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.~.. ~ . . . . 1 .; . (c). Clearances from combustible material to gas appliance vent piping other than approved type B gas vents shall be in ac- cordance with standard practice for safe installation and use as required by section 36. 5. Flue mortar. All flue mortar for flues or vent pipes from gas-burning appliances shall be acid resisting. 6. Draft hoods. Every flue connected appliance except an incinerator, unless its construction serves the same purpose, shall be equipped with an effective draft hood which either (1) has been approved as part of the appliance or (2) complies with nationally recognized standards for draft hoods. The draft hood shall be at- tached to the flue collar of the appliance or as near to the appliance as conditions permit and in a position for which it is designed with reference to horizontal and vertical planes. The draft hood shall be so located that the relief opening is not obstructed by any part of the appliance or adjacent construction. 37 4. Installation of gas vents other than type B. (a). Gas appliance vent piping other than approved type B gas vents shall not pass through any attic or concealed space nor through any combustible floor, and shall not pass through any combustible roof except as specified in subsection 2(c). Where passing through combustible roofs in accordance with subsection 2(c) they shall be guarded at the point of passage as specified for passage through combustible walls or partitions in paragraph (b) below. Vent piping may extend through any existing chimney whether the chimney is lined or not. (b). Gas appliance vent piping other than approved type B gas vents shall not pass through ,combustible walls or partitions unless they are guarded at the point of passage (1) by metal venti- lated thimbles not less than 6 inches larger in diameter than the pipe, or (2) by metal thimbles not less than 4 inches larger in diameter than the pipe with the annular space filled with mineral wool or other approved noncombustible insulating material; or in lieu of such protection all combustible material in the wall or par- tition shall be cut away from the vent pipe a sufficient distance to provide the clearance required from such vent pipe to combustible material-any material used to close up such opening shall be non- combustible. (d). Type B gas vents shall fuel burning appliances. . . not 3. Installation of type B gas vents. (a). Type B gas vents shall be made up with tight joints. (b). Type B gas vents shall be installed with a clearance to combustible material or construction whether plastered or unplas- tered, of not less than one inch, provided that for vents of floor furnaces such clearance shall be not less than 3 inches for a distance of not less than 3 feet from the outlet of the draft hood, measured along the center line of the vent piping. (c). Suitable provision shall be made to prevent mechanical injury to type B gas vents where they extend through walls, floors or roofs. be used with solid or liquid e e e - from combustible material and passage through wall or roof shall comply with the provisions of subsection 4 of this section.