McDonnell & Miller 51-S-2 Switch

The McDonnell & Miller 51-S-2 is a heavy-duty mechanical water feeder and low water cut-off (LWCO) designed for low-pressure steam and hot water boilers. Engineered for systems with a maximum vessel pressure of 35 psi, this unit features a snap-acting SPDT switch that ensures positive electrical contact for burner control or alarm signaling. The 1" NPT connections allow for straightforward integration into existing boiler piping, providing a reliable steam boiler water level control solution. Unlike electronic probes that may foul, this float-operated mechanical water feeder relies on physical buoyancy to maintain precise water levels, preventing dry-fire conditions in residential and light commercial applications.

• Automatic Water Level Maintenance: The McDonnell & Miller 135900 replaces water lost through evaporation or leaks, ensuring consistent pressure.
• Dual Function Design: Acts as both a feeder and a low water cut-off to protect heat exchangers from thermal shock.
• Snap Acting SPDT Switch: Provides instantaneous response to water level fluctuations, minimizing contact arcing.
• 35 PSI Maximum Vessel Pressure: Rated specifically for low-pressure hydronic and steam heating systems.
• Field Serviceable: Designed for long-term maintenance with accessible internal components for routine inspections.

Common Technical Questions

What is the maximum steam pressure for the 51-S-2?
This mechanical feeder is rated for a maximum steam pressure of 35 psi (2.5 kg/cm2) and a maximum water pressure of 150 psi.

Can the 51-S-2 be used for both feeder and cut-off functions?
Yes, the integrated SPDT switch allows for simultaneous control of a feeder valve and a burner circuit or low-water alarm.

Is this compatible with the standard 51 series?
The 51-S-2 is a specific configuration; always verify the part number 135900 to ensure the correct switch assembly and connection sizes for your boiler.

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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.