N.Y.C. Admin. Code § 27-885: § 27-885 Labeling gas vents.

Gas vents for use with gas-fired equipment, and which are not designed for use with solid or liquid fuel-fired equipment, shall be plainly and permanently identified by a metal label etched or embossed to read: "This gas vent is for equipment which burns gas only. Do not connect to incinerator or solid or liquid fuel-burning equipment." This label shall be attached to the vent at a point near where the gas vent system enters the wall, ceiling, or chimney.