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Why Is My Furnace Blowing Cold Air?

Nothing is more frustrating than when the weather turns cold and your heating system stops working. Homeowners may try to fix the problems themselves rather than pay for HVAC service, and sometimes, this issue could result from a relatively minor cause that’s easily handled. However, furnaces are complicated equipment, and DIY repairs could void the system’s warranty, leading to more expensive and complicated issues in the future. If your furnace is blowing cold air or is not adequately warming your home, it’s likely time to call an expert.

Common Reasons A Furnace Will Blow Cold Air and How to Fix Them

A furnace blowing cold air through your air ducts could be due to several reasons, from minor to serious. It’s important to rule out any possibility to ensure the right repair is performed to make your home comfortable all winter.  


It’s true that a furnace blowing cold air could be a simple problem. The first thing you should check is your thermostat. If the fan is set to on, you’ll want to switch it to auto. This will stop cold air from blowing in when the heat is not on. If you flip the switch on your thermostat and the furnace is still blowing cold, it could be a bigger issue.

Dirty Air Filter

Air filters are meant to trap dirt, dust, and contaminants to help keep your indoor air, ductwork, and furnace clean. If the air filter is not regularly replaced or maintained, all of the collected dirt and debris can choke and restrict necessary airflow to the furnace, causing it to work harder and potentially overheating the heat exchanger and shutting it off too quickly. Additionally, the insufficient airflow caused by a dirty air filter means there’s less to heat and redistribute throughout the home, causing the furnace to blow cold air. 

Lack Of or Poor Airflow

Poor airflow can be caused by more than just a dirty filter, so it’s important to look at all possible causes. Blocking vents or return registers with furniture or other items can hinder airflow, both back into the furnace and out into the rooms. Check to make sure nothing is placed in front of or over the vents, as well as ensure they are all open and clear of dust and debris. 

A blower fan motor malfunction may also cause a furnace to blow cold air. Typically, the blower motor is responsible for pushing the heated air through the ductwork, but if all you’re feeling is cold air, it is essential to consult with a professional HVAC technician for an inspection and possible repair or replacement. 

Finally, poor airflow could be caused by undersized ductwork. If the furnace has a larger capacity, but the ductwork is too small, airflow can be restricted, preventing proper warm air distribution and leaving it cold once it reaches the vents. This issue would also require consultation with an HVAC system professional to determine if upgraded ductwork may be necessary.

Leaky Ducts

Holes, cracks, or gaps throughout the ductwork allow heated air to escape before reaching the rooms. If the air ducts were incorrectly installed, they can also cause leaks due to improper sealing. Small, noticeable gaps can be sealed using mastic sealant or metal tape, but more extensive repairs require an HVAC professional. 

If your ducts pass through unconditioned areas that lack proper insulation, like an attic or crawl space, this could also result in heat loss as it cools the air that moves through, distributing that cooler air into your home. Any ducts in these spaces should be insulated with the appropriate materials, so seek professional assistance. 

Like most things, ductwork deteriorates over time, developing leaks and compromising its integrity. The ducts may require complete replacement if they are very old or significantly deteriorated. An experienced HVAC system professional can assess their condition and recommend the appropriate action. 

Failing Parts

Your furnace is made of myriad electrical components. As your system ages, these parts may fail. When this happens, cold air will blow from your system, or your system may not work at all. Finding the right parts to repair your furnace can be tricky, and using the wrong replacement parts could be a costly mistake. Calling a trained HVAC professional is the best way to diagnose what’s happening and make any necessary repairs. They will have access to the necessary parts, which can be replaced quickly without further damaging your system.

Flame Sensor or Pilot Light Issues

A dirty flame sensor is the most common reason a furnace will blow cold air. When the sensor is dirty, it may cause your furnace to start blowing hot air, but then the air will quickly turn cold again and begin blowing into your home. You’ll need to call a qualified professional so they can clean the sensor and check for any other problems.

If your home is equipped with an older model gas furnace, it most likely relies on a pilot light; if the light is extinguished, the furnace will start blowing cold air. You can check to see if it’s on by looking at the pilot light assembly for the flame. Newer systems built within the last 20 years will utilize spark ignition or hot surface ignitors rather than a pilot light. In either case, messing with gas furnaces is not a good idea unless you are qualified, so reach out to an experienced professional. 

Possible Issues for Different Furnace Types

When your furnace starts blowing cold air, identifying the specific type of furnace you have is crucial to understanding and effectively resolving the issue. Different furnace types may encounter distinct problems. Here are some possible issues for each furnace type: 

Electric Furnace

Some possible issues for electric furnaces that would cause it to blow cold air include a faulty heating element or a tripped circuit breaker. The heating element in an electric furnace is essential for producing warm air, but if it malfunctions, it could begin circulating cold air. A professional technician can inspect and replace a faulty heating element for you.

Circuit breakers can be tripped for one reason or another, and if they’ve been tripped and disrupted the power supply to your electric furnace, they can start blowing cold air. Be sure to check your circuit breaker and reset if necessary. If it’s still blowing cold air, consult with an electrician to address any potential electrical problems. 

Heat Pump

Heat pumps rely on refrigerant, and if there are insufficient refrigerant levels or leaks, the heat pump’s ability to absorb and release heat is compromised, resulting in cold air output. Addressing refrigerant leaks and recharge levels as needed is critical, so consult with an HVAC professional to properly identify and fix any issues. 

A heat pump also utilizes a defrost cycle, but ice can build up on the outdoor unit if it’s not functioning correctly. This impedes the heat exchange process, blowing cold air into the home. Seek professional assistance to inspect and repair the defrost cycle mechanism. 

Gas Furnace

Gas furnaces can run into a number of potential issues that would cause a furnace to blow cold air. An accumulation of dirt, dust, or debris on the gas burner or within the orifice can disrupt the combustion process, leading to cold air. Regular maintenance, cleaning, and inspection can remove and prevent clogs. If clogging is persistent, consult with a professional to determine a potential cause and have a more thorough cleaning. 

Problems with the gas supply can also impede the ignition process and result in cold air output. This could include a closed valve, gas line blockage, or malfunctioning gas valve. If at any point you suspect gas line issues, contact a licensed HVAC professional immediately. Gas-related problems require specialized attention for safety reasons. 

In a high-efficiency gas furnace, a clogged condensate drain line can also hinder proper operation, leading to cold air circulation. Be sure to regularly inspect and clean the condensate drain line, but if it seems to clog frequently, consult with a professional to restore furnace functionality and prevent possible water damage.   

Gas furnaces also include at least one heat exchanger comprised of a set of metal coils that are warmed by combustion gas and fire. The heat exchanger transfers the heat from the exhaust fumes or flue gas into your home without passing harmful materials into the air. If you have a cracked heat exchanger, it could be a potentially dangerous issue that can result in a carbon monoxide gas leak. You may not even know if you have a cracked heat exchanger unless you begin to notice specific things, such as a lack of heat, recurring flu-like symptoms, or your carbon monoxide detector goes off. It’s essential to have this issue diagnosed and repaired immediately to prevent potentially dangerous, even fatal, conditions in your home.

Contact Jack Lehr Heating, Cooling & Electric For Furnace Repair

A few common problems may result in a furnace blowing cold air, but having the right experience for a proper diagnosis and repair is crucial. Whether your furnace is blowing cold air or you’re looking to install a new system, Jack Lehr’s technicians have the knowledge and equipment to help. Our certified technicians are trained in all the latest technology.

For over 50 years, we’ve provided affordable heating services to our customers. Our experts can help diagnose your furnace issues and get it back up and running. We will never send a sub-technician to your home, as we believe in providing exceptional service with every visit. All products and parts we install are of the highest quality. 

When you contact Jack Lehr, you can expect high-quality furnace repair and HVAC work at affordable prices. Call us at 610-596-9967 or schedule your service online now.

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