A central air conditioning system distributes the cooled air throughout your house via ducts. How much air each room gets depends on the damper settings. These plates, which are located inside the ducts, are adjustable. They can be opened or closed as much as needed to make the rooms they control comfortable. Different rooms in your home need different amounts of cooled air. That's why you need to adjust the air conditioning dampers to control and balance the air flow.
Open all the air conditioning vents or registers in the house so they are wide open. Find the air conditioning duct trunks that lead out from the furnace. Locate the damper lever on each duct. Each metal lever is located on the side of the duct. Open each damper by moving its lever so it points in the direction the duct travels, not across it. With all the balancing dampers open, the air conditioning air flow isn't restricted. It's operating at full capacity.
Close any balancing dampers that lead to rooms that are sufficiently cooled by the air conditioning. Turn the metal levers so they point across the ducts, perpendicular to the path the ducts follow. You may want to close the dampers on ducts that lead to small or unused rooms in the house as well. They don't require much cooling and will still receive a small amount of cooled air because the dampers don't shut completely tight.
Wait a few days and make mental notes of the temperatures of each room in your house. Check to determine which rooms are comfortable, which are too warm, and which are too cold.
Return to the air conditioning dampers in the furnace ducts. Adjust each air conditioning damper so it provides the right amount of air to each room. Open the dampers in ducts that lead to rooms that are too warm. Close dampers a bit to rooms that are too cool. Make small adjustments; don't open or close any damper completely.
Wait a few more days, and repeat Step 4. You need to keep doing this until every room in your house is cooled to your comfort level. When you're done balancing the air conditioning dampers, use a permanent marker to draw a line on each furnace duct. Extend each line out from the handles so you know where the position should be. Make an "S" beside each line to indicate it's the summer setting.