McDonnell & Miller 51-S-2-HD Cut Off

The McDonnell & Miller 51-S-2-HD (Part Number 136200) serves as a heavy-duty head mechanism only for professional-grade water level control. Engineered for high-capacity low pressure steam and hot water boiler systems, this unit integrates a mechanical water feeder with a low water cut-off (LWCO) function via a built-in Snap Acting SPDT switch (No. 2 Switch). It is designed to handle a maximum vessel pressure of 35 PSI and an inlet water supply up to 100 PSI, making it a rugged choice for commercial mechanical water feeder applications where continuous maintenance of a safe water line is critical. The proportional feed action ensures precise makeup water delivery, while the automatic reset prevents nuisance trippings. This head mechanism allows for direct replacement without disturbing existing system piping, specifically fitting the 51-S series float chambers for efficient field servicing.

• Dual functionality: Combines reliable boiler water feeding with a safety cut-off to protect against low water conditions.
• High-capacity compatibility: Rated for large boiler configurations up to 35,000 sq. ft. of steam.
• Serviceable design: Replaceable head mechanism permits quick restoration of LWCO and feeder performance without total valve replacement.
• Proportional makeup: Delivers water in proportion to demand, reducing thermal shock to the boiler vessel.
• Universal integration: Features a snap-acting SPDT switch for burner control and optional alarm signaling.

What is the maximum pressure rating for this unit?
The 51-S-2-HD is rated for a maximum vessel pressure of 35 PSI (2.5 kg/cm2) and a maximum water supply pressure of 100 PSI (7 kg/cm2).

Does this include the float chamber?
No, the 136200 is the head mechanism only. It is intended for contractors replacing the internal guts of an existing McDonnell & Miller Series 51-S assembly.

How do I wire the No. 2 Switch for cut-off?
The SPDT switch should be wired in series with the burner's control circuit. Terminals C and NC typically handle the cut-off, while C and NO can be used for a remote alarm.

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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.