The 15° Elbow Kit is used to get past rafters or away from an obstruction like plumbing, HVAC, or the roof peak. Up to 2 lengths of chimney pipe (for a maximum run of 72 in. between elbows) can be added between elbows to allow for necessary spacing. An Elbow Strap is included and helps support the weight of the offset from the elbows. If your installation includes 2 lengths of pipe between the elbows a second Elbow Strap (sold separately) is required. CONSTRUCTION MATERIAL Our double wall Class-A insulated chimney system delivers superior safety and performance. The 1.18 in. thick high density ceramic fiber blanket insulation is sandwiched between two 0.019 in. thick 304 stainless steel walls. Compared to our competitors 430 stainless steel or galvanized steel, 304 stainless steel provides superior corrosion resistance while our 0.019 in. thick inner and outer walls are as must as 25% thicker than our competition. CERTIFICATION Certified and listed to the UL 103HT standard and designed for appliances that are listed for use with a 2100°F High Temperature chimney system. It can be used with wood stoves, fireplaces, fireboxes, furnaces, boilers, water heaters, pizza ovens and other residential heating appliances fueled by wood, gas, coal or oil. Note: Do not use with forced draft, positive-pressure appliances.