How to fix warm air heating

Balancing and maintaining a forced-air heating system

Manually adjusted dampers in the ducts of a forced-air system let you regulate the amount of warm air entering a room. If some rooms are cold and others overheated, balance the system by opening or closing the dampers.

Look for dampers at the beginning of branch ducts leading from a main duct (or, in some systems, from the hot-air plenum) to room registers. Follow a branch duct’s path to determine which room it heats; or close a damper to see which room gets cold. Label the damper once you’ve identified it. A damper is closed when its handle is at right angles to the duct; it’s open when the handle is parallel to the duct. Secure a damper in place by tightening its locknut (if any].

Balance the ducts on a cold day. Fully open all dampers and registers.

Set the thermostat at normal room temperature and let it operate the heating system for at least 3 hours. Partially close the damper in the duct leading to a room that overheats. Wait an hour, then see how the room feels; check its temperature with a thermometer held 4 feet above the floor. Open or close the damper as needed; check the room again after an hour. Working from warm to cold, repeat the procedure in each room. Because adjusting air flow in one duct affects the others, fine-tuning the system may take several days. If after balancing the ducts, some rooms remain cold, try increasing the speed of the furnace blower.


Periodically vacuum the room registers using a crevice tool. Once a year, inspect ductwork for leaks; seal them with duct tape.