The FAMCO SQ Square Flue Vacuum Cap has been used for decades with gas, oil, coal, and wood burning chimneys. This chimney cap increases updraft and prevents downdrafts caused from wind. This works by forcing the wind to alter its direction over one of four gores on the cap. The change of direction creates an area of lower density air (partial vacuum) at the outlet of the pipe. Lower density air creates draft within the exhaust pipe, helping to remove exhaust gases and increasing overall efficiency. The positioning of the cap relative to the prevailing winds is not critical but is slightly more effective when placed with its flat side facing the winds. Quality constructed in 28 gauge Galvanized steel. The FAMCO SQ Square Flue Vacuum Cap has a rounded corner square base and draw bolt for secure installation. Comes in a range of sizes; 8×8 inch, 8×12 inch, 8×16 inch, 12×12 inch, and 12×16 inch. Measure carefully. This is a custom item requiring your exact length, width and perimeter measurements for the outside dimension of your flue. FAMCO can custom build certain sizes of the Square Base Chimney Caps. The FAMCO SQ Square Flue Vacuum Cap is manufactured by Famco in the United States and comes with a 1-year warranty against product defects and workmanship. If you have any questions about this product, please refer to the FAQs above or feel free to contact us by phone or email.