Thimbles.
NYC Admin. Code § 27-894
in effect§ 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.
Source: City of New York, Charter, Administrative Code and Rules (American Legal Publishing XML export, codelibrary.amlegal.com)URL: https://codelibrary.amlegal.com/codes/newyorkcity/latest/NYCadmin/0-0-0-52894Fetched: 10/24/2026