizes the blower.
• Pressure Switch (PS) - If the draft motor fails, the pressure switch
prevents the ignition control and gas valve from being energized.
• Flame Sensor - The flame sensor is located on the left-most
burner. If an ignition control fails to detect a signal from the flame
sensor indicating the flame is properly ignited, the main gas valve
closes.
• Rollout Switch (RS) - This switch is located on the burner assem-
bly. In the event of a sustained main burner flame rollout, the con-
trol closes the main gas valve. This is a manual reset type switch.
GAS HEATING FAULT CODES
The gas heating section has built-in self-diagnostic capability. A blinking
LED light on the control board can flash red, green, or amber to indicate
various conditions. The control continuously monitors its own operation
and the operation of the system. If a fault occurs, the LED light indicates
the fault code.
The slow flash speed is 2 seconds on and 2 seconds off. The other
flash codes listed below have the following timing: the LED light turns
on for 1/3 second and off for 1/3 second. This pattern is repeated the
number of times equal to the code. There is a two-second pause
between codes. For example, the 6 Red Flashes fault code means the
LED light flashes on and off six times and then remains off for 2 sec-
onds. This pattern repeats as long as the fault condition remains. The
continuous flash codes listed below mean the LED light flashes on and
off continuously with no breaks or longer pauses.
Slow Green Flash:
Indicates normal operation with no thermostat
calls. Standby mode
Slow Amber Flash:
Indicates normal operation with a call for heat
LED Steady Off:
If the LED light does not flash at all, check for power
to the control board and check for an open fuse on the control board. If
the control board is properly powered and the fuse is not open, the con-
trol board may need to be replaced.
Steady On Any Color:
Indicates a possible control failure. Turn the
power to the furnace off and back on. If the fault code recurs, the control
board must be replaced. The control board is not field-repairable.
Rapid Amber Flash:
Indicates the flame sense current is below 1.5
VDC. Check and clean the flame sensor. Check for proper gas flow.
Verify that the current is greater than 1.5 VDC at the flame current test
pad.
4 Amber Flashes:
Indicates the control is receiving a Y signal from the
thermostat without a G signal. The furnace operates normally during
heating and cooling, but this fault code is displayed to alert the user that
there is a wiring problem. Verify that the G wire from the thermostat is
connected properly.
1 Red Flash:
Indicates flame was sensed when there was no call for
heat. The control turns on both the inducer motor and supply air blower.
Check for a leaking or slow-closing gas valve.
2 Red Flashes:
Indicates the pressure switch is closed when it should
be open. The control confirms the pressure switch contacts are open at
the beginning of each heat cycle. The control prevents the ignition
sequence from continuing if the pressure switch contacts are closed
when they should be open. Check for a faulty pressure switch or miswir-
ing.
NOTICE
The manual reset Rollout Switch (RS) must be reset before allowing
furnace operation.
