McDonnell & Miller 157S Controller

The McDonnell & Miller 157S is a heavy-duty combination low water cut-off and pump controller engineered for commercial and industrial steam and hot water boilers. This specific OEM part #173502 features the 157S series head mechanism (part #173003) equipped with a snap-acting SPDT switch. Designed for high-pressure applications up to 150 psi (10.5 kg/cm²), this controller ensures precise liquid level management, acting as a critical failsafe by interrupting the burner circuit during low water conditions while simultaneously managing boiler feed pump cycles. The float-actuated mechanism eliminated the need for complex bellows, providing more reliable performance in high-temperature environments. For contractors performing maintenance or retrofits, this unit provides a direct replacement for existing 150 series controllers, maintaining system integrity and preventing catastrophic dry-fire scenarios. Its rugged construction facilitates long-term stability in demanding hydration systems, ensuring your boiler water level control remains within strict operational parameters.

Key Technical Benefits

• Dual Functionality: Integrates primary low water cut-off protection with independent pump control to streamline boiler piping and wiring.
• Snap-Acting SPDT Switch: Provides crisp, positive electrical switching to prevent "chatter" and ensure high-voltage signal reliability for burner and pump circuits.
• High Pressure Rating: Rated for maximum steam pressure of 150 psi, making it suitable for a wide range of process steam and heating applications.
• Direct OEM Replacement: The 173502 model allows for seamless installation into existing float chambers, reducing labor time during field service calls.
• Internal Performance: Features heavy-duty internal components designed to resist scale buildup and maintain buoyancy in turbulent water conditions.

Common Questions

Is the 157S compatible with 150 series float chambers?
Yes, the McDonnell & Miller 157S head mechanism is designed to be backwards compatible with most 150 and 157 series float chambers, though port configurations should always be verified on-site.

What is the difference between the 150S and the 157S?
The 157S typically includes an integral water column with 1" NPT tappings for easier installation of gauge glass and tri-cocks, whereas the 150S is the standalone controller unit.

Can this be used for both steam and hot water boilers?
Primarily utilized in steam systems for low water protection, the 157S controller is also effective in large hot water boiler applications to ensure minimum water levels are maintained during operation.

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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.