Close It Down
Close the damper. While running any kind of air conditioner, shut your fireplace damper. An open one "pulls hot air into your house instead of sucking it out," says Tommy Spoto, a master chimney sweep at Chimney Chap, in Copiague, New York. "This is called flow reversal."
Close everything else, too. Whether the air conditioner is on or off, keep windows and doors shut if the temperature outside is more than 77 degrees Fahrenheit (most people start to sweat at 78). Whenever the outside air is hotter than the inside air, opening a window invites heat to creep in.