Just so I am understanding right your firebox exhaust directly up to the chimney? If this is the case I would suspect that you draft heat out of your chimney at a great rate. This set up would not work for me as my exhaust exits through 11 tubes that go front to back through the water jacket then to the chimney. I have a bunch more area to steal heat before it leaves the OWB, but it cools fast when it goes up the chimney (Note I do have a draft fan that kicks on and off to maintain water temp). With this in mind if I tried to slow the gases exiting the burner up the chimney I would have such a creosote mess I probably would not be able to pull the auger out of the stack.