How to Fix a Furnace That's Blowing Cold Air

Posted by James Clark on Nov 17th 2021

How to Fix a Furnace That's Blowing Cold Air

Does your blower motor refuse to stop? If you keep asking yourself, "Why is my furnace blowing cold air?" then this is the article for you.

Heating system repairs are the fourth most common repair made in homes. A whopping 20.44% of people responding said they'd replaced or repaired their heating unit in 2019.

If you're interested in saving yourself the time, money, and effort of a major repair, keep reading.

Why Is My Furnace Blowing Cold Air?

There are a number of reasons why your furnace could be blowing cold air. We'll deal with the more common ones first and then move on to the more unlikely candidates.

Hand-in-hand with this, we'll take a look at why your blower motor won't stop and how this problem relates to your furnace issue.

First, it's important to understand how the two work together. This will help you understand why the problems are so often related.

The blower has one simple job - to blow air around your home. When you set the temperature on your thermostat, you're really asking your heating system to reach that temperature everywhere in the house.

The blower motor is responsible for moving heated or cooled air around until you reach the temperature requested. So, as you can see, the blower is an important part of the whole system.

If it turns out your blower motor is faulty, don't despair. It's relatively simple to replace, and there are some great guides to get you started.

First, though, you'll need to troubleshoot your furnace.

Troubleshooting Your Furnace

Some issues are simple to fix, while others will require a professional to assist you. So let's dive right into figuring out why your furnace is only blowing cold air.

Check Your Thermostat

Sometimes thermostats break. And when they do, your furnace has no idea what to do. For example, it's possible that your thermostat might be incorrectly telling the furnace to blow cold air around your house because of bad signals or connections.

Before you replace your thermostat, there are a few steps you can follow to narrow down the problem:

Check Your Thermostat Fan

You'll need to take a look at the settings you have for your furnace fan. Some models have a fan that runs all the time, even when the system isn't making heat.

Make sure the switch is in the AUTO position, not ON. Once you've set the correct switch position, your furnace should only run the fan when it's actively heating.

Reset and Check the Temperature

It may sound obvious, but sometimes the problem is simply that the temperature isn't set correctly. Make sure that you've set the temperature to something higher than the room's temperature.

Check for Dust

Carefully remove the cover of your thermostat and check for accumulations of dust. Dust build-up can cause problems with your thermostat, so this is another easy step before you go for a replacement.

Check the Wiring

This step will involve electrical work, so make sure you follow safety guidelines while you're working. You will need to remove the cover from the thermostat in order to get to the wiring.

Once the cover is open, you'll notice that there are two sets of wires. One set brings power to the unit, and the other activates the furnace. Read through the steps below carefully before you start working.

  1. Make sure power is off at the breaker
  2. Make sure the power switch is off if there is one
  3. Undo the terminals and make a note of where each goes
  4. Be careful not to allow the wires to fall into the wall
  5. Twist the ends of the wires together
  6. Turn on the power for the furnace

If the furnace burners come on and the blower starts functioning normally, your thermostat is defective and will need to be replaced. While you're replacing your faulty unit, it might be a good idea to upgrade to a digital version.

Check the Breaker

Another obvious check but something that comes up fairly often. Check that the breaker is on and your furnace's power switch is set to ON as well. If your furnace has a pilot light, check out our troubleshooting steps below.

Check Your Filter

Sometimes a dust-clogged, or dirty filter is all that's needed for your furnace to malfunction and start pumping out cold air. What's happening behind the scenes is that the filter isn't allowing air to move around the furnace as well as it should.

This, in turn, makes the furnace heat up far too quickly. That means the unit will stop all heating cycles until it cools down enough to run.

You can check your filter quickly by sliding it out of the filter compartment and visually inspecting it. If it looks dirty, replace it. Filters are generally quite affordable. You should certainly consider replacing your filter before you embark on any more expensive repairs.

Removing the filter is a simple process in most models:

  1. Find your filter compartment
  2. If there is one, remove the access door
  3. Slide the filter out
  4. Inspect the filter, looking for dirt, tears, or clogs
  5. Insert the new filter, making sure it's going in the right way
  6. Close the filter compartment

Once the furnace is cool again, it should start working normally.

Check Your Pilot Light

Depending on the age of your furnace, this step may not apply if you have an electric furnace. Older models still use pilot lights, so check your furnace's manual or contact a professional to ask questions if you're unsure.

If the pilot light goes out, your furnace will start blowing cold air since it's no longer generating any heat. A natural gas furnace uses combustion to provide heat, so you'll need to get your pilot going again.

The method may vary depending on the type of furnace you have, but the general steps are the same:

  1. Make sure the furnace is turned off
  2. Find your pilot light and make sure the switch is off
  3. Wait around 15 minutes to let leftover gas dissipate
  4. Toggle the reset switch to "PILOT"
  5. Use a lighter to ignite the pilot
  6. If the pilot lights, turn the switch to the ON position
  7. Turn your furnace back on

If this doesn't work, it's time to start shopping around for a replacement pilot.

Check Your Control Board

It's important to note that the control board is an entirely different part to the thermostat. The control board is responsible for pushing power to all the various parts of your furnace.

As with the thermostat, make sure you're following good safety guidelines and are working safely.

Although the process isn't terribly difficult, you will need to know how to use a voltmeter. Again, that's not terribly difficult, but it's the only way to know for sure that your control board is the problem.

Gather Supplies

You'll need a few tools to check your control board. However, you very likely already have these tools, so you shouldn't need to go out and buy anything special. The exception is the voltmeter, but they're so helpful that it's an excellent idea to have one for all your furnace troubleshooting anyway.

  1. Voltmeter
  2. Screwdriver
  3. Electrical tape

Diagnostics Port

Many furnace models have a clear glass port with LED lights behind it. This is usually visible in one of the access panels.

The LEDs in the port help you figure out what might be wrong by blinking in particular patterns. Your furnace manual will show you what each of these patterns means.

It's a good first step to check this before you go any deeper. If you can find the problem without having to undo any other parts, you'll thank yourself later.

Remove Access Panels

Many furnace models use screws to secure access panels though some have latches. There should be two panels that you'll need to remove to access the interior components.

Tape the Door

Because we're working on a live and powered-on furnace, you'll most likely need to fool the door switch by placing a small piece of tape over it. As soon as you take off the access panels, you should be able to see the safety switch.

When you remove the panel, you'll notice that the switch pops up, and your furnace turns off.

Check the Indicator

The very first step once you've got the access panels off is to check that the indicator light is blinking. While some furnaces don't have this light, most of them do.

To locate the indicator light, look at the lower-most access panel. You'll see the control board in front of the furnace blower. If the light is blinking, congratulations, you have power, and you can skip all the way to "Check the Terminal Strip" down below.

If the light isn't blinking or you don't have an indicator light, keep reading.

Test the Power

If the indicator light isn't blinking, you now need to check where the power problem is. You'll be checking through each component of the control board to see where the fault is.

Find the Common Wire

The first step is to locate the common wire. You won't be doing anything with this yet but will need to know which one it is later.

The common wire is found on the transformer, a small, rectangular box usually attached to the furnace with screws. It will have four wires coming out of it. Two high voltage, black and white, and two low-voltage wires.

Although the common wire is usually black, you can double-check this by looking at the transformer. It should be labeled "COM".

Test the Power to the Board

Check your control board for a wire labeled "LINE". This is the wire that brings voltage to the control board. Using your voltmeter, touch both the common and LINE wire. You should see 120v.

Test the Transformer

The transformer converts the 120v you saw in the previous step down to 24v. To test whether the transformer is working, use your voltmeter to touch each of the high-voltage wires. Each should read 120v. Next, touch the low-voltage wires - they should read 24v.

If you see 120v coming in but nothing coming out or a different voltage to 24v, you may have a problem with the transformer. You'll need to buy a new one and replace it as soon as possible.

Test the Molex Connectors

Carefully trace each of the two low-voltage wires backward until you reach the control board. You should see them connecting to a Molex Connector. Gently insert your meter leads into the sockets where the low-voltage wires run and take note of the readings. You should see a reading of 24v each time.

Test the Terminal Strip

The very last step is checking to see that the terminal strip is working as it should. Right at the edge of the control board, you should see five colored wires with W, R, Y, G, and C labels.

If you touch your voltmeter to the R and C wires, you should see a reading of 24v. If all of the tests were successful, you can rule out the control board as the source of your problem. 

Check for Water

Your furnace has a condensation drain line that works to get rid of condensation produced during regular operation. If this pipe becomes blocked or otherwise compromised, you may find standing water around your furnace.

Furnace Troubleshooting Made Easy

Now that you can confidently answer the question "Why is my furnace blowing cold air?" and know all the tests to run, there's only one thing left to do.

You need a good, reliable vendor who can guide you to the correct part and make sure it gets to you quickly and easily. So contact our parts department and start the process of fixing your furnace now.

About the Author

James Clark

James Clark is the HVAC Controls Specialist and Ecommerce Manager for Technical Hot & Cold. He has been with the company since 2014 and enjoys helping homeowners save money by providing help articles that walk them through various DIY HVAC repairs. In his spare time, he's playing music with his children and spending too much time working on his lawn.


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