I have a forced air gas heater, and I've wondered if, when the fan is on, some of the air gets ventilated out to the outdoors. Would this never happen? The only ventilation in it is probably combustion gases, they would be separate from the air that's being heated?
If circulating the air just circulates the air, rather than a lot of air leaking to the outdoors, then it's a good way of distributing humidity etc. I don't care if air leaks out to the garage, where the forced air ducts are, because I'm keeping that dehumidified too, and the garage is well sealed.