Carrier HH18HA138 Limit Switch

The Carrier HH18HA138 is a professional-grade HVAC high-limit switch engineered for precision thermal protection in Carrier, Bryant, and Payne gas furnaces and packaged units. As a normally closed (NC) disc-style limit switch, it serves as a critical safety component by monitoring internal heat levels and interrupting the burner circuit if temperatures exceed safe operational parameters. This specific factory authorized part features an open temperature of 135°F and a close temperature of 95°F, effectively preventing heat exchanger damage and potential flame rollout. Designed with an automatic reset function (L135-40), the switch restores system continuity once the equipment cools back to the 95°F set point, eliminating the need for manual intervention after temporary over-temperature events. This OEM replacement part (also referenced as LF18-094) ensures 100% compatibility with specific residential and commercial models, including many 48DJD, 581BTV, and 48HCD series units, maintaining the original engineering specifications for system safety and airflow regulation.

• Exact OEM Specifications: Guarantees a direct fit and correct 95-135F temperature range without field modifications.
• System Protection: Rapidly opens the circuit at 135°F to prevent overheating and protect the heat exchanger from cracks or fatigue.
• Automatic Reset Logic: Features an auto-reset design that clears once temperatures drop to 95°F, minimizing nuisance service calls for manual resets.
• High Compatibility: Fits an extensive range of Carrier/Bryant/Payne furnaces and rooftop units, making it a versatile addition to a service truck inventory.
• Durable Construction: Built to withstand the high-cycle environments of modern high-efficiency heating systems.

What causes a Carrier HH18HA138 to trip?
The most common causes for this switch to open are restricted airflow, a dirty air filter, a failing blower motor, or an oversized furnace. In rooftop units, it often trips due to a blocked return air duct or broken blower belt.

Can I use a higher temperature limit switch as a substitute?
No. Using a limit switch with a higher opening temperature than 135°F can lead to unsafe operating conditions and heat exchanger failure. Always replace with the exact HH18HA138 specified by the manufacturer.

Is this switch a manual or automatic reset?
The HH18HA138 is an automatic reset limit switch. It will close the circuit automatically once the sensor detects the temperature has dropped to approximately 95°F.

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.