ITT McDonnell Miller FS254 Flow Switch 1" 120610

General-purpose flow switch

The ITT McDonnell Miller FS254 is a general-purpose flow switch designed for heating, ventilation and air-conditioning systems. It detects liquid movement in a pipe and can start or stop electrically operated equipment such as pumps or burners. The device suits installers who need a compact, in-line sensor for low-flow applications. Its NEMA 4 enclosure protects the mechanism in damp or outdoor environments.

In-line installation limits

An in-line configuration removes the requirement for a separate pipe tee, reducing the number of joints and potential leak points. The switch mounts directly in the piping run, so the installer must provide a straight section of pipe that matches the connection size. Flow must be steady enough for the paddle to operate reliably; turbulent or pulsating flow can cause false actuation. The sensitivity adjusting screw allows fine-tuning, but the unit cannot compensate for pipe diameters or flow profiles outside its design range.

Capacity and headroom

The switch handles low-flow rates typical of hydronic heating loops and similar HVAC circuits. The adjustable screw lets the installer set the actuation point near the minimum detectable flow, giving headroom for normal system variations. If the application demands higher flow volumes or pressure drops beyond the switch’s calibrated range, the paddle may remain permanently actuated or fail to reset. Buyers who need to monitor large-diameter mains or high-velocity process lines should select a sensor rated for those conditions.