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Why Is My Furnace Blowing Cold Air? A Texas Tech Answers

March 06, 2019

Does this sound familiar? You turn your thermostat to “heat,” expecting your furnace to warm up your home. Instead, the air coming from your vents feels much colder than it should.

If your furnace is blowing cold air, you might have one of the following problems:

  1. A thermostat issue
  2. A faulty fan limit switch
  3. An overheating furnace

In this blog, we’ll go into each of these problems and how a professional will help you fix them.

Need a furnace repair?

Problem #1: A thermostat issue

One of the most common causes of a furnace blowing cold air is a problem with the thermostat.

Below are 3 issues you could be having with your thermostat:

  1. Your thermostat is set to COOL instead of HEAT. You may have accidentally set your thermostat to COOL instead of HEAT, which would explain the cold air coming from your vents. Double check that your thermostat is set to HEAT.
  1. Your thermostat is set to ON instead of AUTO. When set to ON, the fan in your furnace will blow continually, even when the furnace isn’t heating the air. As a result, you may feel cold air coming from your vents. Switch your thermostat to AUTO, so the fan will only run as your furnace is heating air.
  1. Your thermostat has wiring issues. If the wiring connected to your thermostat is malfunctioning, your thermostat could be sending incorrect signals to your furnace, telling it to cool your home instead of heat it. A professional will need to inspect your thermostat and repair any faulty wiring.

Solution

Make sure your thermostat is set to HEAT, not COOL. Then, make sure the thermostat is set to AUTO, not ON. If you still feel cold air coming from your vents, contact a professional to inspect your thermostat.

Problem #2: Your fan limit switch needs to be adjusted/replaced

The fan limit switch is located inside your air handler, and its job is to tell the furnace blower when to turn on.

Under normal operation, this is how your limit switch should work:

  • The limit switch turns the blower on after the burners have warmed up completely
  • The limit switch turns the blower off after the burners have cooled down (to prevent overheating)

However, if the limit switch is misreading the temperature inside your furnace, the blower could be pulling in air and circulating it back into your home without your furnace actually heating the air.

You likely have this problem if...

You feel cold air, followed by warm air, then cold air again before the heating cycle ends. (You’ll know the cycle is over when you don’t hear your furnace running anymore.)

Solution

Have a professional check the fan limit switch and see if it needs to be adjusted or replaced.

Problem #3: Your furnace is overheating

Your heat exchanger (pictured below) is the part that heats your home’s air. If the heat exchanger gets too hot, it will trip a switch, which shuts off the heat exchanger to prevent damage until it cools down.

Furnace Anatomy V3

While the heat exchanger is cooling down, the blower will continue to run (to help cool down the heat exchanger), which could be why you’re feeling cold air from your vents.

Overheating can be caused by several problems, including:

  • A dirty air filter
  • Closed vents
  • A faulty blower motor

Signs you have this problem

  • You have extreme fluctuations in temperature while your furnace is running (hot then cold)
  • You hear your furnace shutting on and off frequently
  • Your furnace turns on and off 3 or 4 times before shutting down for good

Solution

First, check your air filter. If it is dirty, replace it.

Second, make sure all of the vents are open in your house—even in unused rooms.

If that doesn’t solve the problem, contact a professional to inspect your furnace and find what’s causing it to overheat.

Need a furnace repair in San Marcos, TX?

We’ll troubleshoot whatever is causing your furnace to blow cold air, and we’ll give you upfront estimates on how much it will cost to fix your furnace.

To learn more about what to expect when you hire us, visit our furnace repair page.

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