(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.