Symptoms
- Heat starts, then cuts out well before the house reaches temperature
- Furnace may restart on its own after a pause
- Pattern repeats consistently
Likely causes
- CommonDirty air filter causing overheat shutdownRestricted airflow is the most frequent cause of premature shutoffs.
- CommonBlocked or closed vents reducing airflowSimilar effect to a dirty filter: less air moving means more heat buildup.
- PossibleFailing limit switchA switch that trips too early cuts the burners off before a full cycle.
- PossibleBlower motor not running at full speedA weak motor moves less air, causing the same overheat-and-shutoff pattern.
- Less commonBlocked flue or exhaust vent (high-efficiency units)An obstruction can trip a pressure switch and stop the cycle.
What to do
- Replace the filter if it's dirty and check all vents are open.
- Check the outdoor exhaust vent pipe for blockage from snow, leaves, or a bird's nest.
- Turn the furnace off for 15–30 minutes to let it fully reset before restarting.
- Listen to the blower during a cycle; a noticeably weak or straining sound suggests a motor issue.
- If shutoffs continue after these checks, the limit switch likely needs professional testing and possible replacement.
Tools and parts
- Flashlight
Furnace air filterSearch: “furnace air filter size replacement”
When to call a pro
Call an HVAC technician if the furnace keeps shutting off early after filter, vent, and exhaust checks; the limit switch or blower may need repair.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.