Quick answer
- Set the thermostat fan to AUTO (ON blows unheated air between cycles)
- Replace a clogged filter — overheating trips the high-limit switch
- No ignition usually means a flame sensor, igniter, or gas-supply issue
- A cracked heat exchanger is a safety risk — shut it off and call
When this applies
Try this first
Confirm the thermostat is on HEAT with the fan set to AUTO, and replace a dirty air filter. If the furnace still won't produce heat after a cycle or two, it needs a diagnostic.
How it works
How a furnace makes heat
Burners heat a metal heat exchanger; the blower passes house air across it into the ducts. If ignition fails, or a safety device senses overheating or a flame problem, it cuts the burners while the blower keeps running — so you feel cool air.
What to watch out for
Cracked heat exchanger
A cracked heat exchanger can leak combustion gases, including carbon monoxide. It's a safety shut-off, not a DIY repair — stop using the furnace and have it inspected.
Short-cycling on the high-limit
A dirty filter or blocked vents make the furnace overheat and shut the burners off early, then restart — short-cycling that wastes fuel and wears parts. Restoring airflow usually fixes it.
Schedule service
Frequently asked questions
Why does my furnace blow cold air then warm up?
With the fan on AUTO, a short burst of cool air at startup is normal while the heat exchanger warms. If it never warms up, or blows cold for long stretches, that's a fault worth diagnosing.
Reviewed by John Stevenson Plumbing, Heating & Air. Based on the most common no-heat service calls our NATE-certified technicians diagnose across coastal Southern California. Last updated 2026-06-18.