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Why Is My Furnace Blowing Cold Air? A San Antonio Tech Explains

January 22, 2021

Nobody wants to feel cold air coming from their vents on a freezing day. Needless to say, if your furnace is doing the opposite of what it’s supposed to, you want it fixed ASAP.

We’ll help you get to the bottom of this annoying problem.

You could be feeling colder air coming from the vents because of the following issues:

  • Lack of maintenance
  • The furnace is overheating
  • The air ducts are leaky
  • Incorrect thermostat settings
  • Improper installation

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


Want a San Antonio professional to repair your furnace? Our trusted technicians can quickly find what’s causing your furnace to blow cold air and fix the problem.


Problem #1: Lack of Maintenance

Neglecting to clean and maintain your equipment can cause a plethora of problems and prevent the furnace from heating your home. If the furnace has not been inspected or tuned up before it could have a dirty flame sensor. A dirty flame sensor could result in the unit cycling on or off or failing to ignite entirely.

Solution: Get a precision heating tune-up. We will clean and inspect the system to help it operate as efficiently as possible.

Problem #2: The furnace is overheating

You’d think that an overheating furnace would create even hotter air, but it actually has the opposite effect.

You see, every furnace has a safety switch that shuts down the gas burners or heating elements (depending on the type of fuel your furnace uses) if the furnace starts to overheat. This protects important furnace components like the heat exchanger from damage.

However, after the safety switch shuts off the burners or heating elements, the fan will continue to blow cool air over the heat exchanger to cool it down. As this happens, you may start to feel cooler air coming out of the vents.

So, what causes a furnace to overheat in the first place?

The following issues can cause your furnace to overheat:

  • Failed components
  • Dirty air filter
  • Closed vents
  • Blocked exhaust vents

We’ll share how to solve these overheating issues below.

Solution: Schedule a heating tune up to ensure that all critical components of the furnace are working properly.

Not only will this give you peace of mind your furnace will make it through the winter, having the furnace tune up will let it operate more efficiently.

Solution: The first thing you’ll want to do is check your air filter. A dirty air filter restricts airflow, which can cause furnace components to overheat.

If your air filter is dirty (like the one pictured below), then you should replace it with a new, clean one.

Clean (left) vs dirty (right) air filter

Next, you should make sure all of your vents are open—even in unused rooms. If too many vents are closed, your furnace will struggle to pull insufficient cool air. If there is not enough cool air blowing over the heat exchanger (the part that heats your home’s air), then it can overheat and your furnace will shut down.

If the cold air problem persists after you do those two things, then it’s time to call a professional for help. The professional will check for other issues that cause overheating, such as a blocked exhaust vent, and recommend any needed repairs.

Problem #3: The air ducts are leaky

Do you feel weak, slightly cooler air coming from your vents all the time?

If so, the problem could be caused by leaky air ducts.

Most homes have some amount of duct leakage, but if your ducts have severe leaks, then it means heated air from your furnace could be escaping into the attic. This results in weaker airflow, which could give the perception that the air is colder than it should be.

When hot air leaks into the attic, it weakens the airflow coming out of the vents

Solution: Schedule our HVAC professional to inspect your ducts. If they find leaks, the professional will recommend sealing the ducts, or, if the leak is severe, replacing the damaged ducts.

Fixing leaky ducts will not only improve the overall comfort inside your home, but it will also reduce extra wear and tear on your furnace and make it operate more efficiently.

Problem #4: Incorrect thermostat settings

If the thermostat is set incorrectly, your furnace could be pushing cooler air into your home.

Solution: First, go to your thermostat and make sure the thermostat is set to HEAT. If your thermostat is set to COOL, that would explain the cold air coming from the vents.

COOL and HEAT settings on a thermostat

Next, make sure that the thermostat is set to AUTO and not ON. In ON mode, the furnace will push air into your home non-stop, even when the furnace isn’t actively heating air. This could explain why you feel cool air coming from the vents.

In AUTO mode, the furnace fan will only start working when the furnace is heating air. This means when you set your thermostat to AUTO, only warm air will come out of the vents.

The fan settings (ON and AUTO) on a thermostat

Problem #5: Improper Installation

Your furnace could also be blowing cold air because it was installed incorrectly or it’s the wrong size for your home. If you recently had a new furnace installed and are experiencing cold air coming from the vents, an improperly sized or incorrectly installed furnace could be your issue. You should reach out to the company or contractor who installed your furnace to have them check their work and correct it if needed.

Need a furnace repair from a San Antonio tech?

We can quickly fix whatever is causing your furnace to blow cold air. When you hire a Jon Wayne technician, you can count on exceptional customer service and repairs done right—the first time.

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

*All prices shown are estimates and can vary depending on a variety of factors. For an accurate price for your project, call (210) 293-6700.

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