Top flue or rear flue exit?

Which is better and why? I was told every 90 degree bend reduces efficiency 15%. Our stove is set up rear exit right now and I would like to keep it that way. If the plate bolts put up any resistance it may stay rear exit. Is there any way to increase the draft with the 90 degree bend? Lengthen the chimmney? Stove in question is Jotul Combifire #1.

If you've got good draft it doesn't make a big deal, if you don't, then it's more important. You will usually have a good draft if you have a tall chimney, (2 stories?) and live in a cold climate..It also helps if the Chimney is not on an outside wall of the house.

I have since managed to change my flue exit and only broke one bolt! I am still waiting to get a chimney done.