The MARS 39220 Limit Switch is a versatile, high-reliability solution for field service technicians needing an exact or universal replacement for furnace high limit applications. This adjustable snap disc thermostat features a temperature range of 135°F to 175°F with a fixed 40°F differential, allowing for precise calibration to match specific internal plenum requirements. Designed as an open on rise safety device, the 39220 ensures the heating circuit is interrupted if temperatures exceed the set point, preventing heat exchanger damage. Integrated with a universal mounting flange (2-hole oval) and 1/4-inch quick-connect terminals, this unit simplifies installation across a variety of residential and commercial HVAC platforms. Contractors can utilize this SPST limit control to reduce inventory, as its adjustable dial effectively replaces multiple fixed-temperature discs. Rated for 10 Amps at 240 Vac, it provides robust switching performance in demanding HVAC high limit scenarios where consistent thermal monitoring is critical for system safety and operational longevity. Key Benefits Adjustable Temperature Range: Eliminates the need to carry multiple fixed-limit discs by providing a customizable 135-175°F set point. Reliable Safety Interface: Features a snap-acting SPST switch that opens on temperature rise to prevent furnace overheating. Easy Installation: Equipped with an integral oval flange and standard 1/4-inch terminals for fast field wiring. Universal Compatibility: Serves as a direct replacement for various OEM controls including White-Rodgers 3L05-1 and Gem AT-012. Common Questions How do I set the correct temperature on the MARS 39220? The unit features an integrated indicator dial on the control face. Use a small screwdriver or the built-in tab to rotate the dial to the specific cut-out temperature required by the equipment manufacturer. What are the symptoms of a failing limit switch? Technicians typically look for furnace short cycling, the blower fan running continuously, or the system failing to initiate the burner circuit due to an "open" safety loop. Can this be used for fan control? No, this is an open on rise switch designed specifically as a high-limit safety. For fan-on applications, an "increase to close" or dedicated fan/limit controller is required. 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.
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