Short answer
Short cycling, where a furnace runs briefly and shuts off repeatedly, is usually caused by a dirty flame sensor, a clogged filter tripping the high-limit switch, a poorly placed thermostat, or an oversized furnace. The first two are quick fixes; the last is a design problem.
Burners shut off within seconds
If the burners light and go out within a few seconds, the flame sensor isn't detecting flame. Cleaning it is a common, inexpensive repair.
Runs a few minutes, then stops
If it heats for a few minutes and shuts off while the blower keeps running, the furnace is overheating and the limit switch is protecting it.
- Replace the filter.
- Open closed supply registers.
- Check that return grilles aren't blocked by furniture.
- Have the blower and heat exchanger checked if it continues.
Thermostat placement
A thermostat near a supply register, above a lamp or on a sunny wall satisfies too quickly. Moving it or adjusting cycle settings helps.
Oversized equipment
Many furnaces are larger than the home needs. Oversized furnaces heat quickly and shut off, wearing parts and leaving rooms uneven. The fix comes at replacement: a load calculation and a two-stage or modulating furnace.
When to call a technician
- Short cycling continues with a new filter.
- You hear booming at ignition.
- The furnace is locking out.
What the technician does: see furnace repair.
Common questions
Does short cycling waste gas?
Yes. Each start-up is less efficient, and it wears the ignitor and blower faster.
Editorial note: this guide is general information for Kansas City-area homeowners, not a diagnosis. Conditions vary by home, so a licensed HVAC technician should confirm the cause before any repair.