McDonnell & Miller 157S-M Pump Controller

The McDonnell & Miller 157S-M (Part Number 175402) is a professional-grade combination low water cut-off and pump controller designed for commercial and industrial steam boiler applications. This model is engineered with a built-in water column and includes an integral manual reset mechanism, a critical safety requirement for many jurisdictional codes. By utilizing a durable monel bellows, the unit maintains high corrosion resistance and high-temperature performance, ensuring reliable operation up to a maximum pressure of 150 PSI. The internal snap-acting SPDT switch provides crisp, positive electrical contact for both pump control and low-water alarm/cut-off circuits, effectively preventing the common contact "chatter" found in inferior water level controls.

As a veteran's choice for boiler water level control, the 157S-M functions as a primary or secondary safety device. It features 1-inch NPT equalizing tappings and additional 1/2-inch NPT ports for the installation of gauge glasses and tri-cocks. This hardware is compatible with a wide range of steaming capacities and is frequently used to replace legacy mercury-switch models with modern, mercury-free snap switches. For any technician managing heavy-duty steam systems, this controller represents the standard for maintaining precise water levels and preventing dry-fire conditions.

Key Benefits

• Includes an integral water column to simplify piping and provide stable float action in turbulent steam environments.
• Equipped with a manual reset switch ensuring the burner remains off until an operator inspects the system after a low-water event.
• Constructed with a corrosion-resistant monel bellows for extended service life in harsh industrial scale and chemical conditions.
• Features snap-acting switches that eliminate nuisance cycling and provide distinct signals for both the pump and the burner cut-off.
• Universal compatibility with commercial steam boilers up to 150 PSI, supporting a wide range of steaming capacities.

Common Questions

Can the 157S-M be used on hot water boilers?
While primarily designed for steam applications due to the float and column configuration, it is rated for high-pressure environments. However, for dedicated hot water systems, a probe-style or specific 63 series LWCO may be more common depending on the piping requirements.

What is the difference between the 157S and the 157S-M?
The "M" designation identifies the manual reset feature. Unlike the standard 157S which resets automatically when the water level is restored, the 157S-M requires a technician to physically depress the reset button, providing an extra layer of safety to ensure the cause of the low water condition was addressed.

Does this unit replace older mercury switch models?
Yes, the 157S-M is the direct environmental and technical replacement for discontinued mercury-style controllers. It uses bi-metal snap switches that occupy the same footprint and provide superior vibration resistance.

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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.