(a) Exterior metal chimneys. Exterior metal chimneys constructed of steel shall be of adequate thickness to resist all applied loads specified in subchapter nine of this chapter, but shall not be less than the thickness required in table 15-4.
(b) Clearances for exterior metal chimneys. Metal chimneys erected on the exterior of a building shall have sufficient clearance from a wall or frame of combustible construction to satisfy the requirements of table 15-2 and to permit inspection and maintenance operations on the chimney, and shall comply with the following minimum requirements:
(1) Exterior metal chimneys over eighteen inches in diameter, for equipment of any operating temperature, shall have a clearance of at least four inches, and those eighteen inches or less in diameter shall have a clearance of at least two inches from a building wall of any combustible construction.
(2) No portion of an exterior metal chimney shall be nearer than twenty-four inches to any door or window or to any exit, or located where it would be readily accessible to the public, unless it is insulated or shielded to avoid injury to any person coming in contact with the chimney.
TABLE 15-2
Minimum Clearances Required from any Combustible Materials
Minimum Clearances Required from any Combustible Materials
Classification of Equipment | ||
Low Temperature | Medium Temperature | High Temperature |
6" | 24" | 24" |
(c) Enclosure of interior metal chimneys:
(1) Every interior metal chimney or part thereof erected within buildings shall be enclosed with noncombustible construction having a fire resistance rating of not less than that prescribed in table 15-3 in all stories above the one in which the equipment served thereby is located. Where the metal chimney passes through a combustible roof, it shall be guarded by a noncombustible ventilating thimble of galvanized metal or equivalent noncombustible material that extends at least nine inches below and above the roof construction. The thimbles shall be of a size to provide clearances on all sides of the metal chimney of at least six inches for low temperature equipment and at least eighteen inches for medium and high temperature equipment as defined in subchapter fourteen of this chapter. However, if the metal chimney is insulated and protected to prevent a temperature of more than two hundred fifty degrees Fahrenheit on its exterior surface, the thimble clearance may be reduced by fifty percent.
(2) The enclosure around a metal chimney shall provide a space on all sides of the chimney to permit inspection and repair for the entire chimney height. When metal chimneys have a minimum dimension of twenty-four inches in diameter and are completely jacketed with noncombustible insulating material within the required enclosure, access to permit inspection and repair shall not be required.
(3) The enclosing construction shall be of noncombustible materials and shall have a fire resistance rating as required in table 15-3 and shall be without openings, except access openings equipped with opening protectives constructed in accordance with the requirements of subchapter five of this chapter, at floor levels where necessary for complete inspection purposes.
(4) The required fire resistance ratings of table 15-3 for the enclosure of a medium or high temperature metal chimney may be reduced by one hour if the chimney is insulated with an insulation adequate for the temperature of the chimney and having at least a one hour fire resistance rating at all sections of the insulation.
TABLE 15-3
Required Fire Resistance Ratings of Enclosures for Metal Chimneys
Required Fire Resistance Ratings of Enclosures for Metal Chimneys
Chimney Classification | Required Fire Resistance Rating of Enclosure |
Low temperature | 1 Hr. for buildings under 5 stories 2 Hr. for buildings 5 stories or more |
Medium temperature | 2 Hr. |
High temperature | 3 Hr. |
Note: Table 3-4 of section 27-271 shall also be complied with in the construction of enclosures for metal chimneys.
(d) Prohibited location. No interior metal chimney shall be carried up inside a ventilating duct unless such duct is constructed as required by this subchapter for metal chimneys, and only when such duct is used solely for exhaust venting the room or space in which the equipment served by the metal chimney is located. Metal chimneys shall not be installed in air supply ducts.
(e) Unlisted metal chimneys serving residential type or low heat appliances and producing flue gas having a temperature below 350°F at the entrance to the chimney at full load or partial load shall be lined with acid and condensate resistant refractory material, or constructed of suitable stainless steel, or otherwise protected so as to minimize or prevent condensation or corrosion damage as required in reference standard RS 15-14.
(f) Steel chimneys for exterior use shall be insulated and weather sheathed to maintain flue surface temperatures above the acid dew point in all parts of the system in accordance with subdivision (b) of section 27-854.
(g) Steel chimneys for interior use shall be insulated to ensure enclosure temperatures not in excess of 160°F on combustible materials in accordance with subdivision (a) of section 27-854.