Codes from IRC Chapter 18

• R1001.1 General.  Masonry chimneys shall be constructed, anchored, supported and reinforces as required in this chapter and the applicable provisions of Chapters 3, 4 and 6.  In Seismic Design Categories D1 and D2, masonry and concrete chimneys shall be reinforced and anchored as detailed in Section R1003 for chimneys serving fireplaces.  In Seismic Design Category A, B or C, reinforcement and seismic anchorage is not required.  Chimneys shall be structurally sound, durable, smoke-tight and capable of conveying flue gases to the exterior safely.

• M1801.3.2 Flue passageways.  The flue gas passageway shall be free of obstructions and combustible deposits and shall be cleaned if previously used for venting a solid or liquid fuel-burning appliance or fireplace.  The flue liner, chimney inner wall or vent inner wall shall be continuous and free of cracks, gaps, perforations, or other damage or deterioration that would allow the escape of combustion products, including gases, moisture and creosote.

• M1801.3 Existing chimneys and vents.  Where an appliance is permanently disconnected from an existing chimney or vent, or where and appliance is connected to an existing chimney or vent during the process of a new installation, the chimney or vent shall comply with Sections M1801.3.1 through M1801.3.4.

• M1801.3.1 Size.  The chimney or vent shall be resized as necessary to control flue gas condensation in the interior of the chimney or vent and to provide the appliance, or appliances served, with the required draft.  For the venting of oil-fired appliances to masonry chimneys, the resizing shall be done in accordance with NFPA31.