Burning for MANY years, but this is my first with the OAK. I made my own, using 3" galvenized stove pipe for through the wall, and down towards the floor, and then a 4' piece of harmans rip off 2 3/8 expensive pipe for the bend and stove hookup. Wow, HUGE jump in efficency ! No scientific data, but last week when snowing and 33 outside, I kep the house above 65 running on only 2-3. Amazing if you dont need to reheat your combustion air.