The duct system (air distribution system) is the most important and most overlooked part of a heating and air conditioning system. The duct system is the part of the system that is responsible for delivering the correct amount of air to the different rooms or zones of the building and directly responsible for maintaining comfort for the people inside. To correctly design a duct system requires a lot of thought and effort and most are installed incorrectly due to lack of forethought and planning. To properly design a duct system the first step is to perform a room by room manual J load calculation to determine the proper amount of air required for each room or zone. From there different sizes of ducts should be installed in each room based on the requirement of that room. The equivalent length of the duct also affects the size of duct to be installed. Often times people think if a room is uncomfortable that means their heater or air conditioner is too small when in reality the duct system was not properly designed.
The duct system can also be a source of wasted energy. As the air moves through the ducts some of the energy is lost in the attic or crawl space because of the temperature difference in temperature between the air inside the ducts and the space it is installed in. this is especially true when installed in attic spaces due to the extreme temperatures. All ducts should be properly insulated and installed inside heated and cooled spaces when possible. The biggest energy waster relating to ductwork is air leakage. According to energy star 20% of the air that moves through a typical duct system is lost if not properly sealed with mastic and 93% of duct systems fall into this category. When ducts are not sealed they also draw in dust, insects, mold and other unpleasant things into the building.