Furnace Limit Switches
High-limit safety switches, rollout switches, and combination fan-limit controls for gas furnaces.
How to Choose the Right Furnace Limit Switch
Furnace limit switches are safety devices that shut off the gas burner if the heat exchanger gets too hot. They protect against overheating caused by blocked airflow, failed blower motors, or dirty air filters.
Types of Limit Switches
Primary High Limit (Auto-Reset) — Opens at a set temperature (typically 150°F-200°F) and automatically resets when the heat exchanger cools down. If your furnace cycles on and off every few minutes, a tripping high limit is likely the symptom — but the root cause is usually restricted airflow from a dirty filter or failed blower motor.
Auxiliary Limit (Manual-Reset) — A backup safety that requires you to press a button to reset after it trips. These typically open at a higher temperature than the primary limit. If it trips, there's a serious airflow problem that needs to be resolved before resetting.
Flame Rollout Switch — Detects flames escaping the combustion chamber. This is a critical safety device — a tripping rollout switch can indicate a cracked heat exchanger or blocked flue. Never bypass a rollout switch.
Diagnosing a Bad Limit Switch
Before replacing a limit switch, check these common causes of overheating: dirty air filter (replace it first — this fixes 70% of limit switch trips), blocked return air vents, failed blower motor or capacitor, and closed or blocked supply registers. If the switch trips repeatedly after addressing airflow, the switch itself may be faulty — test continuity with a multimeter at room temperature.
Choosing the Right Replacement
Match by temperature rating (stamped on the switch body), number of terminals (2 or 3 wire), and mounting style (snap-disc or probe). OEM replacements guarantee exact fit. Universal switches are available but verify the temperature rating matches your furnace specifications.