Forced-air systems often go out of balance, causing some of the rooms in a home to be too hot or too cold. The furnace is usually not to blame; the problem is caused by ducts and registers that are not properly set. You should balance the system while the furnace is turned on.
To balance a forced-air system, open all the ducts and registers in the system. There may be dampers in various ducts that need to be turned to open the ducts. The damper is open when it's turned parallel with the top and bottom of the ducting.
Next, gather six or seven thermometers and get them all to have about the same temperature reading. You can do this by laying out the thermometers together for about 30 minutes and then noting any discrepancies. Tape the thermometers on the walls of each room so each thermometer is about 36 inches up from the floor, away from the hot air register or cold air return. Wait one hour, then take a thermometer reading in each room when the heat is on. If one room shows a higher temperature than an adjoining room, close the damper or register slightly in the hotter room.
Follow this procedure for each room in your home, opening and closing dampers and registers until the same temperature is maintained in each room or the temperature balance you want is reached. The thermostat to the furnace should be kept at the same reading while you balance the system.
It's also important to know how to improve the flow of warm air in your house, so check out the tips mentioned in the next section.