McDonnell & Miller FS254 Snap Switch

The McDonnell & Miller FS254 Snap Switch (Part Number 120610) is a heavy-duty, general-purpose liquid flow switch engineered for high-reliability monitoring in HVACR and industrial hydronic systems. This unit is the direct replacement for the legacy FS-254 and offers a field-proven solution for starting or stopping electrically operated equipment including automatic burners, circulator pumps, and system alarms. Built with a NEMA 4 enclosure, the FS254 provides a watertight and dust-tight seal, making it ideal for mechanical rooms or outdoor installations where environmental protection is mandatory.

For maximum field flexibility, the FS254 flow switch features a standard 1" NPT connection and includes a complete kit of four stainless steel paddles (1", 2", 3", and 6"), allowing for precise fitment in pipe sizes from 1" to 8". The integrated SPDT snap action switch is rated for 7.4 FLA at 120 VAC and 3.7 FLA at 240 VAC, ensuring robust performance under pilot duty loads. Technicians can easily calibrate flow sensitivity using the external sensitivity adjusting screw to meet specific system GPM requirements. With a maximum operating pressure of 160 psi and a fluid temperature range up to 250°F, this paddle flow switch is the industry standard for protecting boilers and chillers from low-flow conditions.

• Environmental Protection: The NEMA 4 housing allows for reliable operation in wash-down areas or outdoor HVAC applications.
• Universal Application: Includes four stainless steel paddles to accommodate various pipe diameters with a single SKU.
• Direct Replacement: Seamlessly replaces the McDonnell & Miller FS-254 and FS8-W models without piping modifications.
• Precision Control: Features a high-quality SPDT snap switch and EPDM O-ring for a leak-proof, responsive flow sensing.
• Field Adjustable: On-site sensitivity adjustment allows for fine-tuning the trip point based on actual system velocity.

What is the difference between the FS251 and FS254?
The primary difference is the electrical enclosure; the FS254 utilizes a NEMA 4 watertight enclosure, whereas the FS251 is rated for NEMA 1 indoor use only.

Can this switch be used in cooling tower applications?
Yes, the FS254 is frequently used in cooling towers and chiller systems to verify flow before enabling the compressor, provided the fluid is non-hazardous and within the 250°F temperature limit.

How do I adjust the flow rate sensitivity?
Under the enclosure cover, locate the sensitivity adjusting screw. Turning the screw clockwise increases the flow rate required to actuate the snap switch, while counter-clockwise decreases the required flow.

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.