McDonnell & Miller 64-A Low Water Cut Off

The McDonnell & Miller 64-A (Part Number 143700) is a heavy-duty, mechanical low water cut-off engineered specifically for residential or commercial hot water and steam boilers. Designed with 1" NPT quick hook-up fittings, this unit simplifies installation into gauge glass tappings, facilitating proper water level monitoring without extensive repiping. Built to handle a 50 PSI maximum boiler pressure, the 64-A utilizes an automatic reset mechanism to restore burner operation once safe water levels are detected. For contractors, this LWCO for steam boilers is a standard-setting component for protecting heat exchangers from dry-fire conditions. The dual-switch functionality provides a primary circuit for the burner and an integrated alarm or feed valve circuit. Whether you are replacing a legacy low water cut-off switch or performing a new install, the McDonnell & Miller 143700 offers the rugged reliability required for high-cycle hydronic systems and steam applications.

• Steam and Hot Water Compatibility: Rated for a wide range of boiler systems with a maximum working pressure of 50 psi (3.5 kg/cm²).
• Quick Hook-Up Design: Includes 1" NPT fittings for streamlined mounting directly into water gauge tappings.
• Dual Switch Logic: Features independent switches for burner cut-off and low-water alarm or electric water feeder activation.
• Automatic Reset: Eliminates manual intervention by automatically closing the burner circuit when the float returns to a safe operating level.
• Large Float Chamber: Prevents sediment buildup from interfering with the mechanical float movement, ensuring consistent actuation.

Common Technical Questions

Can the McDonnell & Miller 64-A be used on process steam boilers exceeding 50 PSI?
No, this specific model is limited to a maximum pressure of 50 PSI. For high-pressure steam applications, a different series rated for the specific system pressure must be utilized.

What is the purpose of the dual switches in the 143700 model?
The unit contains two separate switches: one to interrupt the burner circuit in a low-water event and another that can trigger an external alarm or signal an automatic water feeder to replenish the boiler.

How often should the blow-down valve be operated on this LWCO?
To maintain reliable performance and clear sediment from the float chamber, technicians recommend performing a blow-down flush weekly during the heating season or according to the boiler manufacturer's maintenance schedule.

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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.