Lennox 66J78 Limit Switch

Experienced HVAC contractors recognize the Lennox 66J78 Limit Switch as a critical safety component for maintaining furnace integrity and preventing heat exchanger damage. This OEM Lennox replacement part is a 36T series disc-style control designed for High-Limit or Secondary Rollout applications. It features a Normally Closed (NC) circuit that opens on temperature rise to interrupt the heating cycle if internal temperatures exceed safe operating parameters. Specifically, this 310-350F AUTO Limit Switch is engineered to actuate (cut-out) at 350°F and includes an automatic reset function that closes the circuit (cut-in) once the temperature drops to 310°F. Since the Lennox 66J78 is identical to 66J7801, it offers seamless compatibility across a wide range of Lennox, Armstrong, and Ducane gas furnaces. Utilizing a genuine furnace high limit sensor ensures precise thermal response and long-term system reliability in demanding residential and commercial heating environments.

• Precise Thermal Protection: Fixed 350°F trip point prevents runaway overheating in the heat exchanger and burner assembly.
• Automatic Reset Logic: Eliminates manual intervention by resetting at 310°F, allowing the system to resume normal operation once cooled.
• OEM Compatibility: Exact drop-in replacement for part numbers 66J7801, 67K55, 67K5501, and 36TX21.
• Robust Disc Construction: Reliable 36T series design provides consistent switching cycles for high-cycle heating applications.

• What causes a 350F limit switch to trip? Frequent tripping usually indicates restricted airflow, such as a dirty filter, blocked return, or a failing blower motor.
• Is the 66J78 interchangeable with the 66J7801? Yes, these part numbers refer to the exact same technical specification and are fully interchangeable.
• Can this be used as a rollout switch? While it functions as a limit control, always verify your specific furnace wiring diagram to ensure the temperature rating matches the original rollout or secondary limit specs.

Disclaimer: While we strive for 100% accuracy, product data and specifications are subject to change. Please contact our team to confirm compatibility or technical details before purchasing.