SECTION X: 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
Main power to the unit must still be interrupted at the main power dis- connect 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 discon- nect.
Blower and burner must never be operated without the blower panel in place.
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 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 SECTION VII, "CONDENSATE PIPING AND FURNACE VENTING CONFIGURATION" for tubing connections.
1.Blockage of vent piping or terminal.
2.Failure of combustion air blower motor.
3.Blockage of combustion air piping or terminals.
4.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:
1.A dirty filter.
2.If the indoor fan motor should fail.
3.Too many supply or return registers closed or blocked off.
The control module will lockout if the limit trips 5 consecutive times. If this occurs, control will reset & try ignition again after 1 hour.
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 will operate on the speed tap wire that is connected to the “HI COOL” cooling terminal on the control board. The fan off setting is fixed at 60 seconds 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 being provided, the pressure switch will close, the ignition control pro- vides 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 will energize 30 seconds after the gas valve opens, if a flame is detected. Normal furnace operation will continue 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 will operate for 15 seconds and the blower motor will operate for the amount of time set by the fan- off delay jumper located on the control board. See Figure 41. The heat- ing 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 will clear the ignition attempt (retry) counter. If three retries occur during a call for heat, the furnace will shut down for one hour. If at the end of the one hour shut down there is a call for heat, the furnace will initiate a normal start cycle. If the problem has not been corrected the furnace will again lockout after three retries.
A momentary loss of gas supply, flame blowout, or a faulty flame probe circuit will result in a disruption in the flame and be sensed within 1.0 seconds. The gas valve will de-energize and the control will begin a recycle operation. A normal ignition sequence will begin after a 15 sec- ond inter-purge. If during the five recycles the gas supply does not return, or the fault condition is not corrected the ignition control will lock- out for 60 minutes.
During burner operation, a momentary loss of power for 50 milliseconds or longer will de-energize the gas valve. When the power is restored, the gas valve will remain de-energized and the ignition sequence will immediately restart.
TROUBLESHOOTING
The following visual checks should 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.
