Why Is My Furnace Blowing Cold Air?

You turn on your furnace expecting warm air, but all you get is a blast of cold air. Now you’re left wondering, why is my furnace blowing cold air? Before you start troubleshooting, it’s important to understand how your furnace works, why it might be blowing cold air, and what you can do to fix the problem.

How Does a Furnace Work?

Your furnace heats your home by circulating warmed air through a system of supply and return ducts. The process starts when the indoor temperature falls below the thermostat setting, which triggers the furnace to turn on. Once the furnace is on, a blower motor circulates air through the supply ducts and into your home. At the same time, air is also being pulled from your home and into the return ducts. This movement of air pulls any cool air that might have snuck into your home back through the return ducts and into the furnace.

The furnace then heats the air before sending it back through the supply ducts and into your home. This process continues until the indoor temperature reaches the thermostat setting, at which point the furnace will turn off.

Why Is My Furnace Blowing Cold Air?

There are a number of reasons why your furnace might be blowing cold air. First, check to see if the furnace is actually turned on. If it is, then the problem might be one of those discussed below.

Furnace Blowing Cold Air Because There’s no Fire

Your furnace blowing cold air may be a result of the ignition not starting. When your ignition does not start a fire in the burner, your furnace cannot heat the air. This problem is usually caused by a broken thermostat, a faulty circuit control board, a lack of fuel, or a bad electric furnace ignition / faulty pilot light.

Broken Thermostat

Your thermostat tells your HVAC system when to turn on and off. If it’s not working properly, then your furnace might blow cold air. A broken thermostat might be the result of a bad battery, loose wiring, or a worn-out switch.

Faulty Circuit Control Board

The circuit control board is responsible for passing out instructions to your furnace’s components. The thermostat sends a signal to the control board when it needs to turn on the furnace. If the control board is faulty, it might not be able to pass along this information, and as a result, your furnace might blow cold air. Your circuit board may fail to carry out its function simply because of a lack of cleaning or maintenance.

Lack of Fuel

If your furnace uses gas and there’s no gas reaching the burner, then your furnace can’t generate heat. This problem is usually caused by a faulty gas valve or an absence of gas. You may just have to refill your gas to resolve the issue. If you make use of propane, you should check your propane tank to make sure it’s not empty. You have to refill it from time to time.

Bad Electric Furnace Ignition / Faulty Pilot Light

Another reason your furnace could be blowing cold air is because of an ignition issue. If you have an electric furnace, then the problem might be with the ignition or pilot light. If the ignition is not working properly, then the burner won’t turn on and your furnace will blow cold air. The same is true for a pilot light that’s gone out. If the pilot light is not lit, then the burner won’t turn on and your furnace will blow cold air. However, if you have a modern furnace, then it probably uses an Electronic Furnace Ignition instead of a pilot light.

Furnace Blowing Warm Air Not Hot Due to a Triggered Safety Mode

Is your furnace providing inconsistent heating? Does it blow cold air sometimes and hot air at others? If so, then your furnace might have entered into safety mode. A safety mode prevents your furnace from overheating and causing a fire. Your furnace enters into safety mode when there’s too much heat buildup in the furnace.

Overheating Furnace

When your furnace overheats, it triggers a safety mode that prevents the furnace from continuing to heat up. This is done by shutting off the gas supply or electric power to the furnace. Your furnace has a main limit whose function is to shut off the power if the temperature in the furnace gets too high. The main limit switch is usually located near the heat exchanger.

Faulty Main Limit

Over time, due to wear and tear, the main limit switch can become weak and might not be able to properly shut off the furnace when it overheats. As a result, your furnace will continue to heat up until it eventually blows cold air.

Cracked Heat Exchanger

Another reason for your furnace overheating and blowing cold air could be a cracked heat exchanger. The heat exchanger is responsible for heating the air that’s blown into your home. If it’s cracked, then the heat can escape, causing the furnace to overheat. A cracked heat exchanger can also lead to carbon monoxide leaks.

Furnace Blowing Cold Air Due to Restricted Air Flow

For your furnace to work properly, there needs to be good airflow. That is why it has ducts and vents. If the airflow is restricted, then your furnace will overheat and enter into safety mode. The most common cause of restricted airflow is dirty air filters.

Dirty Air Filters

Your furnace has air filters that need to be changed on a regular basis. If they’re not changed, then they will get clogged with dirt and debris. This will restrict the airflow from your home to your furnace. As a result, the furnace will not have enough air to heat and redistribute. It will end up blowing cold air.

That is why we recommend that you change your air filters at least once every three months. If you have pets or live in a dusty area, then you might need to change them more often. Another reason you need to change your air filters is that they help to keep your furnace clean. Over time, the dirt and debris will build up on the furnace itself and cause it to work less efficiently.

Blocked Vents and Ducts

Another reason for restricted airflow is blocked vents and ducts. Your furnace needs good air flow in order to work properly. If the vents and ducts are blocked, then the airflow will be restricted and your furnace will overheat. The most common causes of blocked vents and ducts are furniture and dust.

Wrongly Sized Ductwork

If your furnace is newly installed, then the problem of restricted airflow could be due to wrongly sized ductwork. If the return ductwork is too small, then it will restrict the airflow. As a result, your furnace will blow cold air.

Clogged Coil

Does your home have a central AC and furnace? If your answer is yes, then the cooling system’s evaporator coil is placed inside the furnace. While it’s true that the evaporator coil is meant for cooling, all treated air passes through it to get to the supply duct. If the evaporator coil is dirty or clogged, it will restrict the airflow or cut it off completely. This will cause your furnace to blow cold air.

How to Troubleshoot a Furnace Blowing Cold Air

If your furnace is blowing cold air, then you need to troubleshoot the problem and fix it as soon as possible. Otherwise, you’ll be left without heat in your home during the cold winter months.

  1. Check your thermostat setting: The first thing you need to do is check your thermostat setting. If the fan setting is set to “auto” then the furnace will cycle on and off as needed. However, if it’s set to “on” then the furnace will run continuously. If your thermostat is set to “on” then you need to change it to “auto.”
  2. Inspect the air filters: Next, inspect your filters to see if they’re dirty. If they are, then replace them with new ones. Inspect the vents and ducts: You should develop the habit of changing your air filters when they’re dirty or at most every 3 months.
  3. Check and adjust your vents: Check all your vents and registers to see if they’re open. If they’re not, then open them up. Make sure nothing is blocking the vents.
  4. Call a professional HVAC contractor: If you’ve followed all the steps above and your furnace is still blowing cold air, then it’s time to call a professional HVAC contractor like Robert B. Payne. They will be able to diagnose the problem and fix it for you. Gas and electricity can be very dangerous, so it’s always best to leave furnace repair to the professionals.

How to Avoid Furnace Blowing Cold Air

When it comes to furnaces, prevention is key. By signing up for a furnace maintenance plan with Robert B. Payne, you can avoid having your furnace blow cold air. Our experts will come to your home and clean and inspect your furnace on a regular basis.

On any such visit, one of our certified and insured technicians will also tighten any loose bolts, clean the burners and inspect the gas line connections. We will also check each component, especially the thermostat, furnace filter, pilot light, heat exchanger, and main limit switch. Plus, we will oil any moving parts to keep them working smoothly. All these maintenance tasks will help keep your furnace running efficiently and prevent it from blowing cold air.

This will ensure that it’s always in good working condition. Contact us today to sign up for a furnace maintenance plan.

Contact Us to Learn More About Your Furnace

If you have any questions about furnaces or need furnace repair, then don’t hesitate to contact Robert B. Payne. Our team of experts is always happy to help. Call us today at 540-373-5876 to schedule a service call. We look forward to hearing from you!