Section 27-887. Installation requirements

(a)  The gas vent connected to
  equipment with a single vent shall not be less  than  the  size  of  the
  draft hood outlet, and in no case less than two inches in diameter.
    (b)  When more than one piece of equipment is connected to a gas vent,
  the area shall be not less than the area of the largest  vent  connector
  plus  fifty  percent  of  the  areas of additional vent connectors or in
  accordance with the provisions of reference standard RS 15-1.
    (c) Where two or more vent connectors enter a common vertical gas vent
  or chimney, the smaller connector  shall  enter  at  the  highest  level
  consistent with available headroom or clearance to combustible material.
  Two  or  more items of gas equipment may be vented through a common vent
  connector or manifold located  at  the  highest  level  consistent  with
  available  headroom  or clearance to combustible material. The manifold,
  all junction fittings, and the common vent connector shall be of a  size
  adequate for the combined volume of the vent gases.
    (d)  Gas  vents  shall  be  enclosed as provided in subdivision (c) of
  section 27-861 of article two of this subchapter.