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Roy Breshears WIN South East Idaho

The Danger of Unlined Chimney Flues

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The Danger of Unlined Chimney Flues            by: Roy Breshears

Not all chimneys and exhaust flues are created equal. It is important to know certain details about your chimney, not only for the longevity of the structure, but also for the safety of the structure’s occupants.

Gas-burning appliances produce toxic gasses and condensation. These substances will eat away at the masonry bricks inside a chimney. This damage can compromise the safety of your family as well as detract from the value of your home. However, a thorough annual inspection of your chimney by a licensed / qualified contractor can alert you to any current or potential issues before they get worse.

Of course, chimney cleaning and inspection is necessary if a homeowner regularly uses a wood-burning stove or fireplace. An annual chimney evaluation will provide details and information about the overall functionality and safety of both the heating appliance and the chimney. It also is good to have as a baseline in determining if any changes have occurred.


Gas heating appliances – whether a water heater, furnace, or a boiler system – need correctly sized and installed chimney / exhaust systems for venting to occur properly. Although visible soot is typically not apparent when an appliance is running properly, they are still producing very corrosive substances which will harm your chimney. In many cases, these acids will significantly damage the interior of the chimney without necessarily showing any issues externally. This damage can be not only dangerous, but expensive to repair as well.

High efficiency appliances convert most of the heat that is produced by combustion to heat your home. This is good for efficiency; preventing heat that could be used to warm the home from going up and out the chimney. However, some amount of heat is necessary to provide “DRAFT”, which is what makes a venting system work. Ensuring the venting system must is correctly properly matched to the appliance type and size will garner the safest and most efficient operation.


Natural gas is generally considered a clean-burning fuel. Under optimum conditions with both the appliance and the exhaust, that is true. However, there are many variables that make the “clean” nature of natural gas not-so-clean. A major by-product from all the variables is hydrochloric acid, which is highly corrosive. Other acids can also form when the water condensation mixes with residues on the insides of the chimney / flue as well.

Eventually the corrosion from the moisture condensing inside the flue will cause the masonry components to deteriorate and crumble. This debris can cause a blockage and eventually prevent carbon monoxide from leaving the system. This is a very dangerous condition as those gases could easily end up back into the living space of the structure.

Old masonry unlined chimneys are the most susceptible to corrosion, in part due to their age. Additionally, the chimney may have been used to vent appliances using different fuels in the past. Any soot not removed on a consistent basics will speed up the corrosion process. Chimney flues that are unlined, oversized or deteriorated can usually be lined with a UL listed lining system approved for gas appliances. A CSIA Certified Chimney Sweep can provide more information about chimney liners.


Whether your chimney is new or old, the National Fire Protection Association and the Chimney Safety Institute of America recommend annual evaluation, regardless of the type of fuel or appliance being vented. It should be noted that not all liners are compatible with gas appliances due to the type of material from which they are made. It is also imperative to ensure your gas appliance was installed according to the manufacturer’s installation instructions as well as the appropriate National Fire Protection Association Standards.