OEM Lennox Limit Switch — 78W99
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The genuine OEM Lennox 78W99 limit switch is a high-temperature safety control mounted near the heat exchanger of your Lennox furnace. It continuously monitors plenum temperature and shuts off the burners if the unit overheats — typically caused by restricted airflow — protecting the heat exchanger from cracking and preventing a fire hazard.
Signs your limit switch may have failed:
- Furnace repeatedly shuts off on high limit (short-cycling)
- Blower runs continuously and won't shut off
- No heat, with the burners lighting briefly then extinguishing
- Overheating fault codes at the control board
Because the 78W99 is calibrated to a specific trip temperature matched to your Lennox furnace's design, installing the exact OEM part is essential for safe operation. A generic switch with the wrong setpoint can allow dangerous overheating or cause frustrating nuisance trips. Choose the factory 78W99 to keep your Lennox furnace's built-in overheat protection working exactly as engineered.
Shipping Information
- Orders placed before 3PM CST ship same business day
- Standard delivery: 2-5 business days
- Express shipping available at checkout
- Free shipping on orders over $75
- Tracking number emailed upon shipment
Return Policy
- 30-day returns on unused parts in original packaging
- Electrical parts (capacitors, contactors, control boards) are final sale once installed
- Email info@nationalhvacparts.com to initiate a return
- Refunds processed within 5-7 business days