Why is My Furnace Blowing Cold Air?

If your furnace is blowing cold air, it could be due to normal startup operations, incorrect thermostat settings, or issues like a dirty filter, clogged condensate drain, or a malfunctioning flame sensor. While some causes are simple to address, others may require professional attention. Let’s explore why this happens and how you can fix it.

When Cold Air from the Furnace Is Normal

Cold air is possible during normal furnace operation. When the furnace first starts after being off for a while, there will be cool air in the ductwork. Once the furnace blower starts pushing warm air into the ducts, this cooler air will be expelled into your home. This should only last for a minute or two before the heated air arrives. Sometimes, it may take longer, such as when a damaged fan limiter starts the blower immediately instead of waiting for the heat exchanger to reach the proper temperature before turning the blower on.

Problems that Can Cause Your Chicago Furnace to Blow Cold Air

When your furnace continues to blow cold air long after the startup period, there may indeed be something wrong, though it could be as minor as incorrect settings on the thermostat. Here are some of the most common causes of cold air from your furnace:

Incorrect Thermostat Settings

If the thermostat's fan settings are set to “On” instead of “Auto,” the fan will run constantly. This will not only prolong the time it takes to warm the heat exchanger up but also allow the fan to circulate air long after the furnace has shut down, at which point the air will be quite cool. Make sure your thermostat is working and set correctly.

Dirty Furnace Filter

The heat exchanger in your furnace is where the air absorbs heat from the combustion process. By moving the heated air into your home, the furnace keeps the heat exchanger from overheating. Any restrictions to this airflow, such as a dirty air filter, will allow the heat exchanger to overheat, tripping the high limit switch and shutting the burners off to prevent heat exchanger damage. If your furnace shuts down prematurely, check for a dirty furnace filter.

Flame Sensor or Pilot Light Problems

If your furnace turns the blower on but the burners are not lit, cool air may be circulated. In modern furnaces, a flame sensor is used to detect the flame and open the gas valve. If this sensor is dirty, the burners will not light, and the system will shut down. Having the flame sensor cleaned can often solve the problem. If you have an older furnace, the pilot light may have gone out or the thermocouple may be damaged. Try relighting the pilot light and make sure the thermocouple is contacting the flame.

Clogged Condensate Drain

In high-efficiency furnaces, nearly all the heat is scavenged from the combustion exhaust for heating the home. This cools the exhaust enough to form condensation, which must be removed with a dedicated drain. If this drain clogs, it can back up into the furnace, tripping a safety switch that shuts the system down. Have the condensate drain cleaned to restore normal furnace operation.

Furnace Repair in Chicago, IL

If your furnace is blowing cold air, our team at ABC Plumbing, Sewer, Heating, Cooling and Electric can help. We offer furnace repair and maintenance throughout the Chicago area, and our talented technicians can handle any furnace problem you may encounter. Call 888-868-6667 today or contact us online to schedule furnace repairs in Chicago!

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