Symptoms
- Furnace runs for a few minutes then shuts off
- Cycles repeat every few minutes all day
- House never reaches set temperature
Likely causes
- CommonClogged air filterRestricted airflow makes the furnace overheat and shut down as a safety measure.
- CommonClosed or blocked supply/return ventsReduced airflow has the same overheating effect as a dirty filter.
- PossibleThermostat located near a heat source or draftA thermostat reading the wrong temperature cycles the system incorrectly.
- PossibleFailing limit switch or flame sensorA faulty sensor can shut the burners off prematurely even with normal airflow.
- Less commonOversized furnace for the houseA furnace that's too powerful heats the space quickly and shuts off before finishing a full cycle.
What to do
- Replace the air filter if it hasn't been changed in the last 1–3 months.
- Walk the house and make sure supply and return vents aren't blocked by furniture, rugs, or closed dampers.
- Check that the thermostat isn't near a heat vent, lamp, or sunny window that could cause a false reading.
- Turn the furnace off for 30 minutes to let it fully reset, then turn it back on and observe a full cycle.
- Check the outdoor exhaust vent (for high-efficiency furnaces) isn't blocked by snow, leaves, or debris.
- If short cycling continues after these checks, the limit switch or flame sensor likely needs professional attention.
Tools and parts
- Replacement furnace filter
Standard furnace air filterSearch: “furnace air filter size replacement”
When to call a pro
Call an HVAC technician if short cycling continues after the filter, vents, and thermostat placement are ruled out; a sensor or control board may need replacement.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.