McDonnell & Miller 42S-HD Controller

The McDonnell & Miller 42S-HD Controller is a high-performance head mechanism only designed for seamless integration into existing Series 42-S float chambers. This replacement head mechanism is engineered for technicians who need to restore steam boiler safety without the labor-intensive process of breaking down equalizing piping. Featuring SPST snap action switches, this controller provides high-reliability low water cut off protection and precise pump controller functionality. The inclusion of a Monel bellows ensures superior corrosion resistance against harsh boiler feedwater chemicals, while the enclosed junction box protects electrical terminations from ambient moisture and heat. Rated for a maximum pressure of 50 psi, the 42S-HD is the industry standard for maintaining a safe water level in residential, commercial, and industrial low-pressure steam systems. By replacing the entire head assembly, you effectively refresh all moving internal components, including the float-operated pump control and burner cut-off switches, ensuring the system meets original field specifications.

• Direct Replacement: Fits existing Series 42-S bodies, allowing for rapid head mechanism swaps without disturbing gauge glass or pipe connections.
• Snap-Switch Technology: Utilizes non-mercury SPST snap action switches for cleaner, more responsive electrical breaks and long-term cycling.
• Heavy-Duty Construction: Features Monel bellows for enhanced durability in diverse water conditions and steam environments.
• Broad Compatibility: Suitable for steam boilers of any steaming capacity requiring combination low water cut-off and pump control.
• Streamlined Maintenance: Replacing the head assembly is the most efficient way to resolve internal component wear or damaged switch assemblies.

Can I use the 42S-HD to replace an older mercury switch 42-HD?
Yes, the 42S-HD is the designated upgrade path for older 42 series controllers originally equipped with mercury switches. It mounts directly to the existing float chamber.

Does this unit include the float chamber body?
No, the 42S-HD is a head mechanism only. It includes the internal mechanical and electrical components, the head plate, and the gasket, but requires an existing bell or float chamber for installation.

What are the electrical ratings for the pump and burner circuits?
The snap action switches are rated for 7.4A at 120VAC and 3.7A at 240VAC. For motors exceeding 1/3 HP, a starter or relay must be utilized to prevent switch damage.

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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.