The DuraVent 55FSFLEX-25FasNSeal 5.5 diameter Flex 25ft chimney flex pipe has a unique mechanical locking band and triple-lipped gaskets for easy installation. This flex pipe is used to vent Category II,III and IV gas burning appliances. It is recommended to be only used with FasNSeal components only. (Please note that this product can ONLY be used in vertical or near vertical runs) Silicone affects the performance and the durability of steel pipes. Products under the FasNSeal series by DuraVent are silicone-free. This latest designed technology enables these products to last a longer time and reduce the chance of malfunction or ventilation system breakdown. The welded seams guarantee leak-free products. For more than 50 years, DuraVent (formerly known as Simpson-DuraVent) has been trusted in the ventilation production through constant advancement of breakthroughs in commodity improvement and development. Having a wide selection of engineered venting supplies, DuraVent has gained its position as the manufacturing leader through producing innovative, reliable, and high-quality ventilation products. Get DuraVents reliable products here at PlumbersStock.com!