N.Y.C. Admin. Code § 27-883: § 27-883 Gas equipment connected to chimneys.

Automatically controlled gas-fired equipment connected to a chimney that also serves equipment for the combustion of solid or liquid fuel shall be equipped with an automatic pilot. A gas-fired equipment vent connector and a chimney connector from equipment burning another fuel, located on the same floor, may be connected into the same chimney through separate openings, or may be connected through a single opening if joined by a suitable fitting located as close as practicable to the chimney. If two or more openings are provided into one chimney, they shall be at different levels.