When two or more flues are located in the same chimney, what is the minimum thickness for the masonry wall between them?

The minimum thickness for the masonry wall between two or more flues located in the same chimney is 4 inches. This requirement is designed to ensure adequate separation and promote safe ventilation for the flues, reducing the risk of heat transfer and potential fire hazards. A 4-inch thickness provides a sufficient barrier to accommodate the materials used in chimney construction while also allowing for the thermal expansion and contraction that may occur. This standard aims to enhance safety and efficiency, making it an essential aspect of chimney design as outlined in the building codes. Adhering to this thickness helps prevent operational issues, such as flue gas migration between the separate flues, which could lead to dangerous situations in a residential setting.