R1003.13

Section R1003.13 is titled 'Multiple flues' and covers requirements for masonry chimneys. The requirement for chimneys with multiple flues serving separate appliances to have the flues separated by a masonry wythe which is a minimum 4" thick and bonded into the walls of the chimney is established. The purpose of this section is to prevent backdrafting of exhaust gases into the building. This section has an exception which allows for multiple flues serving a single appliance to not require separation by a masonry wythe, but does require the joints of each flue to be vertically staggered a minimum of 4". The purpose for this exception is to allow for not constructing a separating wythe where an appliance is designed to use two flues and the risk of backdraft of exhaust gases is low.

The model shows a section of a chimney (with the mortar between the bricks excluded for clarity) including two clay flue liner sections (which are exhausting separate appliances), being separated from each other by a wythe of brick, which has interlocking units with the exterior wall units at every other course.