The Masonry Institute of America, which is the #1 source for published information on Masonry, states that adjacents flues should terminate from 4" to 12" different in vertical height as shown above. The reason is to avoid smoke from exiting one flue and then being drawn down into the living area by an unused flue which is downdrafting. This is a common occurence and may result in multiple problems...from smoky smells in the house to actual poisoning of the residents!
Yet most chimneys do not meet this standard. To make matters worse, the installation of a chimney cap - whether two single caps or one multi-flue cap - makes the situation worse. (Video Demonstration)
Document #1 from Masonry Institute
of America
Document #2 from Masonry Institute of America
MIA Web Site
Our testing has shown that the 4" minimum is not enough and that a separation of at least 7" is required to properly combat this problem. Therefore, all Extend-A-Cap products can add 7" or more to the height of one flue.