You can use a vertical flue with a Garn but there is a catch. For any flue using Class A chimney, like the Garn does, there can be only one 90* turn. That means if the pipe exiting stright out the back of the Garn is below grade, you'd have to excavate down to it and build a "chase" to grade level for the pipe. It also makes accessing the tee used for the 90* turn problematic when it comes time to clean it. If you can put it on the "daylight" wall of the basement it is no problem.