Heating and cooling ductwork recirculates a balanced volume of conditioned air. When the furnace is operating, supply ducts deliver heated air under positive pressure and return ducts convey it back to the air handler under slightly negative pressure. The optimal result is neutral air pressure in all rooms. Blockages in heating ducts usually manifest as some rooms being perpetually colder than others. Blocked or partially blocked ducts also create pressure imbalances. A room under positive pressure when return ducts are blocked pushes heated air out through every structural crack and gap. Negative pressure due to blockage in the supply ducts draws cold outside air into the house, causing the furnace to run longer to meet thermostat settings.
Check the obvious causes first. Ensure the supply and return vents in each room are open and not blocked by furniture that may have been moved, carpets or other obstructions. Listen for the sound of the blower fan in the air handler running when the furnace starts to verify the blower is operating and air is being circulated through the ducts.
Inspect the system air filter to verify that it is not completely clogged and obstructing air flow.
Visually inspect all accessible spans of ductwork with a flashlight, including the attic and the crawl space beneath the home. Look for duct segments that have deteriorated and become disconnected. Where soft, flexible ductwork is installed to bend around corners, look for kinks that may have developed or areas where the ducts have sagged, possibly blocking internal airflow.
Check any duct damper controls to ensure they are all in the “Open” position. These controls are usually a sliding handle or rotating knob protruding from the exterior of a span of rigid duct.
Inspect the air conditioner evaporator coil in the air handler for blockage. Although the evaporator has no role in heating, it is positioned directly in the furnace's air path. An A/C evaporator coil clogged by dirt or mold that formed during the summer cooling season may obstruct furnace airflow during the winter.