Component
Limit switch
The limit switch stops the burner when the heat exchanger area gets too hot, protecting the furnace from airflow failures. It is a temperature switch in the furnace's airflow path; a limit that keeps opening means the furnace is overheating, not that the switch is bad.
Short answer
When the limit opens, the burner stops while the blower continues, and the furnace restarts after it cools: the classic short cycle with a limit code. The limit is doing its job; the cause is airflow. Check the filter and open registers, and have a technician measure temperature rise and static pressure to find why the furnace is running hot.
Where limits live
Furnaces have a main limit near the heat exchanger and, on many, a manual-reset limit or rollout switch in the burner area that responds to flame rollout rather than airflow. Carrier's manuals are explicit that rollout switches must never be jumpered and are reset only after the cause is corrected.
What opens a limit
Restricted airflow (filter, blower, ducts, registers), an oversized furnace for the ducts, or a burner problem that puts more heat into the heat exchanger than the airflow removes. High-fire operation on two-stage furnaces stresses limits most because it needs the most airflow.
Testing
A technician measures temperature rise against the rating plate's certified range and static pressure across the system, checks the blower's delivered airflow, and verifies the switch opens at its rated temperature. Replacing a good switch does not fix overheating.
Related
Questions people ask
Is a furnace with a tripping limit dangerous?
The limit is protecting the furnace from overheating, which is safe behaviour. Repeated overheating can damage the heat exchanger, so the airflow cause should be found promptly.
My furnace resets itself after cooling. Do I need a repair?
The limit is auto-resetting by design; the underlying overheating remains. Without a repair you keep short-cycling, which wears the furnace.
Can a limit switch fail closed?
Yes, which is dangerous because it disables the protection. This is one reason annual maintenance tests the safeties; a stuck-closed or stuck-open switch is replaced.
Need this looked at?
Call BC Wide with the brand, model number and anything on the display. Or
Request service online with this page attached.