Without your furnace's flue, your furnace would have to either keep all the toxic gases it generates inside itself or release them directly into your home. Corrosion in the flue's interior panels is somewhat difficult to detect and the solution, total panel replacement, is tedious and expensive. Instead of letting this discourage you from trying at all, read these two signs that your furnace flue is corroded and needs repair.
Dust Starts Forming On The Exterior Of The Flue
No matter what you do, some small holes will form on your flue a short time after it's installed. Even when your flue's interior is protected by a strong layer of sealant, holes can be formed by localized flareups of gas pressure. Most of these holes are barely larger than a microscopic organism and completely harmless; to make the air around your furnace toxic, you'd need much larger holes.
Nonetheless, once your flue's interior is in need of repair, these holes will start spewing dust at a slow but steady rate. If you start seeing this dust, and it returns after you see it for the first time, you know that you need to replace your flue's interior panels.
Interior Flue Panels Quickly Fill With Debris Again After Being Cleaned
Since you have to completely shut off your furnace and use a screwdriver, it'll take a little more effort to check your flue's interior directly. Fortunately, as a reward, you'll be able to see whether your flue is going to need its panels replaced long before dust starts forming on the exterior.
Even if your flue is fine, you might still see a little debris buildup in the flue. This is normal. Once you clear any debris with a hand-held vacuum cleaner and wipe dust away with a handkerchief or paper towel, wait to access the flue again for at least a couple weeks. If no significant amount of debris builds back up, you're in the clear. However, if a robust layer of dust and some larger pieces of debris are visible, it means that the sealant on your flue's interior panels is mostly gone.
The panels will have to be replaced with new ones that can take a strong layer of sealant. If you try to apply sealant to a rougher and older panel, you'll produce a very inconsistent protective layer with a lot of large holes.
Despite the relative unimportance of your furnace's flue, matters concerning it should be taken very seriously. Just as with any part with your furnace, a major problem with it will significantly hamper your whole HVAC system. Contact a heating repair expert for help with any issues you encounter.