Limit Switch
Updated
The limit switch is a safety control that watches the temperature inside your furnace. If the furnace gets hotter than it should, usually because air is not moving through fast enough, the switch shuts the burners off before the heat can damage the heat exchanger.
It also tells the blower when the air is warm enough to start pushing into the ducts, which is why there is a pause between the burners lighting and warm air arriving at the vents. A limit switch that trips repeatedly is almost always reporting an airflow problem, a clogged filter or blocked return, rather than failing itself.
Common Questions
What does a limit switch do?
The limit switch shuts the burners down when the furnace runs hotter than its design allows, protecting the heat exchanger from heat stress it was never meant to absorb. It also delays the blower until the air is warm, so the vents do not start with a cold blast.
Why does my furnace keep shutting off before the house is warm?
Repeated shutdowns mid-cycle usually mean the limit switch is tripping on real overheating, and restricted airflow is the usual reason, most often a clogged air filter, sometimes blocked returns or closed vents. The switch is doing its job; the airflow is the problem to fix.
Should a tripping limit switch just be replaced?
Replacing the switch without finding out why it trips usually buys a repeat of the same problem, because the switch is reacting to overheating rather than causing it. A furnace repair visit checks airflow, blower operation, and the switch itself, in that order.
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