The frigid Michigan winters make it essential that your furnace is working properly. If your furnace does have issues and isn’t working correctly, your home will quickly become unbearably cold, and your pipes could be at risk of freezing. One problem we encounter fairly frequently is that the furnace will suddenly start blowing cold air.

This issue often means that the furnace isn’t actually running for some reason. Most furnaces will automatically default so that only the blower runs if the furnace malfunctions or has any issues, and this will result in the blower starting to circulate cold air throughout the home. However, your furnace may also be blowing cold air due to an issue with your thermostat or your home’s ductwork. Here is an overview of the different issues that can cause a furnace to blow cold air and how each issue can be diagnosed and repaired.

Incorrect Thermostat Settings

If you do ever notice cold air coming out of your vents when your heating should be running, the first thing to do is check your thermostat settings. There is always a chance that someone accidentally switched the thermostat over to cool instead of heat, and this will obviously result in cold air blowing out of your vents since your furnace isn’t on.

You should also make sure that the fan setting on your thermostat is on “Auto.” If the fan is set to “On” instead of “Auto,” it will run continually even after your furnace shuts off. Even after the furnace shuts down, it will still retain some residual heat. However, the air that blows out of your vents will quickly start to become colder since the blower is merely drawing cold air in and circulating it through the home.

Dirty or Broken Flame Sensor

The flame sensor is an essential safety feature found on all modern furnaces and is designed to prevent gas from continuing to flow if the unit doesn’t light or goes out for any reason. The sensor is basically a probe that works by sensing the heat from the furnace burners. Over time, the sensor can become coated in dirt and grime to the point where it can no longer detect that the burners are lit. Flame sensors can also simply wear out due to age.

Either way, if the flame sensor isn’t working as it should, the furnace will only stay lit for a few seconds before the gas valve closes and the unit shuts off. However, the blower will usually continue to run even though the furnace is off, which means that it will start blowing cold air out of your vents.

Clogged Furnace Burners

The ports on your furnace burners can also become dirty to the point where they start to clog up. If the burners are completely clogged, the furnace may not light at all since gas won’t be able to flow out of the ports. Clogged burners can also prevent the gas from fully combusting. Incomplete combustion produces far less heat than when the gas burns fully, and this can lead to your furnace producing very little heat and the air coming out of your vents feeling cold.

Gas Supply Issues

If your furnace doesn’t turn on, it could also be that the unit isn’t receiving gas for some reason. It could be that the gas supply to your home is shut off or that the main gas shut-off valve to the furnace is closed. The furnace’s internal gas valve can also break and get stuck closed. Again, a lack of gas will prevent your furnace from turning on, but the blower will still usually run and blow cold air.

Malfunctioning Electronic Igniter

If your furnace was manufactured after 2010, you can be almost certain that it uses some type of electronic igniter to light the gas burners instead of a standing pilot light. As with any other furnace component, the electronic igniter can sometimes fail or stop working properly. This will result in your furnace never lighting even though your blower is still running.

Pilot Light Isn’t Lit

If your furnace is older and uses a pilot light, you will want to make sure that it is still lit. Pilot lights can sometimes get blown out by a strong draft or go out because the gas supply was temporarily interrupted. If you can’t get the pilot light to ignite or it won’t stay lit, you will then need to have a technician either repair or replace it before your furnace can run again.

High-limit Switch

The high-limit switch is another safety feature that works to prevent potential damage or fire hazards caused by the furnace overheating. If the limit switch senses that the unit is overheating, it will automatically trigger and shut everything down. Once this happens, your furnace won’t run again until the switch senses it has cooled down to the point where it is again safe to operate.

The limit switch will typically trigger both the furnace and the blower to shut off. However, sometimes the limit switch can malfunction and cause the blower to run constantly even after the furnace has shut off. This is definitely an issue that you will need to have checked out by a furnace technician.

Worn-out Draft Inducer Motor

Most all furnaces have something known as a draft inducer that works to draw the combustion fumes out of the unit and into the exhaust flue. The inducer is the very first thing that turns on whenever your furnace needs to run, and it will typically come on around 30 seconds before the furnace ignites. The inducer works to ensure that no latent fumes remain inside the unit’s combustion chamber, which is important as these fumes are inflammable and could prevent the burners from igniting.

When the inducer runs, it produces a strong air current that activates the furnace’s pressure switch. If the inducer motor is burnt-out and it doesn’t run or if the pressure switch is broken, your furnace won’t ever light since the pressure switch is what activates to open the unit’s gas valve.

Air Duct Leaks or Poorly Insulated Ductwork

If your furnace is lit and cold air is still coming out of your vents, it may be that your ductwork is damaged and allowing lots of the hot air to escape. Another possibility is that the ducts in your attic or crawl space aren’t properly insulated. Ducts in these areas should always be completely wrapped in insulation as these areas tend to stay much colder than the main living area of the home during the winter. If the ducts aren’t insulated, all of the cold air surrounding them can quickly cool off the air flowing through the ducts and lead to cold air blowing from your vents.

At AJ Danboise Plumbing, Heating, Cooling & Electrical, our technicians can help you determine why your furnace is blowing cold air and ensure that the issue is fixed properly. We service and repair all types of furnaces and specialize in heating and cooling installation, maintenance and repair. We also provide electrical and plumbing services. Give AJ Danboise Plumbing, Heating, Cooling & Electrical a call today to schedule an HVAC, plumbing, or electrical appointment in Farmington Hills.

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