What Does It Mean if My Gas Furnace Failed a Smoke Test?

Heat Exchanger

• The heat exchanger is the heart of the furnace. It takes the heat produced by burning gas and transfers it to the forced air that flows over it so it can be delivered throughout your house. Cracks and holes can appear in many places. Seams can open up -- gaskets and sealants can leak; rust can thin the wall. When a heat exchanger becomes damaged or cracked, you usually have to replace the furnace.

Dangers

• Carbon monoxide is an odorless, tasteless, colorless and poisonous gas that can be produced when natural gas or propane burns. A cracked or damaged heat exchanger allows carbon monoxide to leak back into your house. Large holes are especially dangerous because the air that blows through these holes interrupts the furnace flame and increases the amount of carbon monoxide that’s produced. Unless you have a carbon monoxide detector installed, you may not even know that there is a leak.

Smoke Bomb

• Next to visual inspection, a smoke bomb test is the most common test used to look for cracks in a heat exchanger. This test involves lighting a smoke bomb inside the heat exchanger and looking to see if smoke escapes into the side of the heat exchanger that contains circulating air. This test is very sensitive and can detect very small cracks that may be inconsequential. Sometimes an air analysis test is also run. This test looks for combustion products, such as soot or carbon monoxide, that might be in the air. A high reading means there is a definite problem with your heat exchanger.

Chemical Smoke

• A chemical smoke test uses a material, such as titanium tetrachloride, to test your furnace’s heat exchanger. This test is done by vaporizing the chemical inside the heat exchanger by heating it above its boiling point. When the vapor leaves the heat exchanger, it condenses, producing a visible “smoke." One problem with this particular test is the corrosive nature of titanium tetrachloride.