That makes sense. I'm guessing that there is an adapter to connect the double wall to the thimble?The elbow on the wall is single wall stove pipe. The elbow on the stove is double-wall stove pipe. Based on the stovepipe clearances alone (18" for single wall, 6" for double-wall), it looks like you need to do the whole connector in double-wall, DVL stovepipe.