Why is Your Furnace Blowing Cold Air? Causes, Troubleshooting & Fixes

Furnaces are excellent when they’re doing the job they’re meant for. They're supposed to make winter bearable so you don't have to suffer through it. So, it’s really not funny or amusing when your furnace plays AC and starts cooling the room instead of warming it up.

However, one of the most common issues that furnace owners face is that their furnace blows cold air rather than hot air. This can be quite frustrating when it was *just* working.

This is not something that can’t be understood or fixed. In fact, a furnace blowing cold air is usually a sign that something is off with one of the internal or external components in your air system. 

These can usually be fixed through repairs or replacements. This guide will give you basic information about why a furnace is blowing cold air and how to fix it. 

However, you can always talk to a professional to make sure you are on the right track. Be aware that there are numerous ways to receive assistance. With the help of an HVAC service, you ought to be able to identify your needs.

Without keeping you waiting, the following are some reasons why your furnace is acting up:

Dirty Air Filter - Reasons Why Furnaces Blow Cold Air

One of the most common filters in homes is the air filter. This is a cloth, paper, or plastic piece that gets rid of dust and other contaminants in the air. The most important thing to remember about air filters is that they need to be cleaned regularly. 

Dirty air filters can actually impact your furnace because they can trap particles and debris on the surface of the filter. This can result in poor airflow and reduced efficiency.

Usually, your furnace heats the air that comes in from the outside by running it through a burner system. This system senses when there is cold air and aims to heat it up by signaling for the gas valve to open and release gas.

However, what happens when there is no airflow going into the furnace in the first place? 

This happens when you have a dirty air filter. By being clogged up with dust, debris, grime, and so on, the air filters placed right behind the vents prevent external air from entering the system. 

The burner, upon detecting this, dislodges its functioning by turning off the gas valve. Ultimately, this leads to cool, room-temperature air flowing into the house. Resolve this issue by cleaning or replacing your air filter regularly.

Wrong Thermostat Fan Setting - Reasons Why Furnaces Blow Cold Air

This might seem like a silly mistake, but it could be the root of all your problems. 

These devices allow people to control the temperature of different areas of the home. When thermostats are in auto mode, they use a sensor to measure the temperature in an area and then send an electronic signal to the heating or cooling unit. This signal allows the unit to change its temperature accordingly.

Most users prefer to leave the thermostat on auto. However, the fan button has an on setting, which you may have accidentally clicked or forgotten to reset. This setting usually keeps the fan running all the time. 

It can get problematic if you turn off the furnace and leave the fan running because there’s no more hot air being dispelled from the furnace. This leads to the fan blowing cold or room-temperature air.

You should also ensure that your thermostat is not accidentally set to cooling mode. This is a common mistake, especially when winter has just begun and you're still getting used to switching from cooling to heating your home.

Poorly Installed or Defective Thermostat - Reasons Why Furnaces Blow Cold Air

While we’re on the topic of thermostats, think back to when you purchased your thermostat. Was it recommended and/or provided specifically to fit your furnace requirements by your technician?

If you DIYed the process from start to finish, there’s a chance that your thermostat is simply not compatible with your furnace. 

If you install the wrong thermostat, the HVAC system may not work as intended and could cause serious problems. If the temperature in your home is too high or too low, the HVAC system may not be able to regulate it properly. The wrong thermostat could also cause other issues with your home's cooling or heating system, such as a shorter lifespan or even failure.

It is also possible that your thermostat is simply bad. Check whether your thermostat is sending its readings back properly to the furnace. If you stand near the vents after setting a temperature, you should be able to hear the furnace and the burner system coming on to meet the newly set temperature.

However, if you hear nothing, it could mean that your thermostat failed to connect with the furnace, which is probably why you are being served cold air instead of hot air. 

If your thermostat is poorly installed, you can correct it by contacting a professional to fix your furnace with a model that is actually compatible with it.

You can also replace your thermostat if it is damaged or malfunctioning. If it’s simply defective or broken, you could also send it out for repair. This is generally less expensive but time-consuming.

Damaged Pilot Light - Reasons Why Furnaces Blow Cold Air

A furnace's pilot light has one of the most important functions inside the furnace. It ignites the burner system. If the pilot light switches on, this is a sign to the burner system that it is okay for the gas valve to turn on, ultimately leading to heat. So it is possible that your furnace is not producing heat because of a defective pilot light. 

Usually, pilot lights are found in older furnace systems, so this might be a sign that your furnace is old, and it is time to consider adapting to modern technology and finding a replacement.

Most furnaces with pilot lights come with instructions on how to light the pilot light again in the user manual. Once you've found the pilot light, turn off the gas by turning the reset switch to the off position. Then, you wait until the extra gas has left the chamber before putting the part back together and lighting a flame.

Problem with Flame Detector - Reasons Why Furnaces Blow Cold Air

On the other hand, modern furnace counterparts have flame detectors instead of pilot lights, which kick-start the heating process. 

A flame detector or sensor is a critical safety component in a furnace. It monitors the airflow around the flame and signals a warning if the airflow is too low or if the flame is out of control. This helps to prevent serious burns or explosions in the furnace.

Flame sensors are also used in furnaces to maintain a preferred temperature. The heat sensor in the detector can tell when the gases in the furnace reach a certain temperature. This lets the furnace work at a lower temperature, which uses less energy to heat up the materials in the furnace.

But if your flame detector is broken because it hasn't been properly maintained, it could cause problems with the way the system works. 

A flame detector could be covered in dust, dirt, grime, and other particles, which could stop it from telling the system to make heat. As a result, the furnace produces cold air.

This problem can be simply fixed by cleaning the flame sensor: 

Step 1: Find a high-limit switch in order to clean the flame sensor rod. It is typically fastened with a metal plate attached to the outside of the furnace plenum. The air is heated in a sizable metal container called a furnace plenum.

Step 2: Locate a high-limit switch by removing the furnace's cover. A flame sensor rod is put through the furnace plenum and into the high-limit switch.

Step 3: To clean the rod with the plenum on your own, take out the two screws holding it in place.

Step 4: Use steel wool or sandpaper to carefully clean it. If necessary, there is another choice to take its place. 

Before replacing the flame sensor rod, it's best to talk to a professional if you don't know what to do or have any doubts.

Clogged Condensate Lines - Reasons Why Furnaces Blow Cold Air

If you own a high-efficiency heat furnace with a condensate pump that disposes of condensate from the heater, you may be dealing with a clogged condensate line. 

When the temperature of the air and fuel mix in a furnace reaches the point where the water starts to boil, condensation happens. This causes liquid water to condense onto the walls of the furnace. 

Condensate lines are drain lines that take the moisture that has built up inside the unit and send it outside. This keeps the moisture from building up and stopping the burner system from working. 

But if the condensate lines are clogged, the draining process will be slowed down, and the water will stay on the inside of the furnace. In turn, this would stop the burner system from doing its job, and finally, the furnace would start blowing cold air instead of warm air.

To fix this problem, you'll need to thoroughly clean out your condensate lines by hosing them down with water or blowing the clog out with compressed air. This should remove all the dust and debris and leave a clear pathway for the moisture to drain out of the furnace.

Air Leaks - Reasons Why Furnaces Blow Cold Air

Finally, air leaks could be one of the reasons you’re feeling cold air rather than warm air fill your home. If you have unsealed gaps or cracks, the hot air flowing into your room will likely escape through these openings.

Then, cold air from outside comes in and takes its place, making the temperature in the whole house lower. Air leaks can also lead to drafts that make the house feel frigid and uncomfortable, as well as cause freezing and damage to items inside.

Ensure that you seal all the cracks and gaps in your home. If there are holes or openings in the ductwork, you can replace the parts or cover them up with high-quality duct tape.

Not Enough Gas - Reasons Why Furnaces Blow Cold Air

Finally, one of the most common causes of your furnace blowing cold air could simply be a lack of gas to ignite. Furnaces use gas to heat the air in houses; it is burned in the furnace to heat up the air in the system, then this heated air is released into the house. 

Make sure the gas valve is open so that gas can get to the burner system and burn so that heat can be made.

If the gas valve is turned to the open position, your system has probably run out of fuel to power it. It is also possible that the gas supply to your home is not working because it has been cut off. 

Usually, every house has its own gas tank or line connected to the city’s gas line. You’ll have to check the tank if you have one. Otherwise, you might have to check up on the gas line to see if your gas supply has been cut off.

Working on Furnaces Safely - Why is Your Furnace Blowing Cold Air?

Before you do any work on your furnace, turn off the power switch and the circuit breaker. Making sure that an electrical item is off is the first thing you should do before working on it. 

A furnace should always be turned off before maintenance for a few reasons: 

  • If the furnace isn't switched off, it might produce sparks that could be deadly. These sparks can set items in the furnace ablaze and start a fire.

  • A furnace should be turned off before use because if it isn't, an emergency might occur and you won't have enough time to escape. You won't be able to get out of the furnace in time if there's a fire inside because of the heat and smoke. 

  • Shocks from electricity can also be dangerous.

When the power is on, you shouldn't work on a furnace since the consequences could be fatal.

The inside of a furnace can be dangerous—not to even mention how dirty they usually are. When working inside a furnace, the most important safety gear to use is goggles and a dust mask. Additionally, you should wear gloves, long pants, and a shirt that covers your arms and legs.

Make sure that you have the proper tools to perform maintenance on your furnace. 

If you lack the proper tools or don’t feel completely confident in your ability to fix the problem, there is no shame in asking for help. Reach out to a nearby HVAC company if you need help fixing your air system.

Conclusion - Why is Your Furnace Blowing Cold Air?

When they perform the function for which they were designed, furnaces are fantastic. They are meant to make winter more tolerable, so you won't have to put up with it. A furnace is not supposed to blow cold air; if yours does, there is something seriously wrong with it. 

We hope that this article has provided you with some useful information regarding how to troubleshoot and repair your own malfunctioning furnace.

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