McDonnell & Miller 7-B Switch

The McDonnell & Miller 7-B Switch (Part Number 110800) is a critical component for specialized boiler control systems requiring potentiometer proportional control. This auto reset low water cut off switch is engineered to work in tandem with Series 93 and 94 pump controllers, providing precise positioning for modulating valves and ensuring consistent water levels in commercial steam applications. Unlike standard snap-action switches, the 7-B utilizes a 135-ohm potentiometer to deliver a proportional sliding signal, allowing for fine-tuned feedwater regulation rather than simple on-off cycling. For technicians troubleshooting high-pressure systems, this replacement switch assembly ensures the LWCO safety circuit maintains its integrity while providing the necessary feedback for proportional burner or valve control. Its rugged construction is built to withstand the rigorous thermal cycling found in industrial boiler rooms, ensuring reliable signal transmission without electrical drift.

• Integrated 135-ohm potentiometer for seamless modulating motor integration.
• Compatible with McDonnell & Miller Series 93 and 94 liquid level controllers.
• Automatic reset functionality prevents nuisance manual intervention after transient level drops.
• Facilitates precise feedwater regulation to improve overall boiler efficiency and thermal stability.
• Field-replaceable design minimizes downtime during boiler maintenance and control upgrades.

Can the 7-B switch be used in place of a standard 7-B-S switch?
No. The 7-B features the proportional potentiometer required for modulating systems, whereas the 7-B-S is a standard snap-switch. Installing the wrong version will result in a loss of proportional signal to your valve actuator.

What is the primary cause of signal failure in these switches?
Typically, potentiometer wear or carbon buildup on the wiper can cause erratic signals. If your modulating valve is hunting or fails to respond to level changes, the sliding contact in the 110800 assembly is the first thing to test for continuity and resistance.

How do I verify the resistance range during commissioning?
Using a multimeter, you should measure a consistent sweep across the 135-ohm range as the float linkage moves the wiper arm. Any dead spots or infinite resistance indicates a failed proportional switch.

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.