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Residential Heating and Air Conditioning Blog

Warning Signs of an Unsafe Gas Furnace


We don’t want to alarm you with the title of this post. In general, gas furnaces are safe to have in homes. If they receive regular annual safety inspections and tune-ups and prompt repair service when needed, they post almost no danger. They’ll continue to heat homes for many years until it’s time to replace them.

However, gas furnaces can become unsafe, and it’s important to know the signs of this so you can shut the furnace off and then call us for furnace repair in Minneapolis, MN. Here are ways you can detect that your furnace has a safety malfunction.

Your carbon monoxide detectors go off

You must have carbon monoxide (CO) detectors in your home if you use natural gas in any capacity. Because CO leaks are not easily detectable (the gas is colorless, odorless, and tasteless), these detectors are a critical early warning if carbon monoxide begins to escape from your furnace. If your detectors go off, leave the house immediately and contact the utility company. Do not return to your home until it’s been cleared. Then you can call us to examine and repair the furnace.

Strange odors

Although natural gas has no odor, manufacturers of natural gas often place a chemical called mercaptan into the supply so people can detect leaks. This is the familiar sulfur “rotten egg” odor that people associate with gas leaks—and for good reason. If you notice this odor around your furnace or coming from the vents, shut off the gas to the furnace immediately and call for technicians. 

Changes in the color of the burner flames

You can look into your furnace through a hole to see the color of the burner flames. Even sealed combustion furnaces will provide a window to allow you to check on the color. The flames should burn a steady blue. If the flames have turned different colors—red, green, and yellow are the most common—it means the burners are combusting something other than natural gas, and that’s a warning of something wrong. Shut off the furnace and reach out to professionals.

Loud, persistent clicking sounds

When your furnace cycles down after heating your home, you’ll hear some clicking from it. This is normal: it’s the sound of hot mechanical components cooling down. You may also hear clicking right as the furnace starts its heating cycle as the burners ignite. But if you hear clicking that’s loud and persistent at other times, especially right after the blower shuts off, it’s a warning of a possible cracked heat exchanger. This is one of the most common dangers in older furnaces, and you need experts to look into it right away.

Signs of corrosion

A gas furnace can corrode even though it doesn’t use water. Reactions between combustion gas and the metal of the furnace can lead to corrosion and rust over time—especially if the furnace is venting poorly. If you notice corrosion anywhere on your furnace, it’s a reason for concern, as this can easily lead to cracks in the heat exchanger. Often, this corrosion indicates the furnace needs to be replaced because of age. You can call our team to examine your furnace and determine the best course forward.

Rely on Residential Heating and Air Conditioning and you’ll “Feel the Difference”! Serving The Twin Cities Since 1991. 

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