If the tributaries are not working, or there is a back up in the main duct, heat backs up in the furnace instead of circulating where it's needed. In a forced hot air system, the hot air from the furnace is blown by a fan through these ducts and tributaries. When the furnace comes on, the fans are timed to blow the warm air.

An improperly designed system will blow cold air into the ducts when the furnace is not on. Some dual-stage furnaces have high and low settings, while the more common single stage furnaces have a blast of hot air each time the furnace comes on. Each place one of the tributaries joins to the main duct, a mechanism controlled by thermostats blocks the heat or opens to allow access to the tributary to regulate heat in different zones or rooms. This is in addition to the registers in each room, which can be closed. The duct system can also be used for air conditioning, air filtration, humidification and purification. When the furnace and distribution system are designed properly for the home, there is no fuel wasted on energy that never reaches its destination. When the fans are improperly timed or the ducts are blocked, heat backs up, potentially damaging the furnace or emitting fumes into the home.

If your home is cold, it may be a more efficient distribution of the heat that's needed, not a larger furnace. A well designed heat system circulates cold air through the furnace, heating it and distributing it through a series of tributaries that lead into individual rooms. If the tributaries are not working, or there is a back up in the main duct, heat backs up in the furnace instead of circulating where it's needed. In a forced hot air system, the hot air from the furnace is blown by a fan through these ducts and tributaries. When the furnace comes on, the fans are timed to blow the warm