Within the past several months (or more?) someone posted on one of the HI forums regarding discolored vent pipe at a water heater or maybe it was a furnace/boiler. I searched a bit and could not find it. I saw the same condition yesterday. I did a little searching in the net and found a few references to this condition, usually along with failed joints. It does not seem to be a very common problem, but could be a deadly issue. I'm looking for any feedback regarding causes/frequency. BTW, the vents extended vertically up through the roof. Maybe 25 feet long. The pipe was not discolored in the attic area. Both water heaters were replaced within the past year, so sections right at the water heaters may have been replaced. The original installation is 15 years old. The water heaters are in a restaurant and may get heavy use. I felt a vent pipe while running and it felt pretty warm, but I don't usually touch vents so I'm not sure how unusual this is. I did not measure the temperature. Click to Enlarge 45.16 KB Click to Enlarge 35.86 KB