Section 27-861. Metal chimneys

(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
 
  ========================================================================
                 ----------Classification of Equipment----------
                     Low           Medium               High
                 Temperature     Temperature         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
      Chimney Classification            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.