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Why Is My Furnace Blowing Cold Air?

You have noticed a slight chill in the house. You put your hand over the vent, feeling the cool air flow — even though your thermostat is set to 68 degrees! There’s many reasons that this could be happening; it could be the filter, ignitor, high limit switch, or flame sensor, for example.

Many of these issues are interrelated, and often happen if the furnace is older or has not been properly maintained. Let’s go into more detail on these problems and why they’re so important.

Furnace Ignitor

The ignitor on your furnace is used primarily for lighting the burners, which create the heat that is blown throughout your home. Without proper maintenance on your furnace — or if you have an older unit — it’s possible that the ignitor will fail to properly light the burners, and is not able to create a flame to heat your home. Our technicians can help test your furnace ignitor to see if it needs replacing.

Flame Sensor

A flame sensor is a safety feature that is responsible for detecting when the burners of your furnace turn on. If it is not working correctly the furnace will not ignite, and will cause your HVAC system to blow cold air through your vents. Dirt can collect around the sensor causing the part to not work properly or fail. Flame sensors can also become fouled from the byproduct of burning natural gas, meaning it may need to be cleaned or replaced.

High Limit Switch

furnace got too hot? It may sound silly, but it’s actually a good thing if your furnace shuts down if it gets too hot. There’s a safety device called a “limit switch” that turns the furnace off if it reaches an unsafe temperature. This can protect against a blowout and complete system failure, so if your furnace blows hot air, then cold air, then no air at all… this could be the reason. Our HVAC experts can thoroughly inspect and diagnose the issue, and the most common cause is a dirty or clogged air filter. If you notice that your furnace overheats on a regular basis, take the time to change the filter and see if the system begins blowing only hot air from that point. If left untended, your furnace’s heat exchanger can become damaged, which can lead to expensive repairs.


The filter in your furnace/HVAC system helps collect dust, hair, and other particles in the air before they travel through your ductwork. It also helps to protect the internal components — such as the blower motor, heat exchanger, and other parts on your furnace. Also, if your furnace filter is completely full of particles, it will cause your furnace to work harder and less efficiently; it isn’t able to blow enough air through the filter. This can prevent your HVAC system from delivering heat throughout the vents in your home. It can cause your energy bill to go up, lower equipment life, and even cause cold air to blow through the vents. Generally, your furnace filter should be replaced every month, but this may vary based on several different factors.

Pilot Light

If you have a gas furnace — by far the most popular option in Illinois — it’s important to regularly check the pilot light. If there is no flame, it can mean that something blew it out, there isn’t gas flowing into the furnace, or the thermocouple is faulty. We know that relighting the pilot light or checking for proper gas flow can be a bit intimidating, and if parts need to be replaced, it’s always best to contact a professional. If the furnace is running properly and the thermostat is set to “Auto,” the most probable culprit is the pilot light.

Thermostat Settings

We know… this is a common theme for many HVAC problems (and many of our blog posts too!) but it’s often the source of many furnace problems. If your thermostat is set to “On” instead of “Auto,” the fan will continue to run even when the furnace isn’t heating the air that’s flowing through it. You may also be running up your utility bills by continually running the fan. So, if the thermostat was accidentally adjusted, switch it back to the “Auto” setting. That way, the furnace will only run when it’s supposed to.

How To Prevent Issues With Furnace Components

As we said earlier, many of these issues can be prevented with proper maintenance. One of the simplest ways to avoid problems with the various components of your furnace system is to make sure that you change your furnace filter every month. If you are unsure how to change your filter — or if you don’t know what filter is best for your unit — please give us a call at (224) 419-5324.

The second most important thing to do is to maintain your furnace on an annual basis. We offer a yearly HVAC maintenance plan, which includes 2 visits by a G&R technician plus friendly reminders when your system is due for maintenance. And you can save money, too — with discounts on repairs while your agreement is active.

Sign Up for an HVAC Maintenance Plan Today

If You’re Still Getting Cold Air From Your Furnace, Call G&R 24/7

If you’re still worried about your furnace, please call our team at G&R Heating & Air right away. Our friendly staff will book an appointment for one of our knowledgeable technicians to inspect and repair your furnace. You can also speak with your technician to sign up for our yearly maintenance agreement, which will ensure the longevity of your furnace and keep your family comfortable all year long. If you need maintenance but don’t need immediate repair, you can also contact us online!

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