This is my recent installation of the Jotul F50tl. I have a 7ft single wall connection to a class A chimney. I have been burning light for the past few days (highest temp reached 450F). We are most comfortable right now with current weather with the stove cruising around 400-450F. I noticed tonight that my bottom 24" section of pipe is starting to discolor about 16" above the stove exhaust exit. Is this common?