These are the typical main components in a furnace:

• Heat Exchanger
• Burners
• Orifices
• Inducer
• High Limit ( Main Limit)
• Rollout Limit (By the burners)
• Aux Limit (typically in blower cabinet measuring return air temp)
• Gas Valve
• Inducer Pressure Switch (2 Pressure Switches on 2 Stage Units)
• Collector Box Pressure Switch (Found only on 90%+ AFUE Furnaces)
• Control Board
• Transformer
• Condensate Drain Trap
• Collector Box
• Blower Wheel
• Blower Motor ( Three Main Types: PSC, ECM Constant Torque, ECM Constant Airflow)