NFPA 211-2019 Standard for Chimneys, Fireplaces, Vents, and Solid Fuel–Burning Appliances.
22.214.171.124 Steel connectors shall be lined with medium-duty fre‐ clay brick (ASTM C27, Standard Classifcation of Fireclay and High-Alumina Refractory Brick) laid in medium-duty refractory mortar (ASTM C199, Standard Test Method for Pier Test for Refrac‐ tory Mortars) or the equivalent. 126.96.36.199.1 The lining shall be at least 2 in. (51 mm) thick for connectors having an inside diameter or greatest inside cross- sectional dimension of 18 in. (457 mm) or less. 188.8.131.52.2 The lining shall be at least 4 in. (102 mm) thick laid on the 4 in. (102 mm) bed for connectors having an inside diameter or greatest inside cross-sectional dimension greater than 18 in. (457 mm). 9.2.5 Metal connectors for high-heat appliances shall conform to the requirements of 184.108.40.206 and 220.127.116.11. 18.104.22.168 Metal connectors for high-heat appliances shall be made of steel not lighter than that designated for chimneys in Table 22.214.171.124. 126.96.36.199 The connectors shall be lined with high-duty freclay brick (ASTM C27, Standard Classifcation of Fireclay and High- Alumina Refractory Brick) or its equivalent having a thickness of not less than 4 in. (102 mm) laid on the 4 in. (102 mm) bed in high-duty refractory mortar (ASTM C199, Standard Test Method for Pier Test for Refractory Mortars) or its equivalent. 9.2.6 Masonry connectors or breeching shall be made of refractory material equivalent in resistance to heat and corro‐ sion to high-duty freclay brick (ASTM C27, Standard Classifca‐ tion of Fireclay and High-Alumina Refractory Brick) not less than 4 in. (102 mm) thick. 9.3 Length. A connector shall be as short and straight as prac‐ ticable. The appliance shall be located as close as practicable to the chimney or vent. 9.3.1 The horizontal length of a connector to a natural draft chimney or vent serving a single appliance shall be in accord‐ ance with one of the following: (1) Not more than 50 percent of the height of the vertical portion of the chimney above the connector for a connector serving a solid fuel–burning appliance
(2) Not in excess of the design specifcations for a connector that is part of an engineered venting system (3)* In accordance with NFPA 54 for Category I or draft hood–equipped gas appliances (4) Not more than 75 percent of the height of the vertical portion of the chimney or vent above the connector for all other fuels 9.3.2 The horizontal length, design, and construction of combined connectors or connectors to a manifold joining two or more appliances to a chimney or vent shall be determined in accordance with approved engineering methods. 9.4 Size. 9.4.1 The connector shall be sized for its entire length in accordance with 9.4.2 and 9.4.3. 9.4.2 The effective area of a connector for a single appliance shall be not less than the area of the appliance fue collar, unless it is part of an engineered venting system. 9.4.3 A connector or manifold serving two or more appliances shall have an effective area equivalent to the combined areas of the appliance fue collars or individual connectors, unless it is part of an engineered venting system.NFPA 211 pdf download.