In most cases where the air duct is certainly huge enough for a small human to get through, that air duct will abruptly start to become narrow or you would run into fans or some other type of obstruction.
Have you ever watched movies or shows where people are sneaking around through the air ducts? I regularly wondered if all of that was even possible. I am aware that the air ducts in my home are certainly flimsy and couldn’t possibly hold a human being. Could it be that the air ducts in huge facilities are so high quality and so large, that it could actually hold people moving around in it? Well, I was so curious about this subject that I decided to do a little research on it. It turns out that in most cases, this is not at all possible, at least not the way they do it in the movies. In most cases where the air duct is certainly huge enough for a small human to get through, that air duct will abruptly start to become narrow or you would run into fans or some other type of obstruction. Also, in a secure facility such as the ones shown in the movies where they have these huge air duct systems that are used with some sort of powerful heating and cooling system, it would be likely that they would plan for advanced security measures to prevent intruders or pests. I did actually come across an article that talked about a man who was breaking into stores through the air duct, but he ended up getting caught in there plus he suffocated and died. If anything, a story like that should deter people from trying to enter into buildings through an air duct system. I would be afraid of the furnace cranking on and quite possibly burning me to death.