Why Is My Furnace Heater Blowing Cold Air? (7 Causes + Easy Fixes)

The furnace heater is blowing cold air. That’s not how furnaces are supposed to work. What is wrong with your furnace? Before you call your HVAC guy, you can check for 7 reasons why your heater is blowing cold air yourself.

According to the Consumer Affairs survey, more than 20% of homeowners (20.44%, to be exact) reported they needed a heating system repair. Many of these had a problem with the furnace not working adequately. Furnace heater blowing cold air is one of the most common furnace-related problems.

Luckily, repairing a furnace that blows cold it can be done by yourself. First of all, you need to diagnose why your heater is blowing cold air, and then you can fix it (or call an HVAC guy, if the repair is too complex).

A professional repairing a furnace that blows cold air due to condensation drain cloggage.
A professional repairing a furnace that blows cold air due to a clogged condensation drain.

To help you out, we will list the 7 most common causes why your heater is blowing cold air. If you will need help finding a professional, you can use the contact form at the end of the article.

Note: These are furnace troubleshooting instructions, but you can also use them if your heat pump is blowing cold air or if your electric heater is blowing cold air.

#1 Cause: Check Furnace Fan Setting (It Should Be Set To “Auto”, Not “On”)

Do you have this situation: Furnace sometimes blows hot air and sometimes it blows cold air? That’s most commonly connected with a wrong fan setting.

Furnace fans usually have 3 settings: “On”, “Off”, “Auto”. A common mistake that causes the heater to blow cold air is having an “On” fan setting. It seems quite obvious that you should set your furnace fan in “On” setting, right?

Well, “On” setting means that the furnace blower will be running continuously (24/7). That means it will run when you’re actually running the furnace (in this case you will get hot air) and when the furnace is not running (in this case you will get cold air).

to prevent furnace to blow cold air set the fan to auto
Blower fan setting to “On” is the most common culprit for furnace blowing cold air.

If by any chance you have the fan set to “Off” (we usually set it to “Off” during the summer because fans can be noisy), the air coming from the vents might be cold as well.

Solution: Set the fan setting to “Auto”. This mode only runs the blower when the furnace is actually generating heat. That means that only when air is hot, the furnace blower will blow it inside your home.

The same thing goes for heat pumps and electric space heaters blowing cold air. An electric heater with “On” fan setting will just blow air, and that air might be cold. Only when you set it to “Auto”, the heated air will be propelled into a room by the fan.

#2 Cause: Pilot Light Problems (Older Furnaces)

A pilot light is used to turn on a furnace. If you want to check if your furnace has a blower light, you can check our article about Do All Furnaces Have Pilot Lights here.

Here’s the short summary: The newer furnaces (made after 2010) have an electronic pilot light that usually doesn’t cause a furnace to blow cold air when the heat is on. The problem is with older furnaces made prior to 2010. These heating systems have a gas pilot light; if extinguished, it will cause the furnace to blow cold air.

not working gas pilot light making the furnace to blow freezing air
Properly working gas pilot light. You will find these kinds of lights in older furnaces (made prior to 2010).

Without a functioning gas pilot light, your furnace will not start heating. Without the heating, the blower will just blow out cold air. You need to check if your gas pilot light has gone out. To check it, just locate the pilot and see if it won’t light or if it won’t stay lit. If this is the case, the cold air coming from a furnace is due to a faulty pilot light.

Solution: Simple in theory. Fix the gas pilot light. You can do this in two ways:

  1. If you’re a technical type, crack open the furnace’s manual. Check where the gas pilot light is located and follow the instructions on how to repair it or replace it.
  2. Call a furnace technician. Fixing a pilot light on your own might be a daunting task. There are a variety of reasons why your pilot is not working properly including a malfunctioning thermocouple, for example. A furnace technician will know what to do. In general, fixing a malfunctioning gas pilot light is relatively cheap to most other HVAC repair work.

This is one cause of the heater blowing cold air that usually needs help from a professional.

Note: If your electric heater or a heat pump is not blowing hot air, the pilot is not the issue. Space heater and heat pumps don’t use pilot lights.

#3 Cause: Clogged Furnace Filter (Restricts Airflow And Heat Exchange)

One simple reason why a furnace is not blowing cold air is that your furnace filters are ‘dirty’ or clogged. This may sound quite innocent but clogged furnace filters can lead to even more serious problems than a furnace not blowing hot air.

Here are two ways how dirty air filters can lead to furnace blowing cold air:

  1. Air needs to go through a furnace filter to enter the heating chamber. After it’s heated up, it also has to pass a furnace filter in the ducts. If filters are clogged, not enough air enters the heating chamber, and you won’t have a lot of hot air. The amount of how air is further reduces when the blower pushes it out. The net result can be cold air coming from the furnace ducts.
  2. Restricted airflow over the heat exchanger will cause this exchanger to overheat. More heat, right? Wrong. When this happens, the furnace will trip a high limit switch (it’s there to protect the furnace), which will in turn shut off the furnace. You will end up with cold air coming from your furnace.

You already know the solution to this problem, right?

Solution: Change the furnace filters.

if furnace is blowing cold air you need to replace furnace filters
Here is an example of how a dirty filter looks like.

First, you need to turn your furnace off. You can do that with the furnace thermostat. Check if the filters are dirty and replace them.

Needless to say, you will need the right furnace filter. Check the size (dimensions) and the MERV rating of the filters and get the same ones.

After you have swapped the dirty filters with the new ones, turn on the furnace and see if it’s still blowing cold air. If the clogged filters were the root cause of the furnace blowing cold air, you will now see that the furnace is running adequately (blowing hot air again).

The same is true if your heat pump is blowing cold air. Change the filters and see the result.

#4 Cause: Dirty Flame Sensor (Furnace Blows Hot Then Cold Air)

Every furnace has a furnace flame sensor. The function of this sensor is to instruct the burner if you need more or less heat. This is a finely tuned device that needs to function properly.

A furnace starts blowing cold air if the flame sensor is dirty. Basically, the faulty sensor tells the furnace that your home is warm enough, and cuts the burner from heating up the heat exchanger. That means that heat exchange is cold but the blower fan is still running; now it’s blowing cold air from your furnace.

Here’s how to test if you have a dirty furnace flame sensor: Turn the furnace off for 5 minutes before turning it on again. If the flame sensor is dirty, the furnace will start blowing hot air at first but it will switch to blowing cold air.

That’s because the heat exchanger will be heated up at first, but then the faulty flame sensor will tell the burner to stop producing heat. The net result is the ‘furnace heats at first but then stops’ problem.

Solution: There is no easy fix for a faulty flame sensor. Do not try to clean the sensor yourself. In this case, it’s advised you seek professional help. Call your HVAC guy.

To adequately clean the furnace’s flame sensor, you need to access it first. You will need to open the gas chamber, and that’s not an easy task. If you’re inexperienced, all those gas chamber parts can be confusing. What’s even more confusing is putting the gas chamber back together after you’ve cleaned the dirty flame sensor. Do not attempt to do this alone.

If the dirty flame sensor was the root cause of your heater blowing cold air, cleaning it up will restore the normal functionality of the furnace right away.

#5 Cause: Clogged Condensation Line (High-Efficiency Condensation Furnaces)

High-efficiency gas furnaces are the best. They have high energy and cost efficiency but the technology – using condensation lines – leaves these gas furnaces vulnerable to condensation line blockage.

The first thing you notice is that the high-efficiency gas furnace is blowing cold air. If that happens, go and check if there is water around your furnace. You might find anything from light condensation to a pool of water around the gas furnace.

Here’s why this happens: High-efficiency gas furnaces create condensate (water, if you will) when functioning properly. This water has to be emptied and that’s why all condensate gas furnaces use a condensation line. It’s a specialized PVC pipe that drains the condensate continuously.

If that consideration line gets clogged, the water will start to accumulate within and around the furnace. To protect itself, the furnace will shut down. If the furnace shuts down and the fan is still blowing, it’s blowing cold air from your furnace.

Anything from dust, mold, dirt and even ice can clog the drain pipe. In limited cases, the condensation pump – that enables the flow of water in the PVC line – might be broken as well.

Solution: Unclog the furnace’s condensation pipe. In this case, it might be best to call a furnace professional.

How to unclog furnace condensate drain seems to be quite easy:

  1. Locate the condensate drain.
  2. Check for clogging. This is usually an accumulation of dust, dirt, mold, ice, and so on.
  3. Remove the clogging. You can use pan tablets for that.

That’s the theory. In practice, locating the clogging is not all that easy. If you have ice in the furnace’s condensation drain, you might have bigger issues than the furnace not blowing out hot air. If the condensation pump is at fault, you definitely need an HVAC professional.

In most cases, this is a job for a professional. You might be able to check and fix a leaking window AC unit as detailed here, but a high-efficiency gas furnace is another thing altogether.

#6 Thermostat Issues (Setting Change Or Wrong Wiring)

All furnaces are controlled via a thermostat. If there is something faulty with a thermostat, chances are your furnace heater will start blowing cold air even when on heat.

There are two main reasons why this might happen:

  1. Recent thermostat change. Thermostats can be tricky. Maybe somebody changed the furnace set to “Cool”, lower the set temperature, or just messed a Nest thermostat via smartphone. Most of these changes are accidental but you can detect them in order to restore the normal function of the furnace.
  2. Wrong wiring. If you have a new furnace that blows cold air when set on heat, the culprit might be a wrongly wired thermostat. You can check here how to correctly wire a furnace thermostat.
    thermostat issue causing furnace to blow cold air
    Example of correctly wired 5-wire thermostat. White wire (W) is for heating and green wire (G) is for running the fan of your furnace.

Solution: You can fix this furnace blowing cold air problem yourself. Check the thermostat if everything is in order. If the furnace is set to “Heat” and not on “Cold”, if the set temperature is high enough, and so on.

If the thermostat is not correctly wired, you will have to call an electrician to fix the wiring.

#7 Unpaid Gas Bills (Fan Will Function Properly, Heating Won’t)

This is usually the last thing that could go wrong but you would be surprised in how many cases the unpaid utility bills cause a gas furnace to blow cold air.

Here’s the thing:

The lack of gas is not immediately noted. Most of the furnace’s function – the blower fan, thermostat, and so on – is powered by electricity. If you haven’t paid an electricity bill, you will immediately know that’s the culprit because the furnace won’t even start.

With gas furnaces, however, everything seems to work just fine. All the lights are on but the gas furnace still doesn’t blow out hot air. If you missed a gas bill payment, chances are your furnace is not getting the fuel it requires. That will result in a gas furnace seemingly operating as normal but it won’t have access to gas supply and it won’t heat.

Solution: Check if all your gas bills are paid.

If You Need A Professional Help:

We have covered all the most common reasons why a furnace might be blowing cold air (or at least not hot air).

If you have detected a problem that you can’t fix yourself, you would need a professional to help you out.

If you haven’t figured out what the culprit might be, a professional can help you out.

Hope all of this was helpful.

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