Open up the clean out and take a good look and use the vacuum in there along with a brush. I'm at a disadvantage here as I don't know what things look like inside that stove. Normal exhaust flow starts just above the burn pot and goes up and then to both sides going through the heat exchanger ( in, over, and around tubes or convoluted plates) and then down behind the side walls (sometimes rear wall as well) and then out to the combustion blower. Ash gets trapped all along the path. if there are ash traps behind the inspection/clean out plates there can be a heavy load of ash in there that requires a good brushing. With the vent set up you have (tee and vertical outside) you can also have ash precipitate out faster in the outside portion of the venting due to the washing effect of condensing volatiles in the exhaust gas stream, the tee will fill up faster in that case. How do you clean the section of the horizontal venting with the 45 degree elbow?