Cleaned my flue again and noticed there is some hard build up that doesnt want to come off. Using a poly brush. Seems to come off the stainless pretty good but the single wall it is stuck. I try to brush once a month and burn a TSP stick twice a month. Its torward the top of the single wall. Wouldnt have noticed it if i hadnt completly pulled apart my flue. Only takes about 15min to get things together, get on roof and lift out insulated so why not cheack it while im there.