5.17. Masonry Institute of America –
“The pouring of smoke from one flue to an adjacent flue occurs where downdraft from interior suction or vertical wind currents force smoke down an inactive flue as it exhausts from the adjoining flue”
5.14. Masonry Institute of America –
The Masonry Institute of America, which is the #1 source for published information on Masonry, states that adjacent flues should have a 4″ to 12″ in difference 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 occurrence 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 – can make the situation much worse.