Furnace Sequence of Operation

Introduction

• Importance of understanding the furnace sequence of operation.
• Initial steps when the furnace isn't working:
  • Check the sight glass on the bottom door for error codes.
  • Note any blinking error codes before taking off the doors.

Preliminary Steps

• Turn off the furnace power switch.
• Understand that removing the doors may reset error codes.
• Explanation of switches:
  • Power Switch: Located on the furnace.
  • Door Switch: Automatically interrupts power when the door is removed.

Error Codes

• Error codes may be displayed on the control board or on the door's back.
• Different furnaces have varying error codes and diagnostic charts.

Components of a Furnace

• Gas Valve: Allows gas flow when energized.
• Pressure Switches: Different for low and high stages.
• Inducer Motor: May be single or two-stage.
• Burners and Igniter: Burners ignite the gas, igniter starts the process.
• Flame Sensor: Ensures flame presence.
• Control Board: Directs power flow and process.
• Blower Housing and Motor: Distributes heated air.

Sequence of Operation

1. Thermostat Call for Heat:
   • Power flows from the breaker to the control board.
   • Thermostat signals demand for heat, initiating furnace operation.
2. Inducer Motor Activation:
   • Starts to create draft through burners and exhaust.
   • Pressure switches prove draft presence.
3. Igniter and Gas Valve:
   • Igniter glows, gas valve opens, burners light.
   • Flame Sensor detects flame to keep burners on.
4. Blower Motor:
   • Engages after pre-heating to distribute warm air.
5. Cycle Completion:
   • Upon thermostat satisfaction, reverses: gas valve de-energizes, burners off, blower cools furnace.

Diagnosing Issues

• Igniter glowing, no gas: Check gas valve, power, or gas supply.
• Inducer motor running, igniter not glowing: Check igniter or pressure switches.

Simulating Problems

• Demonstrated issues like blocked pressure switch hose or dirty flame sensor.
• Explanation of furnace attempts to relight before soft lockout.

Importance of Manuals

• Manuals provide detailed operation sequences and troubleshooting guides.
• Example from Lennox 80 furnace manual:
  • Normal vs. Abnormal Heating Mode: Outlines furnace checks and responses to irregularities.

Conclusion

• Importance of manuals and understanding the sequence for troubleshooting.
• Encouragement to consult manuals for deeper understanding and diagnostics.

Remember to consult the furnace manual for specific error codes and detailed operations specific to your model. It provides valuable insights for troubleshooting and understanding your furnace's operation.