12 Reasons Why Your Furnace is Blowing Cold Air

Your furnace should be blowing hot air, not blowing cold air!

If you’ve found your furnace to be blowing cold air, then you likely have a problem with:

  • Thermostat
  • Ductwork
  • Gas supply
  • Clogged condensate lines
  • Dirty flame sensor
  • Dirty filter
  • Overheating
  • Broken pilot light
  • Cracked heat exchanger
  • Furnace is about to die

Below you’ll find the 12 reasons why your furnace is blowing cold air and how to fix it.

Let’s get started.

12 Reasons Why Your Furnace is Blowing Cold Air

1. Thermostat is “On” and Not Auto

If your thermostat settings are set to “On”, then your furnace will blow cold air even when it’s not heating your room.

The “On” setting tells your furnace to work continuously without checking the temperature and shutting itself down.

Change your thermostat setting to “Auto” so that your furnace will cycle on and off to bring your home to your desired temperature.

2. Your Ductwork is Damaged

Ductwork is how the hot air gets from the furnace to each room in your home.

Over time, your ductwork that was installed improperly can start to have holes, tears and even cracks. Natural wear and tear is another possibility that your ductwork breaks down and starts to bring in cold air.

If there are holes in your ductwork, the cold air will make its way into the rooms and it’ll feel like the furnace is blowing cold air. Unfortunately, you’ll need to replace the air ducts if you’ve found leaks in them.

3. Gas supply is low

Low gas supply in your home can lead to your furnace blowing cold air.

Without enough gas, then your furnace will shut itself off as it will not be able to properly heat your home.

If you suspect this is the case, check if your other gas appliances are still functioning properly.

If you ran out of gas completely, then you’ll need to contact a gas technician to come and replace your supply.

4. Condensate drain lines are clogged

If you notice that there is a leak around your furnace, your condensate drain lines may be clogged.

This can lead to your furnace not working properly as your furnace cannot function when the drain line is clogged.

5. Flame sensor is dirty

The flame sensor on your furnace is a device that checks if your furnace is ignited, otherwise, it will shut down your furnace to prevent any explosions from occurring.

To clean your sensor, you’ll need to find the burner assembly and remove the flame sensor (it should be in front of the pilot light).

Once you’ve taken it out, you’ll need to use light grit sandpaper to remove any grim or dirt that may have accumulated over time.

Be careful as to not grind down the flame sensor and only remove the debris that’s preventing it from working properly.

6. Airflow is restricted

Your furnace’s air filter is responsible for catching dust, grime or soot that may make its way into your home.

If you haven’t cleaned it in a while, then it can become overly saturated and cause restricted airflow.

This is an easy fix and all that is required is to either wash or change out your furnace’s air filter.

7. Pilot light is damaged

If the pilot light isn’t working, then your furnace will not be able to ignite.

Check if your furnace’s pilot light is on, and if it’s not, you can try the reset switch on your furnace to try to light your furnace again.

If the reset switch doesn’t help light the pilot light, then you may need to replace your pilot light altogether.

Your flame sensor can also be damaged and prevent your furnace from lighting.

8. Faulty Circuit Board

Your thermostat or your circuit board could be faulty, leading to your thermostat telling your HVAC system to blow cold air.

If your furnace is blowing cold air and you’ve tried all of the other possibilities, it could be that a faulty circuit board is a culprit.

To diagnose this issue, you’ll have to consult with a furnace repair technician to take a look and find the root of the problem.

9. Furnace is overheating

Your furnace is designed to stop heating if it senses that it’s overheating. Your furnace blowing cold air is a good thing in this scenario as your furnace is giving itself a chance to cool off.

It is possible that your furnace is reaching the end of its life, and that’s why it continues to overheat.

10. Cracked heat exchanger

A cracked heat exchanger could be a cause for your furnace blowing cold air.

A cracked heat exchanger can also lead to carbon monoxide leaks, so it’s best that you have it repaired right away.

11. No fire

While it may seem silly, if there is no fire, your furnace will only blow cold air.

Check your furnace to see if it’s lit, and if not, try to light your furnace by following the video below:

12. Furnace is broken

If you’ve tried everything and your furnace is still blowing cold air, then your furnace may need to be replaced.

You can try to diagnose a few reasons why your furnace won’t turn on. If you aren’t able to turn on your furnace, you should look into having it replaced.

When to Replace Your Furnace?

Most furnaces last around 20 to 30 years.

If you are experiencing problems that are getting worse each year, you may want to consider replacing your furnace.

You may spend more on repetitive repairs than if you would have replaced your furnace in the first place.

If your furnace is constantly making loud noises, the temperature is fluctuating often, you’re noticing higher energy bills, and the flames are yellow and not blue then it’s likely your furnace is in need of a replacement.

The time it takes to install a furnace is not long at all should you need a replacement.

Final Thoughts

Your furnace blowing cold air can be as simple as a fix as changing your thermostat from “ON” to “AUTO” or even can be a significant repair like an entire furnace repair.

Try checking if you have a dirty air filter as that can also be the reason why your furnace is blowing cold air.

The best way to diagnose your furnace is to call on an HVAC repair man.

If you aren’t sure how to proceed with troubleshooting your own furnace, be careful as your furnace can be a hazard to your health and home. It’s always better to call on professionals who are ready at any time of the day.

About Roy Cohen

Roy Cohen has a burning passion for heating, cooling, and ventilation. He aims to help you save money on expensive repairs and bring you the best HVAC products. He has years of experience behind him in HVAC repair and garage maintenance.

You can find Roy at his LinkedIn or his email: [email protected]

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