Youre forgetting something. Your large bus doesnt have a bloody great engine inside heating up all the air that goes through
When air is heated, it expands by a certain amount.
Lets say for example, all air flowing the cowl expands by 25% as a result of absorbing heat away from the engine.
So you effectively have 25% more air that should be coming out the back than the front.........
So unless your exit hole is 25% larger in area than your inlet hole, not all the hot air is going to escape.
follow?