McDonnell & Miller 309200 Switch

The McDonnell & Miller 309200 is an industrial-grade 11-MV Millivolt Switch specifically engineered for low water cut-off (LWCO) control in steam boiler applications. This millivolt replacement switch is the standard for technicians servicing McDonnell & Miller Series 61, 64, 67, 69, 70, 764, and 767 units. Featuring a Two SPST Auto Reset configuration, the 11-MV provides dual-circuit control, allowing contractors to manage both the burner circuit and an optional high-limit alarm or electric water feeder independently. Unlike standard line-voltage switches, the Model 11-MV is optimized for millivolt-potential gas valves and 24VAC control systems, ensuring precise signal continuity without high-resistance interference. Built with a dual pole double break mechanism, this auto-reset boiler switch eliminates the need for manual intervention after water levels return to safe operating parameters, maintaining system uptime and protecting the pressure vessel from dry-fire conditions. For the veteran installer, the 309200 switch offers a direct-fit replacement to restore low water cut-off safety with verified OEM reliability.

Key Benefits

• Broad Compatibility: Direct replacement for McDonnell & Miller Series 67, 61, 64, and 764 LWCO controls.
• Millivolt Optimized: Specifically designed for millivolt gas valves and 24V burner circuits to prevent signal loss.
• Dual Circuit Control: Features Two SPST switches to handle both burner shutdown and water feeder activation.
• Automatic Reset: Resumes normal operation automatically once the water level rises above the cut-off point.
• Snap-Action Reliability: High-precision snap switch mechanics prevent arcing and ensure a clean break in the electrical circuit.

Common Questions

Can I use the 11-MV switch to replace a standard No. 11 switch?
Yes, the 11-MV (309200) shares the same mounting footprint as the standard No. 11 but features specialized contacts for millivolt or 24V service. It is often used in systems with standing pilots where millivolt signal integrity is critical.

How do I identify this switch in the field?
The McDonnell & Miller 11-MV is easily identified by its red terminal panel, whereas the standard No. 11 and 11M (manual reset) versions typically feature a black terminal panel.

Does this switch support manual reset functionality?
No, the 309200 is an auto-reset model. If your local code or specific boiler application requires a manual reset after a low-water event, you would require the 11-M or similar latching variant.

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.