What concerns me is that we can only see about 3 tiles worth into the flue before it goes black. That means either 1.) it's too dark for that flashlight to give us a longer view or 2.) the buildup is so thick that it can't penetrate it.
I've found that a tiny LED light will let me see all 18ft of my stainless flue 10x better than my Maglight.
I've found that a tiny LED light will let me see all 18ft of my stainless flue 10x better than my Maglight.