McDonnell & Miller 150S-M-MD-HD Replacement Head Mechanism with Manual Reset and Maximum Differential

The McDonnell & Miller 150S-M-MD-HD (part number 172810) is a replacement head mechanism designed for Series 150S and 157S combination low water cut-off and pump controller systems. This head assembly features snap-action manual reset switches specifically engineered for high-temperature boiler applications.

Key Features:

• Manual reset operation with snap-acting SPDT (Single Pole, Double Throw) switch
• Maximum differential operation prevents nuisance burner shutdowns in low-pressure applications
• One single pole, single throw switch for pump control
• One single pole, double throw switch for low water cut-off and alarm actuation
• Maximum operating pressure: 50 psi (3.5 kg/cm²)
• Designed for commercial and industrial steam boiler applications
• Compatible with Series 150S and 157S boiler control bodies

This replacement head mechanism replaces obsolete part number 173302 and provides reliable protection for commercial and industrial boilers of any steaming capacity. The maximum differential design is ideal for low-pressure systems operating below 50 psi, where standard differential controls may cause unnecessary system interruptions.

Note: This is a head mechanism only and requires a compatible Series 150S or 157S control body for installation. McDonnell & Miller recommends replacing head mechanisms every 5 years for optimal performance and safety.