Furnace in a house

Why is My Furnace Blowing Cold Air

It is a nice crisp winter night and your furnace kicks on. Instead of your body warming up, you are feeling cold air in the home and this adds to the coldness that is already there. Why is my furnace blowing cold air? There are many reasons your furnace might end up blowing cold air at some point. Checking the thermostat settings, the pilot light, the oil tank, and valve issues are a few areas to start. We will discuss some of the areas to check and suggest ways for you to ensure that this doesn’t happen.

Thermostat settings can affect the furnace.

Follow Up on Your Thermostat Settings

One area that is missed a lot by people is how simple it is to mess up your furnace settings. Be sure to check your thermostat and ensure the proper settings are in place to run heat. In addition, your thermostat could be broken or be low on batteries.

Furthermore, it is important that you operate the fan settings on Auto rather than on. While you are doing that, make sure your air conditioner is also in the “Auto” position as this will keep the humidity low in the home. However, the problem with the air conditioner staying on “Auto” is it can wear out the blower mower faster so weigh the pros and cons.

Pilot light on a furnace.

Check the Pilot Light

Without a pilot light, the furnace will not start the burners. The pilot light is a heat source that kickstarts the ignition of your furnace burners. There is a step-by-step process that you will need to follow, in order to have a successful relight and not cause damage to your furnace. Each furnace is a little bit different so please be sure to review your manufacturer’s guidelines first.

The usual steps are as follows:

  • Ensure that the furnace has no power
  • Find the pilot assembly and turn the reset switch to Off
  • Wait 10-15 minutes to make sure that any remaining gas has escaped the area
  • Turn on the reset switch to the pilot and use a lighter simultaneously to light the gas coming out
  • Once you see the light is working turn the switch to “on”
  • Turn the power to the furnace back on

If it stops working, check the manufacturer’s instructions to reset it. The pilot light will turn off when there is an issue to keep the natural and propane gas from going into the home. This is one of many reasons the furnace may stop working.

Lack of Fuel

Simply put, if you do not have oil in the tank or the gas is off then the furnace will blow cold air. It is recommended that you check the tank (if you have one) regularly as you use the heat. Having the gas company come out and refill your oil tanks can take some time, so it is best to plan ahead.

Potential Valve Issues

If your system is not cleaned on a regular schedule, it is possible that the pilot light or gas valve may be affected. In addition, a valve could get stuck due to debris and dirt. The valves will only give you a problem if the system is dirty and not cleaned regularly. Have your system cleaned by a professional HVAC company on a semi-annual basis.

Ductwork can leak and let cold air blow out from the furnace.

Duct Work is Leaking

Leaking ductwork will allow heat energy to escape the ducts. This heated energy will never make it to your living spaces, and you will not feel the warmth that it is producing. In this case, your furnace will blow cold air, because the heated energy will escape through the leaking ductwork.

Your best bet to avoid this from happening is to make sure a professional HVAC company inspects the ductwork when they conduct their semi-annual inspections.

Dirty Oil or Gas Burners

Dirty oil or gas burners typically happen when you do not have your system checked and cleaned on a regular basis. Dirt and debris can collect in the system and cause the system to malfunction. Neglecting to have your system maintained can end up causing you a lot of expensive repairs to the system.

Possible Compromised Flame Sensor

If you are experiencing a situation in the home where your air blowing out of the furnace starts warm but turns cold quickly afterward, then the flame sensor will need to be inspected. When the flame sensor is covered with dirt and grime, it can wear out and cause the burner on the furnace to shut off. If this occurs, you will need to call an HVAC professional to come out and make the repair.

Additionally, if you notice that the flame sensor is not the normal blue color and actually turns yellow, this may indicate that there is a carbon monoxide leak.

Clean and Check Your Condensate Line

Your HVAC unit has a condensation line for the excess water to exit the system and drain into your yard. It is located next to the external air unit. If the condensation line becomes clogged, the water will back up into the system and cause a shut-off to the system. When this occurs, the system will not blow any hot or cold air into your home.

You will hear a fan running, but not feel any air change. Inspecting this regularly is important to make sure mold and bacteria haven’t developed to clog the line. You can use vinegar to help kill any bacteria or mold that has grown into the condensation line. With the unit shut off, you will also vacuum suck the water out of the condensate line.

Other Items

There could be many other factors to your furnace not blowing cold air such as age or miscellaneous electrical issues. If you are still stumped on your furnace, check out the video below or call an HVAC company to take a look at your system.

Other Maintenance

To keep the furnace working, you do need to do maintenance on the HVAC unit. One of those is cleaning and changing out the furnace filter every 30 days and 90 days at most.

Also, rattling sounds, lack of airflow, or a crushed duct, and dust on the coils to name a few can cause the air conditioner to freeze up. This is why scheduling preventive maintenance is important.

It is important to check the fresh air intake vent that goes from the outside of the home to the inside and pulls clean air in. This is important to have for a furnace. You want to make sure nothing is blocking it and the air can get into the home. It may sound crazy to have this since it does get cold here in Colorado, but it is necessary.

Lastly, you may find that the furnace stops working. There are a few reasons why this may happen such as: thermostat setting, the circuit breaker,

Maintenance of the furnace is the key.

When To Call a Professional

There are two times that you should call a professional when it comes to your furnace and HVAC system. Unfortunately, with everything that is mechanical, parts can break down. When your system starts to seem off, it is best to call a professional HVAC company to come in and do an inspection. They will be able to make any repairs you need at the time you notice any issue. The other recommended time is to schedule a professional HVAC company to come in semi-annually or annually to inspect the system.


Figuring out why your furnace is blowing cold air can be a tricky situation. With many possible reasons as to why this is occurring, it is important to start with the easiest solutions first. It is highly recommended that you call an HVAC professional to do an inspection of your system if you notice the furnace is blowing cold air. Additionally, it is important to schedule a semi-annual or annual HVAC inspection. Colorado Inspection team will check your furnace during a home inspection in Castle Rock, CO and surrounding areas.

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