The MARS 39004 is a professional-grade 390 Series SPDT Changeover Switch designed for automatic seasonal transition in hydronic heating and cooling systems. Specifically engineered for fan-coil units, this summer-winter changeover switch manages thermostat logic based on water temperature, ensuring seamless climate control without manual intervention. It features a high-reliability SL-8 thermostat enclosed in a sealed case for environment protection and consistent snap-acting disc performance. The unit is rated for 120/240V applications and mounts directly to 5/8-inch OD tubing using the included mounting clip. This HVAC changeover control provides accurate temperature repeatability and is built to replace industry-standard components like the Honeywell L6076A and White Rodgers 752-1. With 12-inch color-coded leads and a universal mounting design, it simplifies installation in tight equipment cabinets while maintaining UL and CSA listings for code compliance in residential and commercial mechanical rooms. Key Benefits Universal Compatibility: Direct drop-in replacement for Honeywell L6076A and White Rodgers 752-1 models. Precise Switching: Utilizes a snap-acting disc for immediate, chatter-free transition between heating and cooling circuits. Durable Construction: Features a sealed SL-8 thermostat case to prevent moisture and debris from fouling electrical contacts. Flexible Mounting: Can be installed in any orientation on 5/8" piping to accommodate complex fan-coil piping layouts. Simplified Wiring: 12-inch color-coded leads reduce field errors and speed up integration with existing low-voltage thermostats. Common Questions What is the purpose of an SPDT changeover switch in hydronic systems? In fan-coil applications, the MARS 39004 senses the temperature of the water in the supply line. It automatically switches the thermostat's internal circuit to the correct mode (heating or cooling) based on whether the central chiller or boiler is active. Can this switch be used for high-voltage applications? Yes, the MARS 39004 is rated for 120/240V electrical service, making it suitable for a wide range of line-voltage fan-coil controls and relay signals. Does it include the hardware needed for installation? Each 390 Series switch kit comes standard with a spring-loaded mounting clip designed to secure the sensor cup firmly against standard copper piping for accurate thermal transfer. 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.
