If you are using an air tight stove, then do not use any type of heat exchanger in the pipe. A non air tight stove gets all the air it wants, burns hot- and consequently the temperature in the pipe is high. However and air tight stove runs at a much lower average temperature. Creosote condenses out of the smoke at around 300/350 degrees, and a pipe heat exchanger can readily drop the flue gasses down low enough that the creosote forms just like moisture will on a window. I've seen puddles of creosote on the floor at the back of a stove all because of the use of the heat exchanger. Or worse- build up in the flue, and highly increased risk of a chimney fire. Just ain't worth the extra heat gained.