Forward air outlet Overhead supply ducts Flexible duct panel AFT air outlet Main distribution manifold Mixing chambers Fan  Control cabin distribution duct Main distribution manifold relief valve 250° F duct overheat switch 190° F duct overheat switch Duct temperature anticipator sensor Duct temperature limit sensor Mixing chamber Mixing chamber Gasper fan Cold air supply Cold air supply Hot air supply Drain hose Hot air supply Ground service connection Gasper fan bypass duct To control cabin Muffler To passenger cabin FWD Main distribution manifold Figure 16-60.  The conditioned air distribution system on a Boeing 737. The main distribution manifold is located under the cabin  floor. Riser ducts run horizontally then vertcally from the manifold to supply ducts, which follow the curvature of the fuselage carrying  conditioned air to be released in the cabin. 16-36