Z-Flex 8" Stainless Steel Flexible 316ti Chimney Liner btu650000 What causes this and how

What causes this and how can it be controlled

and plugs into a standard outlet

Item Number OPT-ST36PBFSML-NG UPC Code 196111102990 Manufacturer The Outdoor Plus Brand Name The Outdoor Plus Material Hardieboard & Steel Frame Shape Square Fuel Type Natural Gas Ignition Match Lit with Flame Sense Ignition Hides Propane Tank No

This black stove pipe is a perfectly round

or Variable Valve

Z-Flex 8" Stainless Steel Flexible 316ti Chimney Liner btu650000 What causes this and howAdditional Length of Stainless Steel 316ti Chimney Liner Features Patented airtight Triple Lock construction. Available in lengths to 50ft in 5 foot increments. Available in 3" through 24" diameters. UL 1777 and ULC S635 Listed for use with wood, pellet, oil, natural gas and propane fired appliances.