Section 27-893. Clearances

(a) Minimum clearance. Minimum clearances at vent
  connectors  to combustible materials shall not be less than those listed
  in table 15-10.
    (b) Reduced clearances. These  clearances  may  be  reduced  when  the
  combustible construction is protected as provided in table 15-11.
 
        Table 15-10 Vent Connector Clearance for Gas-Fired Equipment
  ========================================================================
                                    Minimum Distances from Combustible
                                             Materials (in.)
                                  ----------------------------------------
                                   Type B Gas Vent        Other Than Type
     Equipment                         Material             B Materials
  ------------------------------------------------------------------------
  Boiler                          As required by Std.              6
  Warm air furnace                As required by Std.              6
  Water heater                    As required by Std.              6
  Room heater                     As required by Std.              6
  Floor furnace                   As required by Std.              6
  Conversion burner (with
    draft hood)                           6                        9
  Equipment with draft hoods              6                        9
  Equipment without draft
    hoods                            Not Permitted                18
  ========================================================================
 
           Table 15-11 Reduced Clearances for Vent Connectors with
                      Specified Forms of Protection (a)
  ========================================================================
                                     Reduced Clearances Where the Required
  Specified Form of Protection (b)     Clearance with No Protection is:
                                    --------------------------------------
                                               18 in.    9 in.    6 in.
  ------------------------------------------------------------------------
  (a) 1/4 in. asbestos millboard
      spaced out 1 in. (c)                       12       6         3
  (b) 28 gauge sheet metal on 1/4 in.
      asbestos millboard                         12       4         2
  (c) 28 gauge sheet metal spaced out
      1 in. (c)                                   9       4         2
  (d) 28 gauge sheet metal on 1/8 in.
      asbestos millboard spaced out 1
      in. (c)                                     9       4         2
  (e) 1 1/2 in. asbestos cement
      covering on heating appliance              18       9         6
  (f) 1/4 in. asbestos millboard
      on 1 in. mineral fiber bats
      reinforced with wire mesh or
      equivalent                                  6       4         2
  (g) 22 gauge sheet metal on 1 in.
      mineral fiber bats reinforced with
      wire or equivalent                          3       2         2
  (h) 1/4 in. asbestos cement board
      or 1/4 in. asbestos millboard              18       9         4
  (i) 1/4 in. cellular asbestos                  18       9         3
      ====================================================================

  Notes:
    a. Except for the protection described in (e) above, all clearances
  should  be  measured  from  the  outer  surface  of the appliance to the
  combustible material disregarding any intervening protection applied  to
  the combustible material.
    b. Applied to the combustible material unless otherwise specified and
  covering  all  surfaces  within  the  distance specified as the required
  clearance with no protection. Thicknesses are minimum.
    c. Spacers shall be on noncombustible material.