When your furnace runs but the air coming out is cold, it usually means the burners are not staying lit or a safety control has shut down the heat. Some causes are simple; others need a technician. Here is how to tell.
With the fan set to ON instead of AUTO, it blows even between heating cycles — pushing room-temperature air. A quick setting check rules this out.
A dirty flame sensor or cracked ignitor causes the burners to light and then shut off, so the blower pushes cold air.
A clogged filter or blocked airflow trips the high-limit switch, cutting the burners while the fan keeps running.
Most cold-air furnace repairs (flame sensor, ignitor, filter, controls) run $120–$600. Heat exchanger or gas valve issues are quoted after diagnosis.
A furnace that short cycles wears components fast, and some causes (like a cracked heat exchanger) are safety risks. Get it diagnosed before the cold snap leaves you without heat.
Related service: HVAC Repair & Service · Browse all common problems
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