HVAC Troubleshooting
Furnace Blowing Cold Air?
Common Causes & Fixes
Your furnace fan is running but the air coming from the vents is cool or room temperature instead of warm. This is often a simpler fix than you’d expect — about 40% of these calls are resolved with a thermostat check or filter replacement.
Furnace completely dead?
That’s a different issue — see Furnace Not Turning On.
Common Causes
1
Fan Set to ON Instead of AUTO
The most common cause — and a free fix. When the fan is set to ON, it runs continuously even between heating cycles, blowing unheated air. Switch to AUTO so the fan only runs when the burners are actually heating.
2
Dirty Flame Sensor
The furnace ignites but the flame sensor can’t detect the flame due to carbon buildup. The control board shuts off gas as a safety measure — usually within 3–10 seconds. The fan continues running, blowing the residual warm air then cool air. This is the #1 professional repair for this symptom.
3
Clogged Air Filter
A severely restricted filter causes the heat exchanger to overheat, triggering the high-limit switch. The burners shut off but the fan keeps running to cool the exchanger. Result: cold air from vents.
4
Pilot Light / Ignitor Problem
The fan runs on a timer — it starts after a delay when the thermostat calls for heat. If the burners fail to ignite (bad ignitor, no gas), the fan still starts on schedule but there’s no heat to distribute.
5
Ductwork Leak
In older Bay Area homes, ductwork in unconditioned spaces (attics, crawlspaces) can develop leaks. Heated air escapes before reaching the vents, and cool attic or crawlspace air gets pulled in. The furnace works fine but the air arriving at vents feels cool.
Safe Checks You Can Perform
Check the fan setting. Must be AUTO, not ON. This is the fix about 25% of the time.
Replace the air filter. Even if it looks “okay” — replace it. A dirty filter causing heat exchanger overheating is the second most common cause.
Listen to the furnace cycle. Does the burner ignite? You should hear a click, then a whoosh of flame. If you hear ignition but it shuts off within seconds — dirty flame sensor (needs a tech).
Check the gas supply. Gas valve near furnace should be ON (handle parallel to pipe). Verify your stove or water heater works to confirm gas is flowing to the house.
Signs You Need a Professional
Burner lights then shuts off within seconds — dirty flame sensor or failing gas valve. Common repair ($100–$300).
Furnace short cycles repeatedly — heat exchanger overheating from restricted airflow or a cracked exchanger. The crack is a carbon monoxide risk — get it inspected.
Yellow or flickering flame — burners should produce a steady blue flame. Yellow or flickering indicates incomplete combustion and potential CO production. Call immediately.
Why Choose North Breeze
Same-day service
NATE certified
All furnace brands
5.0 rated
Upfront pricing
Parts on truck
Related Problems
Related Services
Frequently Asked Questions
Most common causes: fan set to ON instead of AUTO (blows air between cycles), dirty flame sensor (burner shuts off after seconds), clogged filter (heat exchanger overheats and shuts down), or ignitor failure. Check the fan setting first — switching to AUTO fixes it about 25% of the time.
A flame sensor is a safety device that confirms gas is actually burning. When coated with carbon, it can’t detect the flame and shuts off gas flow within 3–10 seconds. The burner lights briefly then goes out. Cleaning costs $100–$200 and is one of the most common furnace repairs we perform.
Furnace Blowing Cold?
Same-day furnace diagnosis and repair. Most furnace issues are fixed in one visit.
NATE Certified
All Brands
Same-Day