After running my Mt.Vernon AE insert for the last month I decided to change my existing 3" flex/Duravent vent system to a 4" flex liner. I will also be adding an o.a.k. Both the vent and the o.a.k. will be run up thru a 18' masonry flue. I'm now trying to come up with a parts list of what I need to order. I took off the right side cover of the insert and noticed that instead of using an adapter on the top vent outlet the installers mounted a 3"X12" pc of type "L" Duravent pipe. The pipe is labeled 3SPXL12. The pipe is screwed and rtv'd in place. The flow arrow is pointed down instead of up and the pipe is slightly tilted back to clear the top of the insert frame. My guess is this was done to keep the vent as straight as possible and get the pipe above the top of the insert. Is the flow direction on this 12" pc of pipe crucial? With the way the pipe is installed (upside down) I figure I would have to come up with a 3" male to 4" flex adapter to make what I currently have work. Where is an adapter like that even available? From looking in the side opening it doesn't look as if the vent can be run straight up without interfering with the case. If this is true, what is the exact configuration you other Mt. Vernon insert owners are using to offset the vent pipe to clear and get above the top of the insert to go up a masonry flue? Did you have to use an adapter with two 45 degree fittings and a flex adapter or how exactly was it done on your install? If I can safely use the existing upside down, tilted piece I will. Since I will be removing the insert anyway, I want to make sure everything is up to par for next heating season. I especially need advise from owners and installers familiar with the Mt. Vernon AE insert. Thanks in advance for any replies and advise.