Flat oval ducts have smaller height requirements than round ducts and retain most of advantages of the round ducts. However, fittings for flat oval ducts are difficult for a contractor to fabricate or modify in the field. Other disadvantages of flat oval ducts include: difficulty of handling and shipping larger sizes; a tendency of these ducts to become more round under pressure; and, in large aspect ratios, difficulty of assembling oval slip joints. The use of flat oval ducts has also been impeded because of the absence of flow resistance coefficients for many fittings.
