8 Reasons Your Furnace Is Blowing Cold Air (And How To Fix It)


As a licensed HVAC contractor, furnaces that blow cold air are what keep me busy through the winter.

Your furnace is blowing cold air because something’s wrong with an internal component or with your thermostat. If a significant repair is necessary, be sure to get professional help. Gas furnaces can produce harmful and deadly gases if not repaired properly.

It also could be as simple as your furnace just needs cleaned, a thermostat adjustment, or a dirty filter, but sometimes it’s a sign you have a more significant issue that could require a complicated repair.

Keep reading to learn the 8 reasons your furnace is blowing cold air below…

8 Reasons The Heater Is Blowing Cold Air

  1. The Pilot Light Is Out Or Faulty
  2. The Air Filter Is Dirty
  3. Dirty Flame Sensor
  4. Low Gas Pressure
  5. Ducts Have Air Leaks
  6. Condensate Lines Aren’t Working
  7. The Thermostat Is In Auto Mode
  8. Furnace Needs Time To Turn On

Reason #1: The Pilot Light Is Out Or Faulty

Older furnaces use standing pilots. The pilot light is a small flame that stays lit continuously so it can light the main burners when the thermostat calls for heat.

If the pilot blows out or if you have a faulty thermocouple, your heat can be on but blowing cold air.

Relight Your Pilot

Your furnace should have instructions on the front panel describing how to relight your pilot. Follow those instructions or check out the other article I wrote on how to relight your pilot.

Reason #2: The Air Filter Is Dirty

This is one of the most common reasons a gas furnace will blow cold air and won’t shut off. 

Air filters get dirty and have to be replaced or cleaned on a regular basis.

Your furnace has a safety switch, called a high limit switch, that opens and turns the burner off to prevent overheating or heat exchanger damage. Restricted airflow is the primary reason for this switch to open.

Usually replacing the filter and resetting the furnace will correct this problem.

Read Also >> Why Is My Furnace Leaking Water?

Reason #3: Dirty Flame Sensor

Newer furnaces use what’s called a flame sensor. A flame sensor is a small rod that’s placed directly in front of the burners to read when a flame is present.

These sensors can get covered in dust, dirt, or corrosion and will need to be cleaned.

Typically your furnace will try to run several times and the burners will light and continue to go out. Then your furnace blower will kick on and blow cold air and won’t shut off until you reset it manually.

Reason #4: Low Gas Pressure

Occasionally gas supply issues can occur. If your house is not being supplied with enough gas, your furnace will not be able to light properly and will inevitably blow cold air.

There’s no easy fix for this type of problem and you will need to call your gas supplier or an HVAC contractor to diagnose and correct the issue.

Reason #5: Ducts Have Air Leaks

If your furnace is in an unconditioned area, like an attic or crawlspace, there’s a chance it’s drawing cold air into the return.

If your ductwork is damaged or open in this space your furnace is trying to heat and work properly, but can’t make up for the excess cold air. Sealing your duct will prevent this and allow your furnace to blow warm air again.

Reason #6: Condensate Lines Aren’t Working

High efficient furnaces have such a low flue gas temperature that water condenses in the exhaust pipe and inside the secondary heat exchanger. This condensation has to be drained away from the furnace so that it can run properly.

Sediment can clog the lines that remove the water and cause it to back up inside the furnace. Safety switches recognize this water build up and will prevent the furnace from firing, causing it to blow cold air.

Clearing the debris and draining the water should allow your furnace to fire and stop blowing cold air.

RELATED: Why Does My furnace keep turning on and off?

Reason #7: The Thermostat Is In Auto Mode

Thermostats typically have a fan switch that says “on” and “auto”. If it’s set to “on” your furnace blower will run continuously and the air you feel coming out of your vents may feel cool. This isn’t really a furnace problem and can be easily corrected by switching the thermostat to “auto”.

Reason #8: Furnace Needs Time To Warm Up

Some furnaces take a few minutes to heat up the air prior to the blower turning on.

You may think the furnace has turned on, but the blower is just waiting for the signal from fan limit switch to kick on the blower.

So if there is a short delay, it may be totally normal, and you can consult your owner’s manual for more information. But if the furnace still blows cold air after several minutes, contact an HVAC pro.

Frequently Asked Questions

Should I Turn My Furnace Off If It’s Blowing Cold Air?

If you notice your furnace isn’t heating or it’s blowing cold air and won’t shut offyou may feel like shutting your furnace off completely. Usually the air coming out of the vents is room temperature air and not actively cooling the house, so shutting your furnace off won’t make a difference.

The majority of furnaces have a circuit board with diagnostic LEDs. Shutting your furnace down may reset that code and make it more difficult to diagnose.

If the reasons above aren’t what’s causing your furnace to blow cold air, having the LED code can be helpful in figuring out what the problem is.

Why Does My Heat Feel Like Cold Air?

It’s probably just circulating the air from the house. If you turn the temperature up on the thermostat the air should start to heat up. If it doesn’t you most likely have a problem that needs addressed. 

Where Is The Reset Button On My Furnace?

Most building codes require a service switch or disconnect for your furnace. Typically a simple light switch is what is used, and can be used to reset your furnace if you’re having a problem.

What Are The Signs Of A Furnace Going Out?

In most cases, your heat is on but it’s blowing cold air. Your thermostat temperature is usually lower than your set point.

Sometimes your furnace may even be making noises or sounds it doesn’t generally make. If this is happening, call a professional to have things checked out.

How Do You Fix A Furnace That Blows Cold Air?

Follow the reasons above and try to eliminate all the easy issues. If it continues to blow cold air, you’ll need to have your furnace checked out and have the problem diagnosed by a technician.

Read Also >> how cold can a heat pump work?

How Long Should It Take To Heat A House 5°F?

This largely depends on the type of furnace you have, how well your house is insulated, and how cold it is outside. It could take an hour or two to bring the house up 5°F. As long as the temperature continues to go up, it’s usually okay.

Final Thoughts

If your furnace is blowing cold air instead of heat you definitely need to take a look at some things. A lot of simple issues can be taken care of at home, like changing your filter, lighting your pilot, or cleaning your flame sensor.

If the problem is more in depth, definitely ask for help and have a pro take a look. Don’t forget to make sure your thermostat is set to “auto”. It’s awfully expensive to have a service tech switch it for you.

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