Closing vents to redirect air doesn't work and damages your system. When you close a vent, static pressure rises and forces the blower to work harder, but airflow capacity stays the same. A 6-inch duct handles about 100 CFM no matter what, so blocking one room doesn't send more air elsewhere.
Do not close your vents or registers in one part of the house to redirect cool or warm air to another part of the house. Just about any HVAC technician you talk to is going to tell you exactly that.
Never ever close the vents off in your house. The gentleman goes on to state how, um, mainly it's a static pressure issue. Yes, static pressure is a factor in not closing your vents. That's true because you're causing more air to go back to the blower, and then it's working harder to push it out. But mainly, people close vents because they're trying to redirect air to a place they're not at to a place they are at. Your duct system does not work in that fashion. A 6-inch run of duct in a room, uh, is made for 100 CFM of air. It can only hold 100 CFM of air. So if you close the vents off in the the room next to it or the room down the hall, more air is not going to come through the 100 CFM vent.
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