Nope, the more I think about it, the more it doesn't make sense. If it's a mild day and the system switches between heat and a/c, it should be switching between the center vents and the floor vents, not the center vents and defrost. I bought a MityVac so maybe I'll find something out soon. I'm going across country in July and I really really really want a 100% functioning HVAC system. - Mike