McDonnell & Miller 64-B Low Water Cut Off

The McDonnell & Miller 64-B Low Water Cut Off (Part Number 143800) is a heavy-duty mechanical safety control engineered for both hot water and steam boilers. As a veteran-grade float-type LWCO, this unit provides essential protection against dry firing by interrupting the burner circuit if the water level drops below a safe threshold. It features a robust 1" NPT connection and is rated for a maximum boiler pressure of 50 PSI. The integration of dual precision switches allows for simultaneous control of the burner and an auxiliary alarm or electric water feeder, ensuring comprehensive system monitoring. Designed with a packless bellows and an automatic reset function, this mechanical low water cut-off maintains reliable operation in demanding residential and commercial environments where constant vigilance is required to protect the boiler vessel. The included float block stabilizes the mechanism against turbulence, providing the precise level detection necessary for high-performance hydronic and steam systems.

• Universal Application: Compatible with any steaming capacity for steam or hot water boilers.
• Dual Switch Functionality: Operates burner and alarm/feeder independently for enhanced system safety.
• Heavy-Duty Construction: Features a packless bellows and durable float block for long-term field reliability.
• Standardized Connections: 1" NPT equalizing pipes ensure a straightforward swap or new installation on standard manifolds.
• Variable Voltage: Compatible with 120VAC or 240VAC electrical configurations.

Common Questions

How do I troubleshoot a burner that won't shut off during low water?
Usually, this indicates sediment or scale build-up in the float chamber. You must perform a blow down procedure to clear the chamber. If the float is restricted by "mud," it cannot drop to trigger the switch.

What is the purpose of the 143800 float block?
The float block is designed to reduce the effects of water surges and turbulence inside the chamber, preventing rapid cycling of the burner and ensuring more accurate low water cutoff performance.

Can this be used as a primary or secondary safety?
Yes, the Series 64-B is frequently used as a primary LWCO, but it is also ideal as a redundant secondary safety on systems with high BTU inputs or higher operating pressures where safety codes require dual controls.

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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.