Why is my stainless stovepipe changing color?
From time to time, folks will contact us asking whether they've damaged their stovepipe. It looks burned or something. Does this mean I had a chimney fire? Is the pipe's structural integrity compromised?
When you heat stainless steel in the presence of oxygen, a thin layer on its surface will change color. The color will correspond to the highest temperature achieved. This is a phenomenon called tempering. Because solid fuel stovepipe is designed to withstand temperatures of around 2,100 degrees F, it's possible to get the whole rainbow of colors without structurally damaging the pipe.
Our earliest stovepipe customers (around 2016) received stainless steel single-wall pipe, which makes the tempering effect more obvious. We have since switched to black painted single-wall pipe, which provides a more consistent look. However, single-wall clamps are still susceptible to tempering, as well as a small exposed part of the single-to-double adapter. The outer wall of the insulated pipe is unlikely to get hot enough to change colors in normal use.
Since the effect is only on the surface of the pipe, it is possible to remove the color. You could return the pipe to its original condition with an abrasive polishing compound. However, it's probably not worth the trouble. As soon as the stovepipe gets back up to temperature, the fresh steel will take on the tempered color again.
If you have a probe style thermometer mounted 12-24" above your stove, normal flue operating temperatures are around 300 to 450 degrees F. 450 degrees creates a medium brown color. However, it's common to briefly exceed the target temperature from time to time, so purple or blue colors appearing on the lower flue parts are common. The stove body can also get hotter than the target flue temperatures from time to time, so purple or blue colors are more likely to appear closer to the flue flange.
It's generally best to avoid over-firing your stove to reduce the risk of damage to the stove, but achieving flue temps in the 500-700 degree F range is unlikely to cause any damage to the stovepipe itself. If you ever see any part of your stove glowing (around 900 degrees F and above), you should dampen down the air controls to prevent damage to your stove.
Tempering of your stovepipe is a good reminder of how hot your flue can get, and how important it is to properly maintain your flue system to prevent a chimney fire. We recommend regularly inspecting your flue system and mechanically sweeping it any time creosote buildup is more than 1/8". Our rotary chimney cleaning system is a fantastic tool for cleaning small diameter chimneys thoroughly and easily.