Vent, Valve & Volt
Furnace keeps turning on and off

This one needs a licensed pro

Call an HVAC technician: limit switch, flame sensor or sizing

With airflow and the thermostat ruled out, the remaining causes need a technician. Bryant says it's not recommended to leave a short cycling furnace running.

Why this happens

Carrier, Bryant and Trane list the limit switch, flame sensor, a cracked heat exchanger and an oversized furnace among the causes of short cycling. The sources disagree on which is most common, but all of these need a technician to test. Carrier's advice, once airflow problems are ruled out, is to shut the system off and contact an HVAC professional.

What to do right now

  1. Set the thermostat to Off.
  2. Look for a blinking light through the small window in the furnace door and count the blinks. Don't remove the door.
  3. Don't open the furnace to clean or adjust anything inside.
  4. If a CO alarm sounds or anyone feels headachy, dizzy or sick, get everyone outside and call 911.

What to tell the HVAC technician

  • How long each cycle lasts. Trane gives 7 minutes as the minimum normal length.
  • That you replaced the filter, opened the vents and checked the thermostat location.
  • The blinking light pattern, if any.
  • The brand, model, serial and approximate age of the furnace, and whether it was recently replaced.

Build the full list: what to tell the HVAC technician.

Start this troubleshooter over

References (every fact above comes from one of these pages)
  1. Carrier: Furnace short cycling
  2. Bryant: Furnace short cycling
  3. Trane: Furnace short cycling

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