Why Is My Furnace Blowing Cold Air?

January 17, 2024
Furnace replacement in Deptford, NJ

You expect a furnace to blast warm air into your home to bring it up to the temperature you’ve set on your thermostat. When you get cold air instead, it can cause a pit in your stomach and a bit of anxiety as you wonder about the problem and the cost to fix it. Fortunately, not all possible problems are expensive, and some are easy to resolve on your own. Consider these common reasons why your furnace may be blowing cold air and how to resolve it.

The Heating Cycle Just Started

When the cool air comes from the supply vents can make a big difference. At the beginning of a heating cycle, the furnace has to push out all of the air that cooled since the last heating cycle. This should only last a short time, usually less than a minute depending on your heating system and how far from the furnace you’re feeling the cool air. If it continues for several minutes, it’s not likely just purging cool air and you should consider the following as possible causes.

Improper Thermostat Settings

Cool air can result from a simple improper setting on your thermostat. There’s the obvious consideration that it’s set to Cool instead of Heat and your house got warm enough to initiate a cooling cycle. However, what’s more common and likely over the winter is having the fan set to the wrong mode.

Most thermostats offer an On and Auto function for the circulating fan. When you set it to On, the circulating fan runs all the time, pushing cool air out between heating cycles. Check to make sure your fan is set to Auto.

Neglected Air Filter

Air filters act as the first line of defense against airborne contaminants that can damage a furnace’s internal workings. It’s easy to forget to replace the filter, allowing it to get clogged. Clogged filters restrict the airflow through the system, which causes the system to overheat and the burner to shut down. Even though it’s not producing additional heat, the circulating fan will continue to help return the system to a tolerable temperature and prevent damage. The result is that it may feel like there’s cool air coming from the supply vents.

Other Airflow Problems

There are other sources of airflow problems your furnace may encounter that will cause similar issues as clogged air filters. The most common are issues that arise from neglected furnace maintenance, such as a dirty circulating fan wheel. Another common issue in aging systems is the circulating fan not spinning rapidly enough to move the proper amount of air. A technician will clean the fan wheel during routine maintenance and test it to ensure that it’s operating optimally.

Bad High-Limit Switch

The high-limit switch is one of the safety mechanisms your furnace has that detects a temperature that’s too high at the heat exchanger. Overheating the heat exchanger increases the risk of developing a crack, which allows carbon monoxide to leak into your home. A high-limit switch can go bad before it’s time to replace your system. The result is the system prematurely terminating the heating cycle while leaving the circulating fan running, just like an actual overheating situation.

Improperly Adjusted Burner Flame

When a furnace burner is properly adjusted, it’ll produce a translucent blue flame that efficiently burns all of the fuel. When it’s not burning efficiently, it won’t produce the same level of heat, making the air coming from the vents feel cooler than expected. The indications that the burner isn’t burning efficiently are a yellow or orange flame and excessive soot in the burn chamber. A maintenance technician will test the furnace burner and adjust it to ensure that it’s producing an efficient flame.

Improper Damper Adjustment

Some homes have zoned systems, allowing only those zones that need heating to receive heated air. Inside the ducts are dampers that control which zones receive heated air and how much. Some systems have electric dampers that are controlled by a thermostat. However, others have manual dampers that need adjusting between when you run the AC versus your furnace.

JAMCO Heating & Air Conditioning has been the trusted HVAC provider of choice to keep homes around Deptford comfortable since 1986. Our team provides air conditioning and heating repair, maintenance and installation, including the popular ductless mini-split system. Call to schedule a heating repair or maintenance visit with one of our NATE-certified technicians.

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