Furnace Blows Cold Air Sometimes? 7 Reasons And How To Fix

If your furnace blows cold air sometimes, you most likely have dirty air filters or blocked air vents. However, it could also be something as minor as the wrong thermostat setting or as complex as electrical circuity failure.

Nevertheless, you need to act quickly. Otherwise, you may find yourself in the cold for longer than you thought. Below, we discuss the most common reasons why furnaces blow cold air and recommended fixes.

How the Furnace Blows Warm Air

Before we get to the common reasons for blowing cold air, perhaps it would help you understand how the furnace generates warm air in the first place and why cold air coming out of the supply vents is an anomaly.

Generally, when you push the furnace button to turn it ON, the ignition system kicks on. Ignition systems vary depending on the type of furnace. However, whichever the design, the pilot system will light the burners.

This happens automatically as soon as the thermostat calls for heat. The gas valve will open, allowing gas to reach the burners. Then the burners burst into flames when the gas comes into contact with the pilot flame.  

The burner produces the heat necessary to warm your rooms. However, this heat goes through an exchanger that sits between the burning flames and the air flowing from the return vents to the supply vents.

The exchanger serves multiple functions, including containing the flame to prevent unwanted fires and extracting maximum heat from the flame.

Meanwhile, return vents allow cold/room-temperature air from the home to enter the furnace while supply vents supply warm air to your home via the ductwork. A filter sits right behind the return air vent to trap dirt and debris from the air entering the furnace.

As the furnace runs, a flame sensor in the burner area keeps track of the flame and immediately cuts the gas supply and shut off the furnace if the burner flame goes out or becomes too feeble.

Several safety switches, including a flame rollout switch and overheating switch, can also shut down the furnace if unsafe conditions are detected during furnace operation. Otherwise, the furnace will keep blowing warm air into your home.  

Reasons Why the Furnace Blows Cold Air Sometimes

 The furnace can blow cold air occasionally for various reasons, ranging from air leaks to short cycling and everything in between. The following are the seven most common causes and how to fix them.

1. Thermostat Issues 

The first place to check when your furnace is blowing cold air is the thermostat. That’s because it’s the thermostat that tells the furnace when to run and when to stop. So, if the thermostat setting is below room temperature, your furnace will blow lukewarm air.

The blower fan will continue to run even when heating is off, thus creating even chillier conditions inside the house.

Solution: Check whether the thermostat is functioning normally. If it’s not working or jumps values, consider replacing it. However, if it’s functioning well, set it correctly and wait a few seconds to see if the original issue resolves. If not, replace the thermostat anyway.

2. Blocked Air Filters 

Furnace filters are located just behind the return air vent to trap dirt, debris, and other airborne particles in the air entering the furnace from your home. This helps keep your supply air cleaner and healthier.

However, these can become blocked after extended use. When this happens, airflow into the furnace is curtailed or blocked altogether, resulting in overheating, which, in turn, can cause the furnace to “cycle off” to prevent fire. Thus, the fans will run (blowing cold air), but the burners will be off.

Solution: The only solution to blocked air filters is regular maintenance. Never go an entire week without checking your furnace filter. If it’s dirty, clean it. In fact, you need to clean it at least once every two weeks. More importantly, make sure to install a fresh filter at the beginning of every heating season.

3. General Overheating 

Any other reason that causes the furnace to overheat can also cause it to shut down, and therefore blow cold air. For instance, if the furnace experiences a flame rollout, in which the flame floats out of the designated zone, the furnace may overheat and shut down. Overheating can also result from a defective heat exchanger.

Solution: Try restarting the furnace to see if the problem goes away. If it doesn’t, troubleshoot it for overheating. Most furnaces have alarm systems or error codes that will alert you when there’s overheating and even indicate the cause.

Fix the underlying issue and restart the furnace.

4. The Pilot Light is Off  

The heating process cannot start if the pilot light is off. So, you’ll keep getting cold air from your furnace until you relight or fix the pilot system. So, why is the pilot off? It could be several reasons.

First, maybe a strong wind blew it out. Alternatively, maybe the pilot orifice is blocked. If not, then perhaps the pilot system itself is defective.

Solution: The first step is to check that the pilot light is functional. Does it work? If it’s a standing pilot, it should have a small flame. Can you see the flame when you remove the furnace door?

Alternatively, you hear a spark as the pilot lights up or see a hot surface ignitor glow. If you can’t see any of these, you need to inspect the pilot system and clean, repair, or replace it. 

5. Defective Flame Sensor

If your flame sensor malfunctions and can no longer sense the furnace flame, it will call for furnace shut down, meaning you’ll get cold air at the supply vents. The sensor rod can malfunction for many reasons. For instance, an aging sensor rod can lose its sensitivity.

Or the rod could be covered in a layer of dust. Alternatively, it may be just out of position.

Solution: Begin by wiping it with a softball of steel wool. Then put it back correctly and see if it works. If it doesn’t, it may be time to replace it.  

6. Leaky Ducts  

Warm air from your furnace flows through a series of ducts before exiting into your rooms. Now, imagine if your ducts are leaky! The warm air would escape through the leaky areas, meaning the air that finally reaches your home may not be as warm as you’d want. 

Leaks can also allow cold air to enter the ductwork and mix with the warm air, effectively lowering the supply air temperature.

Solution: The only solution here is to seal the leaks. Probe the entire length of your ductwork, identify the leaks, and seal them. You may need professional help. 

7. Gas Supply Issues 

The furnace relies on a pre-defined gas pressure to function optimally. If the pressure levels drop, heating levels may drop, or the furnace may shut down altogether.

Common reasons you may have low or no gas supply include supplier issues, gas pipe leaks, and problems with your pressure valve. You may also experience leaks if components along the supply or furnace lines aren’t sufficiently tight.

Solution: Begin by confirming that it’s not a supplier issue. If so, turn off the furnace at the breaker, switch off the gas valve, and call an HVAC professional to probe the entire gas line and furnace connections for leaks and fix existing issues.

Summary

Now you know why your furnace blows cold air sometimes. Again, we strongly recommend professional intervention whenever you don’t feel too confident in your DIY abilities.  

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