The FAMCO Vacuum Cap VCA Chimney 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 three 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. This Chimney Cap has a round base for installation over a round pipe. Quality constructed in .020 - .025 gauge Aluminum.