Rusting on Type B Flue Pipes

[Nick Ostrowski]

I often see type B flue pipes above roof surfaces that have varying degrees of surface rust. Some of them have it fairly light while others have quite a bit of rusting. However, none have ever crumbled when I give them the squeeze test with my hand. This is only for the exterior flue pipe and not the sections located inside the house or attic spaces.

What do you guys typically recommend for this condition?

[Bob Harper]

Nick, the corrosion is caused by condensation of acidic flue gases indicating a weak draft. B-vents are rarely protected by a chase but rather exposed directly to the cold and wind, thus exacerbating the condensation of flue gases. If a CAT I vent is drafting properly, the flue gases will exit the termination under sufficient velocity to avoid condensation of those acids on the exterior of the vent but rather dissipate.

If you've inspected the vent internally and found no signs of corrosion to the aluminum liner, then perhaps a light scrubbing with steel wool and touch up with cold galvanized paint will do for now. However, I would strongly recommend having a pro certified in combustion analysis inspect and test the equipment. There could be draft interference issues for one. I would also want to ensure there is no backdrafting, especially if there is a draft hood equipped appliance below.

If there is an 80% induced draft furnace attached, the addition of a 4-5 minute post-purge timer will drastically cut down on flue gas condensation and dry out the vent internally.
HTH

[Eric Barker]

In my experience, once these vents get to about 20-30 years of service there can be notable corrosion that is usually out of sight.