Fix it
Why is my furnace blowing cold air?
Air moving but not warming means the blower is doing its job and the burner is not. The causes sort neatly by whether the air is always cold, cold at first, or cold only sometimes.
What it usually is
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The thermostat fan set to ON instead of AUTO
Set to ON, the blower runs continuously and circulates room-temperature air between heat cycles. It feels cold from the vents even though nothing is broken.
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Ignition failure — the burner never lit
A failed igniter, dirty flame sensor or interrupted gas supply leaves the blower moving unheated air. Listen at the furnace: clicks without the whoosh of ignition tell the story.
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Overheating tripping the limit, burner off, blower purging
After a clogged-filter overheat, the control keeps the blower running to cool the exchanger — deliberately blowing cold air. If it alternates warm then cold, this is the pattern.
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A heat pump in defrost or with a failed reversing valve
Heat pumps periodically run cool air during defrost; constant cold air with the outdoor unit running may be the reversing valve stuck in cooling.
What you can check yourself
Check the thermostat fan setting first, then the filter. Watch an ignition attempt with the room lights off — glow, click, flame? Verify the gas valve lever at the furnace is parallel to the pipe and, on older units, that the pilot is lit.
When it’s a job for a pro
Igniters, flame sensors, gas valves and reversing valves are meter-and-manifold work. If the furnace overheats on a clean filter, the technician should inspect the exchanger before the unit runs another season.
Igniter replacement typically runs $150 to $400; flame sensor service $100 to $250; reversing-valve work on a heat pump is more substantial at $450 to $1,200.
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