McDonnell & Miller 750-T-120 Cut Off

The McDonnell & Miller 750-T-120 (Part Number 176206) is a heavy-duty 120Vac electronic low water cut-off designed for high-reliability sensing in hot water boilers and steam boiler secondary protection. Engineered with a 20,000 ohm sensitivity, this GuardDog series control utilizes a probe-type sensor to monitor water levels, effectively preventing "dry fire" conditions that cause catastrophic vessel failure. The automatic reset configuration ensures that the burner circuit restores immediately once the water level recovers to the probe, making it an ideal choice for commercial boiler safety where manual intervention for every nuisance trip is impractical. This unit features integrated LED status indicators—green for power and red for low water—providing immediate visual diagnostics for technicians during troubleshooting. The built-in test switch allows for a localized functional check of the internal relay and burner circuit without draining the entire system, streamlining mandatory annual safety inspections and maintenance protocols.

• Automatic Reset Logic: Eliminates downtime after transient low water conditions or power interruptions, assuming the probe is submerged.
• Universal 120V Compatibility: Standard voltage requirements for seamless integration into most industrial and commercial burner control circuits.
• System Verification: The manual test switch provides a rigorous check of the burner cut-off relay, ensuring the safety loop is intact.
• Durable NEMA 1 Enclosure: Protects the internal electronics from dust and debris typically found in mechanical rooms.
• LED Diagnostics: Simplifies service calls by indicating exactly when the control has reached a low water condition versus a power loss.

Can this be used as the primary cut-off on a steam boiler?
Strictly speaking, the 750-T-120 is intended for primary or secondary use on hot water boilers. For steam applications, it is typically used as a secondary LWCO, as many local codes require a manual reset model for primary steam protection.

What does it mean if the red LED is solid?
A solid red LED indicates a confirmed low water condition where the probe is no longer sensing the liquid. If the water level is visibly sufficient, check the probe for scale buildup or ensure that Teflon tape was not used on the threads, as the probe requires a metal-to-metal ground to function.

Is this model compatible with automatic water feeders?
Yes, because this is an auto-reset model, it can be integrated with automatic water feeders. However, ensure the feeder's delay settings are coordinated with the LWCO to prevent rapid cycling of the burner.

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.