Why Do Metal Pipes Create a Wet Smoke?

About Wet Smoke

• Wet smoke appears heavy or dense and very dark in color. Most often wet smoke occurs in wood-burning appliances when green or wet wood is burned. This is because the moisture content of the wood is higher than it should be. Burning wet wood increases the amount of creosote produced during the combustion process and can lead to chimney fires. If your furnace or wood stove is producing wet smoke, it usually has little to do with whether your chimney has metal pipe inserts or not. Wet smoke can be an indicator that you should have your chimney cleaned and your venting system inspected. In a gas-fueled furnace, wet smoke may be an indication of incomplete combustion. If the combustion process is not completed, the harmful combustion gases cannot escape safely outside and build up inside the pipes instead.

Metal Pipes

• When a chimney is lined with a stainless steel insert, which is a pipe that runs up inside the chimney and acts as a liner, this changes how the condensation escapes your chimney. The moisture is not absorbed by the porous masonry of the chimney, but instead it gathers on the surface of the steel. If the chimney is properly maintained and vented, this moisture will eventually evaporate and disappear with the combustion gas. If the chimney malfunctions, the moisture may re-enter your interior pipes, and this can result in leaks and damage to walls and ceilings.

Wood Combustion

• The first stage of combustion in a wood stove is the heating of the wood and evaporation of the moisture inside. Heat causes the temperature of the wood to increase, and the water inside boils and eventually evaporates. Combustion gases contain this moisture as well as carbon dioxide, carbon monoxide and various acids found in creosote. When the moisture is driven from the wood, it has to escape somewhere. Typically it is drawn up the stove pipes and into the chimney, where it escapes with combustion gases outside. Burning wet wood increases the content of moisture in combustion gases and creates a darker and denser smoke.

Gas and Oil Combustion

• When gas such as propane or fuels such as oil are burned, moisture is present from both the intake air necessary to ensure combustion and from the fuel itself. As the water vapor is drawn up into your chimney, condensation occurs. According to the Chimney Safety Institute of America, the average furnace can put about 1 1/2 gallons of water into your chimney every hour. This was visible as steam or wet smoke escaping out of the chimney in older, inefficient furnaces. Newer high-efficiency gas and oil furnaces condense inside the flue instead of the chimney so wet smoke coming from these newer appliances is rare.