the ventless heaters are the ones that throw more humidity into the air. The direct vent unit has a sealed combustion chamber that draws in outside air for combustion. So it doesn't take any air out of the inside and most importantly, being sealed, it is safe to use around in a garage with the potential for gasoline leaks and the like. My buddy has a hod dawg unit. It works, but it is noticeably noisier and blows out the warm air from the ceiling level, where the direct vent blows warm air out from the lower part of the furnace. This heats up the floor area quicker. The hot dawg has an open flame. Most residential codes want any open flame to be a certain height off the floor. I don't remember the exact number, but for some reason, 10' comes to mind. Most residential garages don't have ceilings high enough to mount one to meet code. I bought my first direct vent unit in 1985. The oldest one I have at my present house is from 1992, and still works fine with no maintenance. Pretty trouble free.