White-Rodgers 3L09-4 Limit Switch

The White-Rodgers 3L09-4 Limit Switch is a 1/2-inch bimetal disc control engineered for critical thermal protection in residential furnaces and air handling units. Designed for high-speed contact separation, this SPST board mount limit switch functions on a fixed thermal profile, featuring a 150-degree F open temperature and a 130-degree F close temperature. With a 3.12-inch insertion length, this plenum high limit control ensures the sensing element is positioned directly in the airflow for rapid response to overtemperature conditions. As an auto reset HVAC limit switch, it restores the circuit once temperatures drop below the 20-degree differential threshold. This component is a direct OEM replacement for Rheem/Ruud 47-21711-19 and various Therm-O-Disc 36T series models, providing technicians with a reliable solution for restoring system safety and preventing heat exchanger damage during blower failure or airflow restriction.

• Precise Thermal Calibration: Fixed 150°F trip point ensures consistent burner cutout before reaching dangerous internal temperatures.
• Optimized Insertion Depth: The 3.12" probe length allows the bimetal disc to accurately sample air temperature in diverse plenum configurations.
• Broad Compatibility: Serves as a versatile HVAC replacement part for multiple 1/2" bimetal disc board-mounted applications.
• High-Speed Switching: Snap-action contacts minimize electrical arcing and promote long-term switch reliability under inductive loads.
• Automatic System Recovery: Integrated auto-reset functionality eliminates the need for manual intervention once system cooling occurs.

What causes a furnace limit switch to trip repeatedly?
Repeated tripping usually indicates restricted airflow, often caused by a dirty air filter, blocked return vents, or a failing blower motor. If the switch opens at 150°F, it is performing its safety function to prevent overheating.

Can I use the 3L09-4 to replace a switch with a different temperature rating?
No. For safety and proper equipment operation, always match the original manufacturer’s open and close temperature specifications exactly.

How do I verify if my limit switch is defective?
Using a multimeter, technicians should check for continuity across the terminals when the switch is cool. If the switch remains open at room temperature, the internal bimetal disc or contacts have likely failed.

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.