Burners Not Lighting
If the blower runs but no heat comes out, the burners may not be lighting at all. The ignition sequence attempts, fails, and the blower runs anyway on some models, or the system locks out and only the fan operates. A technician tests the igniter, gas valve, and flame sensor in order.
The Overheating Shutdown
A furnace that blows warm for a few minutes then cold is often overheating and tripping its high-limit switch. Restricted airflow from a clogged filter or blocked vents is the usual cause. The furnace protects itself by shutting the burners off while the blower keeps running, which feels like cold air.
Thermostat and Duct Confusion
Sometimes the air is not actually cold, just cooler than expected because the thermostat is miscalibrated or the ducts leak heat into an attic. A technician measures supply temperature against the furnace rating to separate real problems from perceived ones.
What to Check Before Calling
Replace a dirty filter, confirm all supply vents are open, and verify the thermostat setting. If cold air continues after those checks, the problem needs diagnostic tools. Call (571) 561-3454 and describe the pattern: constant cold, or warm-then-cold.
Questions About Furnace Blowing Cold Air
Is cold air from vents an emergency?
Not usually, unless the house is dropping temperature fast in freezing weather. But it will not fix itself, so do not wait weeks.
Could it be the thermostat?
Yes. A miscalibrated thermostat or one losing its call signal can produce lukewarm or mistimed heating.
Why does it start warm then go cold?
That pattern points to the high-limit switch tripping from overheating, most often caused by restricted airflow.
