McDonnell & Miller 64-HD Cut Off

The McDonnell & Miller 144400, also known as the 64-HD Head Mechanism, is a heavy-duty replacement unit designed for Series 64 and Series 764 low water cut-offs. Engineered for both hot water and steam boilers, this mechanical oversight component facilitates an auto-reset function to ensure continuous system protection without manual intervention. As a float-type LWCO mechanism, it is rated for a maximum boiler pressure of 50 PSI. This unit serves as the internal operating assembly, allowing contractors to perform a quick-change replacement by simply unbolting the existing head from the float chamber, which eliminates the need to cut into or disrupt the system's 1-inch NPT equalizing pipes.

Built with a packless bellows and high-performance dual precision switches (including the No. 11 switch), the 64-HD provides dependable signaling for burner circuits and external alarms or electric water feeders. Its robust construction makes it a standard for residential and commercial boiler maintenance, offering a reliable solution for aging mechanical low water cut-offs where the original float chamber remains intact. This mechanism ensures accurate water level sensing and dry-fire prevention in high-demand hydronic and steam applications.

• Drop-in Compatibility: Specifically designed as a replacement head mechanism for McDonnell & Miller 64 Series and 764 Series units.
• Streamlined Installation: Designed for an easy bolt-on replacement that avoids the labor costs of repiping the entire boiler safety control.
• Dual Switch Logic: Features integrated No. 11 auto-reset switches to manage both the burner circuit and an optional alarm or electric water feeder.
• Industrial Grade Durability: Equipped with a packless bellows seal to prevent leaks and ensure long-term mechanical reliability at 50 PSI max pressure.
• Field Serviceable: Allows for immediate restoration of low water cut-off functionality while reusing the existing cast iron float bowl.

Is the float assembly included with the 64-HD head mechanism?
Yes, the McDonnell & Miller 144400 includes the float assembly, head gasket, and the internal switch mechanism required for a full head-unit overhaul.

Can I use this 64-HD mechanism to replace a manual reset unit?
The 64-HD is an auto-reset head mechanism. If you are replacing a manual reset (Series 64-M) unit, verify your local code requirements and wiring configuration, as this model will automatically restart the burner once water levels return to safe operating heights.

What is the maximum pressure rating for this LWCO part?
The McDonnell & Miller 144400 is rated for a maximum boiler pressure of 50 PSI (3.5 kg/cm²), making it suitable for most low-pressure steam and hot water heating systems.

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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.