McDonnell & Miller 5-M Switch

The McDonnell & Miller 5-M Switch (Part Number 110400) is a critical safety component designed for heavy-duty steam boiler control. This manual reset general purpose switch is engineered specifically to interface with Series 93, 193, 94, and 194 combination low water cut-off and pump controllers. Featuring a 2-SPDT (dual single-pole, double-throw) configuration, the 5-M allows for the simultaneous management of independent circuits—typically the burner circuit and a boiler feed pump or high-water alarm. Operating effectively at 120Vac or 240Vac, this switch provides the mechanical intervention required for secondary low water protection, ensuring the burner remains locked out until a technician verifies the water level. The high volt-ampere rating of 345 VA ensures reliable switching performance even in demanding commercial boiler rooms. For contractors maintaining float-actuated controls, the 110400 is the standard for restoring precise low water cut-off functionality and protecting the pressure vessel from dry-fire conditions.

• Dual Circuit Logic: The 2-SPDT switch design enables independent control over pump activation and burner shutdown from a single float mechanism.
• Manual Reset Safety: Prevents automatic burner restart after a low water event, forcing a manual inspection of the boiler water level.
• Direct OEM Replacement: Drop-in compatibility for McDonnell & Miller Series 93 and 194 head mechanisms, ensuring original calibration is maintained.
• High Electrical Capacity: Rated for 345 VA at 120/240Vac, providing robust pilot duty performance for motor starters and gas valves.
• Field-Proven Reliability: Heavy-duty construction designed to withstand the high-temperature environment of commercial steam systems.

How do I wire a McDonnell & Miller 5-M switch for a dual-voltage system?
The 110400 is rated for dual-voltage applications (120Vac or 240Vac). Always use the common, normally open, and normally closed terminals as identified on the terminal panel to match your specific burner circuit or pump starter requirements.

Can I replace an auto-reset No. 5 switch with this manual reset 5-M model?
Yes, the 5-M can physically replace a standard No. 5 switch on compatible Series 93 or 193 units to upgrade the system to manual reset functionality, which is often required for secondary LWCO devices by local codes.

What are the symptoms of a failing boiler pump control switch?
Erratic burner cycling, failure of the pump to activate at the designated water line, or the switch failing to lock out after a blow-down test typically indicate worn contacts or mechanical binding within the switch assembly.

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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.