Why Is My Furnace Blowing Cold Air?

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Heating expert implementing a furnace repair in Edmond, OK

Frustrated by the cold air coming from your vents in OKC?

We don’t blame you! The last thing you want is more cold air coming into your home on a freezing day.

If your furnace is running but you’re feeling cold air, you may have one of the following furnace problems:

  • A thermostat that’s set incorrectly
  • A Broken fan limit switch
  • A faulty ignition system
  • An overheating furnace
  • Leaky air ducts

Most of these problems will require a professional to diagnose and repair your furnace. We’ll go into more details about each of the problems below, explain what causes them and why a professional needs to handle the repair.

But first, we’ll share 2 DIY repairs you can try before calling a professional.

Want a professional to repair your furnace? We can help!

Before you call a pro, try these 2 DIY fixes

  1. Check the thermostat
  2. Check the air filter

Step #1: Check the thermostat

We know this may seem obvious, but first you’ll want to make sure your thermostat is set to HEAT and not COOL. Sometimes a thermostat accidentally gets switched to the incorrect setting, which would explain the cold air.

Next, make sure your thermostat is set to AUTO, not ON. This setting controls your furnace fan. If the thermostat is set to ON, the fan will blow constantly—even when your furnace isn’t heating the air. That could explain why you’re feeling cold air coming from your vents. Setting your fan to AUTO will ensure that the fan will only run while the furnace is actually heating air.

Note: If the screen on your thermostat isn’t working or the buttons are unresponsive, try changing the batteries. If that doesn’t fix the problem, contact a professional to repair the thermostat.

Step #2: Check the air filter

If your thermostat is set to the correct settings but you’re still getting cold air, you’ll want to check your air filter next.

A dirty air filter restricts airflow to your furnace, which can cause your furnace to overheat. When the furnace starts to overheat, a safety device shuts off the burners, which would explain why your furnace is pushing cold air into your home instead of warm air.

If your filter looks like the one on the right in the picture below, it’s dirty and should be replaced ASAP.

HVAC air filter examples of a clean air filter vs dirty air filter in Edmond, OK.

Clean (left) vs dirty (right) air filter

Cold air problems a professional will need to fix

After you’ve tried those DIY fixes, if you still feel cold air coming from your vents, you’ll need to contact a professional. 

Some common causes of cold air coming from a furnace include:

  • A broken fan limit switch
  • A faulty ignition system
  • An overheating furnace
  • Leaky air ducts

Let’s look at each of these problems in more detail.

Broken fan limit switch

Earlier we mentioned that your furnace has a fan, which blows heated air into your home. The fan is controlled by a limit switch that tells it when to turn on and off.

During normal operation, the fan should only turn on after the burners have warmed up completely so that only warm air is blown into your home. But if there’s an issue with the limit switch, sometimes the fan turns on too soon, which causes cold air to be blown into your home. If this is your problem, the cold air should slowly turn to warm air after a few minutes.

A professional can test the limit switch to make sure it’s working properly, and replace the switch if it’s broken.

Faulty ignition system

The ignition system in your furnace facilitates the combustion process, which is how your furnace creates heat to warm your home. Newer furnaces have an electronic ignition system; older furnaces have a pilot light. 

If there’s an electrical issue with the electronic ignition system, or the pilot light in your old furnace won’t stay lit, your furnace can’t deliver warm air into your home.

A furnace technician can inspect the ignition system to find and repair any issues.

Overheating furnace

Though it may sound counterintuitive, an overheated furnace can actually result in cold air coming from your vents.

Here’s why: The part of your furnace that actually warms cold air is called the heat exchanger. The heat exchanger is made up of metal coils that are heated by the combustion process. As cold air blows over these hot metal coils, heat is transferred to the cold air, turning it warm. This warm air is blown back into your home via the fan to raise your indoor temperature.

But sometimes the heat exchanger can get too hot. If your heat exchanger overheats, your furnace will shut down the heating process to prevent damage to the heat exchanger. Sometimes the fan will continue to run after the heat exchanger shuts down (to help cool it down), which could explain why you’re feeling cooler air.

The most common causes of an overheated furnace include:

  • Dirty air filter (which we discussed above)
  • Malfunctioning fan
  • Blocked exhaust vents

If you’ve checked your air filter and know that’s not causing the cold air problem, you’ll need to contact a professional to inspect your furnace and find what could be causing it to overheat.

Leaky air ducts

If your ductwork has large holes or leaks on the supply side (the side that delivers air to your home), the warm air inside the ducts will escape via the leaks. This could cause the air from your vents to feel cooler than normal.

An HVAC professional can inspect your ductwork to find and repair any leaks or disconnected ducts. 

Need a furnace repair from an Edmond tech?

At Prudhom, we take great pride in the fact that we will never sell you on a repair you don’t actually need. Our technicians will give you honest, upfront pricing on all furnace repairs and provide clear explanations throughout every step of the project.

Learn more about what to expect when you hire us by visiting our furnace repair page.