A Manual D duct design is the ACCA [Air Conditioning Contractors of America] method used to determine the overall detailed duct lay-out of the home which includes the sizing of each individual duct and register and the placement within each room. To design a duct system, the HVAC system designer must have first completed a room-by-room Manual J load calculation. The 2006 International Residential Code, IRC section M1601.1, requires that “Duct systems serving heating, cooling and ventilation equipment shall be fabricated in accordance with the provisions of this section and ACCA Manual D or other approved methods.” Most states require that energy code compliance of a residential construction project must be reported in order to qualify for the appropriate construction permits.

All to often, whole HVAC central systems along with the accompanying duct systems are created using rule-of-thumb methods in lieu of using the approved Manual J, Manual S and Manual D methodologies. This practice is the predominant reason for complaints of temperature differentials throughout a home as well as complaints of excessive noise caused by air velocity that exceeds the maximum allowed by Manual D.

In order for home owners to be comfortable a duct-work system must be designed to carry the proper amount of air, at the correct speed, into the right room. What problems come from incorrectly-sized ducts? If the ducts are the wrong size then the wrong amount of air will enter the room and may cause:

  • The room to be too warm or too cool
  • The air to be too drafty and disturb people while they sleep, eat, read, etc.
  • The air to be too noisy and drown out conversations, TV or radio programs, etc.
  • The air to be too slow – the conditioned air will not circulate or mix well in the room
  • The fan to work harder, possibly fail sooner, and use more energy to move air
  • The furnace or air conditioner safety devices to stop equipment operation
  • Pressure differentials that may increase energy costs by pushing out conditioned air or drawing in un-wanted air

The Building Technologies Program of the Office of Energy Efficiency and Renewable Energy U.S. Department Of Energy states, “The efficiency of air distribution systems has been found to be 60-75% or less in many houses because of insufficient and/or poorly installed duct insulation and leaks in the duct system”. Moreover, “efficient duct system installations can reduce equipment size, further saving money for new or replacement equipment”.

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