Lochinvar & A.O. Smith 100109726 Lower Probe

The Lochinvar & A.O. Smith 100109726 Lower Probe is a precision-engineered OEM replacement part designed for high-efficiency commercial water heaters, specifically the Cyclone Mxi series (BTH 120-300). As a critical water heater temperature sensor, this lower probe assembly provides real-time thermal data to the control board, allowing for precise HVAC temperature control and modulation. Operating within the lower tank section, it monitors the coldest water in the vessel to ensure accurate firing rates and prevent thermal stacking. Constructed with high-grade thermistor technology, the 100109726 is essential for resolving fault code A5-401 (high temperature limit) or standby errors caused by open or shorted sensor circuits. This probe replaces legacy part numbers 192609 and 9004241215, ensuring seamless integration with existing Lochinvar boiler parts and A.O. Smith commercial controls. Professional installation is recommended to maintain optimal BTU output and prevent ignition failures associated with inaccurate temperature readings.

• Direct OEM Compatibility: Specifically designed for A.O. Smith Cyclone BTH and Lochinvar commercial units for exact fitment.
• Precise Thermal Feedback: Delivers accurate resistance-to-temperature signals to the digital control system, optimizing fuel efficiency.
• System Protection: Acts as a primary safety diagnostic to prevent water heater overheating and tank damage.
• Durable Construction: Built to withstand the corrosive environment of a pressurized commercial water heater tank.
• Error Code Resolution: Corrects common A.O. Smith temperature probe faults and sensor-related lockout cycles.

What are the symptoms of a failing lower probe?
Technicians often see "Probe Open" or "Probe Shorted" error codes on the display. Inconsistent water temperatures or short-cycling of the burner are also primary indicators that the 100109726 sensor has drifted out of calibration.

Does this probe work on residential models?
This specific part is typically found in high-efficiency commercial gas water heaters. Always verify the model/serial number against the manufacturer’s parts manual to ensure this is the correct temperature sensing probe for your specific chassis.

Can I use an aftermarket sensor instead?
For Lochinvar and A.O. Smith commercial systems, using the genuine 100109726 OEM probe is necessary to ensure the control board’s ohm-to-temperature curves match perfectly, preventing nuisance shutdowns.

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.