Rusted Metal Flue Pipe: The Science Behind Your Inspection by Tom Feiza, Mr. Fix-It, Inc. HowToOperateYourHome.com
Holes in metal flue pipes that extend from gas appliances to a
Figure 1: Signs of Condensation
chimney can create a hazard if they fail. They may also indicate significant gas appliance and home performance issues. What is the science behind the stained, rusty metal flue pipes? Why did they fail? Why does a home inspector care? Problems Galore Figure 1 shows telltale signs of moisture issues on two metal flue pipes from gas appliances to a masonry chimney. You can see stains and drips from moisture. There are pinholes in the metal flue and extensive rust. Pinholes show up on the outside of a metal flue pipe as slightly tan, white, or rust-colored. These flue pipes are failing. If you look closely, you can see the lower pipe is newer but still showing signs of stains and rust. I believe a severely corroded metal flue pipe was replaced but the draft and condensation problem causing the corrosion was not corrected.
Figure 2: Burn marks, rust, melted plastic, or melted foam insulation
The science? The system is not properly removing the products of combustion with adequate flow. The natural “draft” of the chimney and hot combustion gas is not working. It is not venting properly. The flow of combustion gas is slowed and it cools. Combustion products contain lots of moisture. As the combustion gas is cooled below the dew point temperature, moisture forms inside the metal flue. The moisture is mildly acidic from the products of combustion. The acidic water stains and rusts through the galvanized coating and through the metal. This is a hazard all inspectors should be concerned with—a failing flue pipe and possible backdrafting of combustion products into a home. Signs of Condensation The products of gas combustion should stay warm enough to flow up the chimney without condensation. Watch for signs of poor draft of combustion gas, such as condensation and rust on the metal flue pipe from the appliance to the chimney (See Figure 2). Burn marks, rust, melted plastic, or melted foam insulation on pipes at a draft diverter are sure signs of backdrafting (See Figure 3; pg. 18 and Figure 4, pg 20).
Figure 3: Gas Water Heater with Signs of Backdrafting
page 20 8
18 Working RE Inspector Winter/Spring 2023