McDonnell & Miller FS251 Snap Switch

The McDonnell & Miller FS251 Snap Switch (Part Number 120611) is an industrial-grade liquid flow switch engineered for heavy-duty HVACR and process piping applications. Built with a NEMA 1 enclosure for indoor protection, this SPDT flow switch provides reliable mechanical flow sensing for pipe sizes ranging from 1" to 8". The unit features a standard 1" NPT threaded connection and includes a comprehensive set of 1", 2", 3", and 6" stainless steel paddles to ensure precise field adaptation. Designed for a maximum operating pressure of 160 psi and fluid temperatures up to 250°F, it serves as a robust general purpose flow switch for managing pumps, burners, and signaling devices. The integrated sensitivity adjusting screw allows technicians to fine-tune the flow actuation point, while the dual electrical knockouts facilitate flexible wiring in tight mechanical rooms. This model is the standard replacement flow switch for common legacy series, providing a high-capacity 7.4 FLA switch rating at 120 VAC.

Key Benefits

• Universal Compatibility: Includes four stainless steel paddles, making it adaptable for 1" through 8" pipe diameters on the job site.
• Precise Control: Features a single pole, double throw (SPDT) snap-action switch for versatile Normally Open (NO) or Normally Closed (NC) wiring configurations.
• Durable Construction: Equipped with an EPDM O-ring seal and Monel bellows to ensure long-term performance in hydronic heating and chilled water systems.
• Field Adjustable: The sensitivity screw provides easy calibration for specific flow velocities, preventing short-cycling of equipment.
• Broad Application: Suitable for starting or stopping boilers, circulator pumps, cooling towers, and water treatment systems.

Common Questions

What is the maximum flow velocity for the FS251?
This switch is rated for a maximum liquid velocity of 10 feet per second. Exceeding this can lead to paddle fatigue or system damage.

Can the FS251 be used in outdoor environments?
The FS251 features a NEMA 1 enclosure, which is designated for indoor use. For outdoor or wash-down applications, the NEMA 4 version is required.

Is this switch compatible with low-flow systems?
Yes, the FS251 is specifically designed for high sensitivity and can be adjusted via the internal screw to detect lower flow rates than standard general-purpose switches.

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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.