5715184-BIM-A-0519
30
Johnson Controls Ducted Systems
SECTION IX: SAFETY CONTROLS
CONTROL CIRCUIT FUSE
A 3-amp fuse is provided on the control circuit board to protect the 24-
volt transformer from overload caused by control circuit wiring errors.
This is an ATO 3, automotive type fuse and is located on the control
board.
BLOWER DOOR SAFETY SWITCH
This unit is equipped with an electrical interlock switch mounted in the
burner compartment. This switch interrupts all power at the unit when
the panel covering the blower compartment is removed.
Electrical supply to this unit is dependent upon the panel that covers the
blower compartment being in place and properly positioned.
ROLLOUT SWITCH CONTROLS
These controls are mounted on the burner assembly. If the temperature
in the area surrounding the burner exceeds its set point, the gas valve
is de-energized. The operation of this control indicates a malfunction in
the combustion air blower, heat exchanger or a blocked vent pipe con-
nection. Corrective action is required. These are manual reset controls
that must be reset before operation can continue.
PRESSURE SWITCHES
This furnace is supplied with two pressure switches, which monitor the
flow through the combustion air/vent piping and condensate drain sys-
tem. These switches de-energize the gas valve if any of the following
conditions are present. Refer to "CONDENSATE PIPING AND FUR-
NACE VENTING CONFIGURATION" for tubing connections.
•
Blockage of vent piping or terminal
•
Failure of combustion air blower motor
•
Blockage of combustion air piping or terminals
•
Blockage of condensate drain piping
LIMIT CONTROLS
There is a high temperature limit control located on the furnace vesti-
bule panel near the gas valve. 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
The control module locks out if the limit trips 5 consecutive times. If this
occurs, the control resets and tries ignition again after 1 hour.
SECTION X: NORMAL OPERATION AND
DIAGNOSTICS
NORMAL OPERATION SEQUENCE
The following describes the sequence of operation of the furnace. Refer
to Owners Manual for component location.
Continuous Blower
Cooling/heating thermostats have a fan switch that has an ON and
AUTO position. In the ON position the thermostat circuit is completed
between terminals R and G. The motor operates continuously on the
speed tap wire that is connected to the HI COOL cooling terminal on the
control board. To obtain a constant air circulation at lower flow rate,
change the high-speed wire to another low-speed wire.
Intermittent Blower - Cooling
Cooling/heating thermostats have a fan switch that has an ON and
AUTO position. In the AUTO position the thermostat circuit is completed
between terminals R and G when there is a call for cooling. The motor
operates on the speed tap wire that is connected to the HI COOL cool-
ing terminal on the control board. The fan-off setting is fixed at 60 sec-
onds to improve cooling efficiency.
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 proper amount of combustion air is
provided, the pressure switch closes and the ignition control provides a
17-second 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 seconds 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 seconds and the
blower motor operates for the amount of time set by the fan-off delay
jumper located on the control board. See Figure 33. The heating cycle
is now complete, and ready for the start of the next heating cycle.
If the flame is not detected within 7 seconds of the gas valve opening,
the gas valve is shut off and a retry operation begins. Also, if the flame
is lost for 2 seconds during the 10-second stabilization period, the gas
valve is shut off and a retry operation begins. During a retry operation,
the vent motor starts a 15-second inter-purge and the ignitor warm-up
time is extended to 27 seconds. If the flame is established for more than
10 seconds 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 one hour. If there is a call for heat at the end of
the one-hour shut down, the furnace initiates a normal start cycle. If the
problem has not been corrected, the furnace locks out again 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 1.0 sec-
onds. The gas valve de-energizes and the control begins a recycle
operation. A normal ignition sequence begins after a 15-second 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-
utes.
During burner operation, a momentary loss of power for 50 milliseconds
or longer de-energizes the gas valve. When the power is restored, the
gas valve remains de-energized and the ignition sequence immediately
restarts.
IGNITION SYSTEM SEQUENCE
1.
Turn the gas supply ON at external valve and main gas valve.
2.
Set the thermostat above room temperature to call for heat.
3.
System start-up occurs as follows:
a.
The induced draft blower motor starts and comes up to
speed. Shortly after inducer start-up, the hot surface igniter
glows for about 17 seconds.
b.
After this warm up, the ignition module energizes (opens) the
main gas valve.
c.
After flame is established, the supply air blower starts within
approximately 30 seconds.
TROUBLESHOOTING
The following visual checks must be made before troubleshooting:
1.
Check to see that the power to the furnace and the ignition control
module is ON.
2.
The manual shut-off valves in the gas line to the furnace must be
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.
CAUTION
Main power to the unit must still be interrupted at the main power
disconnect switch before any service or repair work is to be done to
the unit. Do not rely upon the interlock switch as a main power dis-
connect.
Blower and burner must never be operated without the blower
panel in place.
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