High Limit Switch
The furnace's overheat safety.
Furnace Blowing Cold Air: A Diagnostic Order That Finds It Fast
Why a gas furnace blows cold or lukewarm air, in the order to check it: thermostat fan setting, lockouts, flame proving, limits, gas supply, and duct leaks.
Furnace Temperature Rise: How to Measure It and What the Reading Means
How to measure furnace temperature rise correctly, where the normal range comes from, how to estimate airflow from it, and what a high or low rise tells you.
Furnace High Limit Switch Tripping: Causes and How to Fix It
Why a furnace's high limit opens, how to confirm it with temperature rise and static pressure, and when the switch itself is the problem.
Furnace Short Cycling: 7 Causes and How to Pin Down Which One
Diagnose a gas furnace that lights and shuts off too soon, from flame sensing and limit trips to oversizing and thermostat settings.