Draft is effected in a negative way with to long a stack as they should be running positive pressure like a oil fired evaporator.
One thing that verified a suspicion of mine was that deeper flues are not that much of an advantage. Maybe in certain applications but not in a extemely hot and efficient evaporator. I have always felt that if the BTU's were there it didn't make that much difference with the deeper flues. One probably can go to shallow of a flue, but if your sap is achieving boiling temperature with a 7 inch, it will not get any hotter and thus evaporate any faster. The churning of the surface is what is releasing the steam and if the surface is violently moving it just can't boil any higher of a temperature.
Success is not final,failure is not fatal.It is courage to continue that really counts
“I have not failed. I’ve just found 10,000 ways that won’t work.”
– Thomas Edison