A/C Vent question: Keep them closed or open?

Hi All, I recently purchased a town-home in the Northern VA area, and have a question regardng AC efficiency.
Is it more energy efficient to close off A/C vents in rooms that we don't use frequently (spare bedroom, basement, etc)? Or does it make more sense to open them all up and let the house breathe?
The previous owner kept most of the vents closed in rooms that were not often used, like the laundry room. I went ahead and opened them all up to circulate the air.
Is it more energy efficient to close them off? I own a 3 floor 2 bedroom 1500 sq ft. townhome end-unit.
Thanks!
Andrew
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Every forced-air heat/AC system in the world tries to recirculate air by blowing it out one side of the blower and sucking it back thru "returns" to blow it again. The system can't blow any more air out than it takes in. If the system is "balanced", that is if the output registers have equal capacity as the air returns, you might as well leave all registers open. If your town home is like MANY and has return(s) that is smaller than the sum of the registers, you might as well close some registers and keep your minimal flow in the rooms you use most. When I say capacity, I generally mean the cross sectional area of the registers and returns. Use Pi r squared for a round duct. When I say generally, it because duct length is a factor, too. If you have round ducts, the diameter may be printed on the outside of the plastic tube as 6,8,10,12, 14 or 16 inches, meaning a radius of 3,4,5,6,7, or 8 inches. That's about all I can tell you in one sitting.

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Hi Drew, hope you are having a nice day
On 27-Jul-03 At About 23:35:10, Drew wrote to All Subject: A/C Vent question: Keep them closed or open?
D> From: andrew snipped-for-privacy@yahoo.com (Drew)
D> Hi All, I recently purchased a town-home in the Northern VA area, and D> have a question regardng AC efficiency. D> Is it more energy efficient to close off A/C vents in rooms that we D> don't use frequently (spare bedroom, basement, etc)? Or does it make D> more sense to open them all up and let the house breathe?
An A/C system is designed to run with a certain amount of air. if you cut back too much it can cause damage to the compressor. it is best to leave all of the diffusers open.
-=> HvacTech2 <=-
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