McDonnell & Miller 7-BM Switch

The McDonnell & Miller 7-BM Switch (Part Number 110900) is a heavy-duty, potentiometer proportional low water cut-off switch specifically built for Series 93, 193, 94, and 194 pump controllers. Designed for boiler systems requiring modulated feedwater control, this manual reset switch utilizes a 0-135 ohm slide wire resistor to provide a continuous signal to a proportional domestic or industrial modulating water valve. The single pole double break configuration ensures precise multi-function operation, acting as an alarm trigger or valve actuator while providing critical low water cut-off (LWCO) protection. For steam applications up to 150 PSI or 250 PSI depending on the head mechanism, the 7-BM eliminates the need for bellows through its magnetic repulsion technology, preventing common failure points in high-pressure environments. When the water level reaches the cast line, the burner circuit breaks, requiring a manual technician intervention to restore operation, ensuring the highest level of safety monitoring for commercial steam boilers.

• Precise Modulation: The 0-135 ohm potentiometer allows for constant boiler water level maintenance by providing a variable signal to the feedwater valve rather than simple on-off cycling.
• Enhanced Safety: The manual reset feature ensures the system remains locked out after a low-water event, forcing a manual inspection of the boiler before restart.
• Internal Reliability: Magnetic repulsion operation removes the mechanical stress of bellows, extending the service life of the switch assembly in high-temperature steam applications.
• Direct OEM Fit: Designed as a drop-in replacement for McDonnell & Miller 110900, ensuring zero-clearance issues with Series 193 and Series 194 head mechanisms.

• How do I verify the modulation range during commissioning? Check the resistance across the slide wire; the valve should be closed at 2-11/16" above the cast line and full-open as the water level drops toward the 7/8" mark.
• Can I swap an auto-reset 7-B for this 7-BM? Yes, the 7-BM is physically compatible but adds the required manual reset safety protocol often mandated by local boiler codes for larger steam plants.
• Why is my burner not firing after the water level is restored? Confirm the manual reset button has been depressed. If the circuit remains open, verify the wiper arm alignment on the potentiometer slide wire.

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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.