I use a bathroom fan with a 3" outlet. I use 3" galvanized pipe straight from the bathroom fan, through the back of the barrel, at the bottom of the barrel, and shoved against the front of the barrel to seal it. Then brick goes up the sides of the barrel and a grate over the pipe. I have a bunch of 3/8" holes drilled in the galvanized pipe under the grate. I have a fan speed control to control it.