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M-1201.3 Size of chimney or vent: The size of the chimney or vent, [in other than multiple connections and] serving a single appliance, except engineered systems and Category I gas-fired appliances without draft hoods, shall have a minimum area equal to the area of the appliance connection.

M-1201.3.1 Category I appliances without draft hoods: Chimneys and vents serving Category I gas fired appliances without draft hoods shall be sized in accordance with the appliance and chimney or vent manufacturer's installation instructions.

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M-1201.5 Cutting or notching: A structural member shall not be cut, notched or pierced [in excess of the limitations specified] except as provided for in the building code [listed in Chapter 21, unless proven safe by structural analysis].

M-1201.6 Existing chimneys and vents: Where a fuel-fired appliance is permanently disconnected from an existing chimney or vent system serving multiple fuel-fired appliances, such 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 remaining appliance or appliances served with the required draft. For Category I gas fired appliances, the resizing shall be in accordance with Sections M-1201.3 and M-1201.3.1 or Sections M-1214.4 and M-1214.4.1 for the remaining appliance or appliances.