Section 27-819. Furnace supports

Floor furnaces shall be installed only in
  floors of noncombustible construction having at least a  two  hour  fire
  resistance  rating, except as provided for one- and two-family dwellings
  in section 27-823  of  this  article.  Floor  furnaces  shall  have  the
  following clearances:
    (a)  Pit  clearances. Floor furnaces, when other than gas-fired, shall
  be mounted independently of the floor grille with a six  inch  clearance
  at  the  bottom and a twelve inch clearance at the sides except that the
  clearance on the control side shall be at least eighteen inches.
    (b) Pit waterproofing. When there is likelihood of water rising  above
  the  bottom  clearance  of the unit, the pit shall be constructed with a
  watertight enclosure with the sides extending at least four inches above
  the ground level.