N.Y.C. Admin. Code § 27-894: § 27-894 Thimbles.

(a) When passing through combustible constructions, bent connectors constructed of type B gas vent material shall be installed so that the clearances required by the standard are maintained.

(b) Vent connections made of single-wall metal pipe shall not pass through any combustible walls or partitions unless they are guarded at the point of passage by ventilated metal thimbles not smaller than the following:

(1) For equipment conforming to standards – four inches larger in diameter than the vent connector.

(2) For equipment having draft hoods – six inches larger in diameter than the vent connector.

(3) For equipment without draft hoods – twelve inches larger in diameter than the vent connector.