Continuing the discussion from Student Discussion: The 2021 International Residential Code® (IRC®) Exam Study Course: Where must a chimney connector enter a masonry chimney? How is the size of a chimney flue that is connected to more than one appliance calculated?
A chimney connector has to enter the masonry chimney through a properly fitted thimble, and it can’t pass through any floor, ceiling, or combustible wall to get there — basically, it needs a direct, safe path into the chimney without cutting through material that could catch fire. When more than one appliance is tied into the same flue, you can’t just add up the flue areas needed for each one individually; instead, the flue has to be sized using the combined appliance input ratings and the methods laid out in the fuel gas code or NFPA 31, which factor in things like appliance type, connector height, and total vertical run to make sure the shared flue can actually handle the draft for everything hooked up to it.