You turn up the heat on a chilly evening, the fan starts, and cold air comes out of the vents. It is one of the most common heating calls we get every fall in La Crescenta, Glendale and the San Fernando Valley. Sometimes the fix is a thermostat setting. Sometimes the furnace is protecting itself from a real problem. Here is how to tell the difference.
1. The fan is set to “On” instead of “Auto”
When the thermostat fan is set to On, the blower runs all the time — including between heating cycles, when the burners are off. That air feels cool. Switch the fan to Auto so it only runs while the furnace is actually heating.
2. The furnace is just warming up
For the first minute or so of a cycle, the air coming out may feel lukewarm while the heat exchanger warms up. That is normal. If the air stays cold after several minutes, something else is going on.
3. A dirty air filter caused the furnace to overheat
A clogged filter restricts airflow. The furnace overheats, a safety limit switch shuts the burners off, and the blower keeps running to cool things down — so you feel cold air. Replace the filter and see if normal heat returns. If it keeps happening with a clean filter, call a technician.
4. The burners are not lighting
If the blower runs but the burners never light, the cause is often a dirty flame sensor, a failed hot surface igniter or a gas supply issue. Check that the gas valve next to the furnace is open and that other gas appliances work. If you smell gas, leave the home and call the gas company or 911 from outside.
5. The condensate line is clogged (high-efficiency furnaces)
High-efficiency furnaces produce water that drains through a small line. If the line or trap clogs, a safety switch stops the burners. You may notice water near the furnace or a flashing error code on the control board.
6. Leaky or disconnected ductwork
If the furnace is heating but some rooms still get cool air, ductwork may be leaking or disconnected in the attic or crawl space. Warm air escapes before it reaches the vents, and cold attic air can get pulled in. This is common in older Glendale and Burbank homes with original ducts.
7. It is a heat pump in defrost mode
If you have a heat pump instead of a gas furnace, it will occasionally switch to defrost mode on cold mornings to clear frost from the outdoor coil. During defrost the air inside can feel cooler for a few minutes. Short defrost cycles are normal; constant cold air is not.
What you can safely check
- Thermostat on Heat, fan on Auto, set temperature above room temperature
- Fresh air filter
- Furnace power switch on and breaker not tripped
- Gas valve open
- Supply vents and return grilles open and unblocked
When to call a professional
Call a technician if the furnace keeps blowing cold air after these checks, shuts off repeatedly, shows an error code, or you notice a burning smell, a yellow flame or a carbon monoxide alarm. Do not keep resetting a furnace that keeps shutting itself down — the safety switch is telling you something.
Our heating repair technicians diagnose the real cause and give you the price before any work begins. A fall furnace tune-up catches most of these problems before the cold weather arrives. For more troubleshooting, see Furnace Not Working? 9 Common Causes.
Get your heat back today
Orion HVAC serves La Crescenta, Glendale, Burbank, La Cañada Flintridge, Tujunga, Santa Clarita and the San Fernando Valley. Call (323) 736-3737 or book online. You get the price before any work begins, and every repair is backed by our parts and labor guarantee.
