How to Adjust Dampers in Ducts on Your Own

Start by turning on your thermostat's fan-only mode. Open all the registers on the supply grates in each room. The return air duct design means the return grates won’t have any way to open or close them, so they’ll continue pulling air in fan-only mode. Only the supply grates can be opened or closed.

Inspect the ductwork extending from your furnace or air handling unit. Look for a handle or lever on the outside of the ductwork to identify the location of the dampers. You may need a flashlight and step ladder to be able to find and reach the handles for the dampers.

Ideally, your dampers would have labels on them, designating which rooms they serve. Sometimes, looking for the labels first makes it easier to find the nearby handles.

Open all the dampers that you can find. Turn the handle until it’s parallel to the direction of the duct. This should be the open position. If the handle is perpendicular to the direction of the duct, this should be the closed position.

If you rarely or never move the dampers, the handles may be difficult to turn. You can use some pliers to help you move the handles into the desired position.

If your dampers have labels, figure out which ones you want to close. In winter, you may want to close dampers to the upper floors of the home. This allows the furnace to feed the lower floors efficiently. In summer, try closing the dampers to the lower floor or basement so cool air can more efficiently reach the upper floors of the home.

If they don’t have labels, you can use a trial-and-error process to determine which dampers control which rooms. You then can add labels. 

Close a damper and use the direction of the ductwork down the line to try to figure out which rooms it serves. Go to the grates in those rooms and see if air is leaving the supply grates. If no air is coming out when you have the fan-only setting on, the closed damper serves this room. Continue with this process until you figure out the map of the ducts. 

After you figure out how to adjust dampers in the ducts, create the desired setup. Let them run for a couple of days and monitor the overall system performance. If you’re unhappy with the airflow, make some changes to which dampers you have open and closed and observe the results for a couple more days. Continue with this process until you’re happy with the results.