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

Furnace Blowing Cold Air

Furnaces are excellent- when they’re doing the job they’re meant for. They’re supposed to make winters liveable so that you don’t freeze to death. So, it’s really not funny or amusing when your furnace decides to play AC and starts cooling the room instead of warming it up.

Unfortunately, however, a furnace blowing cold air instead of hot is one of the most common problems experienced by furnace owners.

But 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 components in your unit. 

These should usually be fixed through repairs or replacements. You can refer to this guide on basic information about basic causes and solutions to fixing the problem of a furnace blowing cold air, but you can also always consult a technician to make sure you are on the right track.

Without keeping you waiting, here are some reasons why your furnace is acting out, followed by some tips on how to fix these common issues. 


dirty furnace filter

Your furnace generally works on the principle that it heats up the air received from outside 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 where 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 that are placed right behind the vents are preventing 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 out into the house. Resolve this issue by cleaning and washing your air filter on a regular basis.

Wrong Thermostat Fan Setting

This might seem like a silly mistake, but it could be the root of all your problems. Usually, users prefer to set the thermostat to ‘Auto’ so that the unit runs the fan only when the furnace or the air conditioner is also in operation.

However, the fan button itself has an ‘On; setting which you may have accidentally clicked, or forgotten to reset. This setting usually runs the fan on all the time, and 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 set to the ‘Cool’ mode accidentally. This is often a mistake a lot of people make, especially when winters have just started, and you’re still getting used to shifting from cooling to heating up your home.


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

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, and this is probably the reason 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 up with a model that is actually compatible with it.

You can also replace your thermostat if it is damaged or bad. If it’s simply defective or broken, you could also give it up to be repaired. This is generally less expensive and time taking.


A furnace 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 for the burner system that it is okay for the gas valve to turn on as well, 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, well, your furnace really is old, and it is time to consider adapting to modern technology and find a replacement.

Usually, most furnaces with pilot lights come with user manuals that offer directions on how to relight the pilot light. Once you’ve located the pilot light, you have to stop the gas by switching the reset toggle off. Then, you wait for the excess gas to leave the chamber before resetting the component and starting a flame.

Related: Also Check Our Guide On Furnace Pilot Light Won’t Stay Lit


On the other hand, modern furnace counterparts have flame detectors instead of pilot lights which kick-start the heating process. This triggers the gas valve to open and heat up the air by sensing that the flame is on.

However, if your flame detector is faulty in the sense that it is improperly maintained, this might cause problems in the smooth functioning of the working system. 

A flame detector may be covered in dust, dirt, grime, and debris, and this might be preventing the flame sensor from blocking its ability to direct the system to create heat. In turn, this causes the furnace to blow cold air.

This problem can be simply fixed by cleaning the flame sensor. If you don’t want to get personally involved in this, you can always call a professional technician to clean and sanitize the flame sensor and get the furnace back on track.


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

These pipes are basically draining lines that remove moisture collected in the unit and exhaust it all outside so that the moisture doesn’t gather and prevent the burner system from activating. 

However, if the condensate lines are clogged, this will withhold the draining process, leaving the moisture to stick to 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 resolve this issue, you’ll have to thoroughly clean out your condensate lines by hosing them with water or blowing compressed air. This should get all the dust and debris out and leave a clear pathway for the moisture to drain out of the furnace.


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

In turn, cold air from the outside is taking its place instead, resulting in lower temperatures throughout the house.

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


Finally, one of the main causes behind your furnace blowing cold air could simply be the fact that your furnace does not have any gas to ignite. Ensure that the gas valve is open so that gas reaches the burner system and burns accordingly to create heat.

If the gas valve is turned to the open position, it is probable that your system has 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 gas 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.


A furnace is not supposed to blow cold air, and if yours is- there is something seriously wrong with it. This article, hopefully, helped you figure out where you are running into an issue, and what solutions you can resort to in order to get your home’s heating system back on its feet. 

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