McDonnell & Miller 150S-M Pump Controller

The McDonnell & Miller 150S-M is a commercial-grade float-type combination low water cut-off and pump controller designed for steam boilers in high-pressure industrial applications. Engineered for systems with a maximum pressure of 150 psi, this unit features a manual reset mechanism, ensuring that the burner circuit remains locked out following a low-water event until a technician can verify the system's safety. The internal Monel bellows provides superior corrosion resistance, while the snap-acting switches are specifically rated for high-temperature service to prevent sticking and ensure reliable boiler feed pump control. This model includes a single pole, single throw (SPST) switch for pump activation and a single pole, double throw (SPDT) switch for secondary low water cut-off and alarm signaling. For contractors managing steam plants, the 150S-M (Part Number 172702) offers critical dry-fire protection and precise water level regulation, maintaining stable operation in boilers of any steaming capacity.

Key Benefits

• Mechanical Redundancy: The manual reset feature provides an essential layer of safety, requiring a physical check of the boiler before resuming firing after a low water condition.
• Industrial Durability: Equipped with a Monel bellows and high-temp snap action switches, this controller is built to withstand the rigorous cycles of pressurized steam environments.
• Versatile Signaling: Dual switch configuration allows for simultaneous pump control and integrated low water alarm or burner cutout.
• Universal Compatibility: Designed for 1-inch NPT connections, making it a drop-in replacement for older mercury-style 150 series units in most header configurations.

Common Questions

Can the manual reset be bypassed for automatic operation?
Technicians should not attempt to bypass the manual reset. If an automatic reset is required for your specific application, you should specify the standard 150S model. The 150S-M is purpose-built for applications where safety lockout is mandated by local code or facility protocol.

Is this a direct replacement for the old mercury switch 150-M?
Yes, the 150S-M (172702) is the mercury-free successor to the original 150-M. It utilizes the same casting and 1-inch NPT tapping, though the snap switch head mechanism provides more precise differential control and eliminates hazardous materials from the boiler room.

Why is my pump staying on even when the water level is high?
This is often caused by sludge buildup in the float chamber or a water-logged float. Regular blow-down of the float chamber is required to prevent sediment from obstructing the mechanical linkage. If the bellows or float are compromised, a head mechanism replacement may be necessary.

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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.