Furnace keeps turning on and off

Trane gives 7 minutes as the minimum normal furnace cycle length. A furnace that shuts off sooner, over and over, may have an airflow or thermostat problem you can fix, or a venting or heat exchanger problem that needs a technician right away.

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Before you start. If a carbon monoxide alarm sounds, or anyone has a headache, dizziness or nausea, get everyone outside and call 911. If you smell gas, leave and call 911 or your gas utility from outside.

A hand sliding a new pleated air filter into a furnace filter slot

Every question and answer on this page, in order

Question 1: Do you notice any of these?

Check the carbon monoxide alarms and look around the furnace and its vent pipe for black soot.

A CO alarm, or headache, dizziness or nausea in the house: Possible carbon monoxide: get outside now
Soot near the furnace, or a burning or metallic smell: Shut the furnace off: possible cracked heat exchanger
None of these: go to question 2

Question 2: How does the air filter look?

Slide the filter out and hold it up to a light.

Gray or dusty, and little light comes through: Likely cause: a clogged air filter
Clean, or changed in the last month: go to question 3

Question 3: Walk the house. Are supply vents or return grilles closed, or covered by furniture, rugs or drapes?

Yes, some are closed or blocked: Likely cause: blocked or closed vents
All open and clear: go to question 4

Question 4: Where is the thermostat?

Carrier and Trane both note that a thermostat that warms up too fast can shut the furnace off early.

In direct sun, near a supply vent, or near a lamp, TV or other heat source: Likely cause: the thermostat is reading false heat
Away from all of those: go to question 5

Question 5: From outside, look at where the furnace vent pipe leaves the house. Is it blocked?

Look only. Don't remove anything from the pipe or change it in any way.

Yes, by snow, leaves, a nest or something else: Blocked vent pipe: shut the furnace off and call a technician
It's clear, or I can't find or see it: Call an HVAC technician: limit switch, flame sensor or sizing

Possible results

Need the words for the phone call? Build a what-to-tell-the-technician checklist, or read when to call an hvac pro.