Start with the simple checks
When your furnace blows cold air, the cause is often small. Before you call anyone, spend a few minutes on the basics. They cost nothing, and they rule out the easy answers.
Walk to the thermostat first. Make sure it is set to heat, not cool or off. Then check the fan setting. If the fan is set to on instead of auto, the blower runs all the time, including between heating cycles. The air that comes out during those pauses is cool, and it can feel like the furnace is broken when it is not.
- Thermostat mode is set to heat.
- Temperature is set above the current room temperature.
- Fan is set to auto, not on.
- Thermostat batteries are fresh, if your model uses them.
- The furnace power switch is on and the breaker has not tripped.
A dirty air filter
A clogged filter chokes the airflow. When the furnace cannot move enough air across its heat exchanger, it can overheat. Most furnaces have a safety limit switch that shuts the burners off when that happens. The blower keeps running to cool things down, and what comes out of your vents is cold.
Pull the filter and hold it up to the light. If you cannot see light through it, replace it. Our post on how often to change air filters covers a simple schedule. A fresh filter is the cheapest fix on this list.
Ignition, pilot light, or flame sensor trouble
A furnace needs a flame to make heat. If the pilot light is out on an older unit, or the igniter is worn on a newer one, the blower can still run while no heat is produced. The result is cold air from the vents.
A dirty flame sensor causes a similar problem. The sensor tells the furnace that a flame is present. If it is coated with residue, it may not sense the flame, so the furnace shuts the gas off for safety after a few seconds. You may hear the furnace start, run briefly, and then blow cool air.
If you smell gas, leave the house and call your gas utility from outside. Do not try to relight anything. For any ignition problem you are not sure about, call a licensed technician.
Other causes that need a technician
Some causes are not do-it-yourself jobs. A failed igniter, a faulty control board, a bad limit switch, a stuck gas valve, or a problem inside the heat exchanger all need proper testing. So do blocked condensate lines on high-efficiency units, which can shut a furnace down.
Ductwork can also be the issue. Leaky or disconnected ducts lose heat before it reaches the room, so the air at the vent feels weak or cool even when the furnace is working. If only some rooms are cold, that points toward airflow or ducts rather than the furnace itself.
A good diagnosis finds the real cause instead of guessing. Vetter Air's furnace repair is licensed and insured, and our technicians are NATE certified. The full diagnosis is only $79, and the service call is only $29 when repairs are done.
How to prevent it next winter
Most cold-air calls trace back to wear that could have been caught earlier. A yearly check lets a technician clean the sensor, inspect the igniter, and look over the safety controls before the first cold snap. Our guide to keeping your furnace healthy with maintenance walks through what to expect.
Change your filter on schedule, keep vents and returns clear of furniture, and listen for new noises when the furnace starts. A change in sound or smell is often the first hint of a problem.
When to call Vetter Air
If you have checked the thermostat and filter and the air is still cold, it is time to call. We offer 24/7 emergency care, day or night, so a cold night does not have to wait. Learn more about our furnace repair service, or call (830) 624-7079.
We serve New Braunfels, San Antonio and San Marcos, plus the Texas towns in between. Active duty military, veterans and seniors receive 15% off repair services, so mention it when you book. Every job is backed by our satisfaction guarantee.
Frequently asked questions
Why is my furnace blowing cold air right after it turns on?
Some cool air at startup is normal while the heat exchanger warms up. If it stays cold, check the filter and thermostat, then call a technician. An ignition or flame sensor problem is a common cause.
Can a dirty filter make my furnace blow cold air?
Yes. Poor airflow can overheat the furnace, and the limit switch then shuts off the burners while the blower keeps running. Replacing the filter often fixes it.
Is it safe to keep running a furnace that blows cold air?
Turn it off if you smell gas or notice anything unusual, and call for help. Otherwise, run it only while you check the simple items, and do not keep forcing a unit that keeps shutting down.
How much does a furnace diagnosis cost with Vetter Air?
A full diagnosis is $79, and the service call is $29 when repairs are done. Veterans, active duty military and seniors get 15% off repair services.