Lochinvar & A.O. Smith 100109925 Switch

The Lochinvar & A.O. Smith 100109925 Blocked Inlet Switch is a critical safety component designed for high-efficiency gas-fired boilers and commercial water heaters. This normally closed (NC) pressure switch is calibrated to a -1.0 inch water column set point, ensuring the combustion blower maintains proper airflow for safe ignition. If the intake system encounters excessive static pressure due to blocked air intake pipes, debris in the mesh screen, or improper venting lengths, the switch opens to break the safety circuit. This prevents incomplete combustion and dangerous carbon monoxide accumulation by halting the heating cycle immediately.

Engineered for the Lochinvar Knight boiler and A.O. Smith Cyclone BTH series, this OEM replacement part (also known by cross-reference numbers 9004540215 and 193293) features a robust diaphragm and high-temperature resistant housing. When troubleshooting Air Pressure SW fault codes or a lockout on blocked inlet, a continuity test across the 100109925 terminals is the standard diagnostic procedure. If the switch remains open while the unit is off or fails to maintain the circuit during the pre-purge cycle, a replacement is necessary to restore reliable HVAC system performance and safety interlock functionality.

• Precise Pressure Sensing: Calibrated to -1.0" W.C. to accurately detect intake restrictions before they impact combustion.
• OEM Compatibility: Direct fit for Lochinvar Knight and A.O. Smith Cyclone models, ensuring seamless integration with the existing low voltage connection board.
• Fail-Safe Design: Operates as a normally closed switch to ensure the combustion process only proceeds under verified safe air supply conditions.
• Durable Construction: Built to withstand the thermal stress and pressure fluctuations typical of commercial water heating applications.

• What causes an "Air Pressure SW" error code? This usually indicates the switch has detected a vacuum higher than -1.0" W.C., often caused by obstructed air intake pipes, ice buildup at terminations, or a faulty blower assembly.
• How do I test the 100109925 switch? With the unit powered off, use a multimeter to check for continuity. A functional switch should show zero ohms (closed). If it remains "open" (OL) without a load, the internal diaphragm or contacts have failed.
• Can I use this switch for the exhaust side? No, this is specifically a blocked inlet switch. Exhaust monitoring typically requires a blocked outlet switch with different pressure ratings.

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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.