McDonnell & Miller FS-1 Switch

For contractors managing low-flow hydronic or process systems, the McDonnell & Miller FS-1 Liquid Flow Switch (Part Number 113200) is the industry standard for high sensitivity flow sensing. This 1/2" NPT in-line flow switch is engineered specifically for applications where traditional paddle-style switches fail to detect minimal movement, such as in high-efficiency heating, air conditioning systems, and viscous fluid lines. Featuring a snap-acting SPDT switch, the FS-1 provides reliable pump control and equipment protection by making or breaking an electrical circuit the moment flow starts or stops.

The McDonnell & Miller 113200 utilizes an in-line configuration that eliminates the need for a pipe tee, simplifying labor during HVAC flow switch installation. Its sealed switch compartment protects internal electrical components from liquid contact, ensuring long-term performance in demanding hydronic system monitoring environments. With an externally accessible sensitivity adjusting screw, technicians can fine-tune the setpoint to meet precise low flow rate detection requirements without dismantling the housing. This unit is rated for pressures up to 100 psi and fluid temperatures reaching 225°F, making it a rugged choice for mechanical contractors across commercial and industrial sectors.

• Enhanced Sensitivity: Specifically designed to detect low and moderate flow rates that larger switches overlook.
• Simplified Piping: Direct in-line 1/2" NPT connections reduce the footprint and complexity of the install.
• Application Versatility: Compatible with water, fuel oil, and various viscous liquids used in process work.
• Reliable Switching: The SPDT snap-acting mechanism ensures crisp electrical transitions for 120V or 240V circuits.
• Field Adjustable: Easy-to-reach adjustment screw allows for rapid calibration to specific GPM setpoints.

• How do I adjust the sensitivity on the FS-1? Use the adjustment screw located on the housing. Turning it clockwise increases the flow rate required to activate the switch, while counter-clockwise decreases it.
• Can this switch be installed in any orientation? For maximum reliability, the FS-1 should be installed in a horizontal run with the arrow on the casting following the direction of fluid flow.
• Is the FS-1 suitable for potable water? This specific model is primarily intended for general-purpose heating, cooling, and process fluids; check local codes regarding lead-free requirements for potable systems.
• What is the maximum pressure rating? The unit is built to withstand operating pressures up to 100 psi (7 kg/cm²).

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.