87° Elbow, Centrotherm-type Concentric Venting Pipe. Takagi/State/AO Smith/Reliance item number 100266132.
This gadget is part of the Direct Venting Systems for Takagi/State/AO Smith/American Heating/Reliance models 110C, 310C and 510C Gas Tankless Water Heaters. This uses the Takagi Proprietary APNC35 Concentric Vent Design which allows for an extremely long maximum venting length of 43 feet and up to 4 elbows.
Venting is a "snap". Female ends have locking collars with gaskets. Unlock the collar
and slide the male end into the female end. The inside of both intake and exhaust pipes
on the female end have gaskets that will seal to the male ends. Once inserted, lock the
collar to secure the two pipe sections.