The exhaust is pretty restricted with all the twist and turns Fire need to breath. It's like you had a 300 lb linebacker sitting on your chest it would be about the same thing. If the fire cant breath it don't work so well to produce heat because it can't get rid of the smoke backing up. The smoke will displace the O2 in the FB. restricting combustion.
I'm a Proxy Vegetarian> Cows eat grass & I eat cows.