Clogged Furnace Flues

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The flue on your furnace is designed to vent carbon monoxide and other gases made during the combustion process outside. If the flue is clogged or damaged, it can't do its job, and this puts you and your family at risk. Every vented heating appliance in your home should be inspected and cleaned regularly to prevent carbon monoxide poisoning.

Signs of Clogged Flues

The flue pipe on your stove can become blocked by ash, birds and small animals or masonry debris. When one or more of these things falls into your chimney, it prevents the proper venting of combustion gases. Gas furnaces build up scale, which forms when gas byproducts and moisture combine, and these can block the flue. If you see rust or water streaking on the vent, flue, or on your chimney, something may be blocking the flue. Moisture builds inside your furnace pipes when the air can't properly circulate, and this can result in rust.

Soot around your furnace is also an indication of a problem. Soot can build up to in your venting system and block it. If your furnace is vented through a masonry chimney, look for white residue on the brick. This is a sign that mineral salts are coming through the masonry because too much moisture is inside the chimney. Although flame color doesn't always mean the presence of carbon monoxide, a change in the color of your flame, such as blue becoming yellow, indicates that the level of carbon monoxide has increased, and this may be caused by a blocked flue.

Potential Problems

A blocked or damaged furnace flue prevents combustion gases from escaping outside. Instead these gases are released into your home. Once inside, they recirculate, and then carbon monoxide becomes part of the air taken in during the combustion process. This increases the amount of carbon monoxide present. The cycle continues and can build up to deadly levels of carbon monoxide in your home.

Symptoms of Carbon Monoxide Poisoning

Symptoms of carbon monoxide poisoning resemble the flu, without fever, in the beginning. If you or your family experience headache, nausea or vomiting, dizziness, fatigue, difficulty breathing or loss of consciousness, go outside your home immediately. If the symptoms lessen or go away but return once inside, you may have carbon monoxide poisoning. Open all doors and windows and turn off any combustion appliances. Seek medical attention. Have your furnace inspected and the flue cleaned immediately by a professional. Have carbon monoxide detectors installed in your home.

Cleaning to Prevent Clogs

Your furnace needs to have its venting system cleaned every year. The buildup of particulates is not as significant as it is with a fireplace flue, but over time, they can reduce the efficiency of the flue. This is dangerous and can lead to carbon monoxide poisoning. Contact a qualified professional to have the venting system cleaned and inspected for damage or leaks even when the furnace appears to operate properly. Only a trained technician can detect blockages or buildup inside your furnace flue and correct the problem.


Renee Miller

Renee Miller began writing professionally in 2008, contributing to websites and the "Community Press" newspaper. She is co-founder of On Fiction Writing, a website for writers. Miller holds a diploma in social services from Clarke College in Belleville, Ontario.