Furnace Blowing Cold Air : 11 Reasons Why And How to Fix It

No matter what side of the world you live in, you probably aren’t fan of freezing cold weather during winter. When you go outside to work or to do grocery shopping, you probably get even colder thanks to icy wind. And what is worse than having to go outside during winter? Coming back home just to find your furnace blowing cold air instead of warming everything up.

Sometimes even the new furnaces can malfunction and blow cold air instead of warm. All of this leads to a bad surprise if you haven’t noticed it right away. Most of the times, you can fix everything by yourself. In other occasions, you will have to call some professional help.

Below are several commonly reported reasons why is your furnace blowing cold air. This way you can know when to try finding a solution by yourself, and when to call for some assistance.

Furnace Blowing Cold Air : 11 Common Reasons Why And How to Fix It

1. The Thermostat Fan Is Turned Incorrectly

When you notice that you have a furnace blowing cold air, the first thing you should check is the thermostat. When the thermostat is on, it might set the air to the room temperature. This will make you have the subjective feeling of cold air. Also, a thermostat turned to ON will make the blower work non-stop. This means that it might blow even when the heater is not working. The solution to this is simple. Just switch the thermostat from ON to AUTO settings. This should solve the problem.

2. Dirty Flame Sensor

The biggest sign that your flame sensor is dirty or compromised is when the warm air quickly turns cold. When dirt is covering a flame sensor, it can cause the burner to repeatedly turn off. This means the flame sensor needs cleaning. Sometimes it’s necessary to replace the sensor completely. It is best if you call HVAC professional to help you with this problem.

3. Air in the Gas Lines


Another common issue can happen if there was any gas line work in the neighborhood. This can cause air to come inside of gas lines. When that happens, your heater will malfunction until the air is out. This can sometimes be a serious problem, but usually you can solve it by yourself. When you wonder why is your furnace blowing cold air, try doing the simplest thing… Turning it on and off again. When you start several cycles, the air will most likely get out. This will cause your heater to start working properly again.

4. The Pilot Light Is Out

Some old furnaces have a continuous pilot light. These models have an ongoing issue as the pilot light tends to go out. This can happen if a gust of air got in the unit and lighted it out. You might try to relight it yourself, but this tends to be a dirty job. Calling a professional is also a good option here.

5. The Gas Can’t Reach the Ignitor

With most gas furnaces models, you can see the light of the ignitor. Its color can tell you a lot about the state of the device. For example, if the ignitor is glowing white, this should mean its working properly. White means it’s warm enough to ignite the gas. If you’re seeing this, and your furnace is still blowing cold air, then the gas can’t reach the ignitor. The reasons for this are many. Usually this means that the pilot nozzle is closed. Another more serious reason is a broken gas valve. The solution for this depends the model of your heater.

If the glow is red, this means that the ignitor can’t get hot enough to heat up the gas. You should buy a new ignitor. Be sure to find the one that is right for your unit! On the other side, if the light is blue, this means that the gas does ignite, but shuts down quickly. The common reason behind this is dirty flame sensor. This also happens if something is blocking the airflow. Try vacuuming the furnace vent.

6. Thermostat Isn’t Compactible with the Furnace


If you have installed the new thermostat lately, this might’ve caused the issue. Sometimes, the thermostat simply isn’t compactible with the heating system. Sadly, this cannot be solved on its own. You should contact an HVAC expert that will help you find the solution.

If you have the same thermostat for a while, check the batteries. If they are low, this might be the cause of your furnace blowing cold air.

7. Clogged Air Filter

If you haven’t changed your air filters lately, this might’ve caused the issue. When the filters are dirty, the warm air can’t go out of the device. Instead it stays inside of the unit, warming it up until it overheats. This will shut down the burner. Clogged air filter is amongst the most common reasons why is your furnace blowing cold air. Luckily, the solution is simple. Just change the air filters. You will most likely be able to do this by yourself. Just be sure that you have a replacement filter. If not, you can buy the one from the manufacturer or on some website such as Amazon.

8. It’s Going through a Defrost Cycle

If turned on for a while, some heaters might go through a defrost cycle. This is especially common if you are keeping yourself warm using a furnace that operates on heat pumps. Why does this happen? If the temperature outside is too cold, the moisture in the air freezes. The system has to solve this, so it will start a defrost cycle. This causes the unit to stop heating. Some devices will stop working temporarily until the cycle is completed, but not all. When a fan is still working but the heating isn’t, it will end with your furnace blowing cold air.

There really isn’t a solution as this isn’t a problem. Don’t call the professional right away. A lot of companies will take money pretending to solve this non-existing ‘issue’. The duration of a defrost cycle depends on factors such as outside cold and the model of the unit. However, if it lasts for way too long or it happens often, then it’s likely that your heater is malfunctioning.

9. Oil Filter Is Clogged

If you have an oil furnace, you have another filter to worry about. Oil filters in these units can get clogged, just like air filters on gas heaters. Various dirt and debris can compromise the units work. This can cause the ignition to malfunction. Since the fan is still working, and the heating is off, you will end with your furnace blowing cold air. Similar to the air filters, you can replace them yourself. However, oil filters are messy and you will probably find it too hard to reach them. We suggest you call the expert to help you with this one. Hiring a professional to change the oil filters isn’t expensive, and it might save you the trouble.

10. Inadequate Gas Supply

The reason is simple, but the solution in most cases isn’t. If you notice there is no gas small in the heater, this probably means that the gas supply isn’t working. This can cause your furnace to lock down for safety reasons. On some occasions, the gas is simply turned off. You can turn it on by yourself and the furnace should start working soon. If the gas supply is on but there is no gas coming out, it means there is a bigger problem. Inadequate gas supply usually means that something severed the gas line. This is a serious issue and a professional should deal with it.

11. Blocked Condensate Line

When you have a high-efficiency furnace, this is a common problem. Often times, when you have a furnace blowing cold air, you will notice a pool of water around it. This usually means that something is blocking the condensate line. How is this connected? High-efficiency furnaces condensate, and that water goes down the drain line. Sometimes, that drain line is blocked, which will cause the water to go back up into the furnace. This will cause an overflow and the furnace will shut down the heating to prevent water damage. On rare occasions you might see the blockage yourself and remove it. It is better to call a professional to ensure everything is fixed properly.

 How to Prevent Your Furnace from Malfunctioning?

The only way you can ensure your heater will keep on working for many years is to do regular maintenance. This should be done at least yearly. Regular maintenance won’t just keep your furnace from actually cooling down the place. It can also help you stay healthy by stopping the growth of bacteria and mold.

Furnace Blowing Cold Air: Bottom Line

We know this might be expensive, but it’s better safe than sorry. Of course, you shouldn’t waste your money on every single repair out there. As we have listed above, you can solve many of the problems on your own. Just be sure that you know what you are doing and that you are staying safe.