OEM Lennox Limit Switch — 33H78

The genuine OEM Lennox 33H78 limit switch is a high-limit safety control that protects your furnace from overheating. Mounted near the heat exchanger, it continuously senses supply-air temperature and opens the circuit to shut off the burners if the plenum gets too hot — typically caused by restricted airflow. Once temperatures return to a safe range, the switch resets and normal operation resumes.

Common failure symptoms include short cycling on heat, the burners shutting off while the blower keeps running, no heat despite the fan operating, or a furnace that trips out repeatedly. A limit switch stuck open causes no ignition, while one stuck closed defeats the safety function and can lead to dangerous overheating and heat exchanger damage.

Why OEM matters: The 33H78 is engineered with a precise temperature set point matched to your specific Lennox furnace. Using the correct factory switch guarantees the burners cut off at the safe design temperature, protecting the heat exchanger and your home. Aftermarket substitutes with incorrect trip temperatures can cause nuisance shutdowns or, worse, fail to protect the unit. Install the OEM 33H78 for dependable overheat protection.