Be compliant with your local building code by using this CNDS Condensate Trap Kit for Takagi/State/AO Smith tankless water heaters.
This gadget is part of the Direct Venting Systems for Takagi/State/AO Smith/American Heating models 110C, 310C and 510C Gas Tankless Water Heaters.
For the 110C, 310C, and 510C models, according to the manufacturer's installation manual, a condensate collector and trap (this page) are required to be installed when the venting run has more than 8 feet (2.4 m) of equivalent vent length, not including thru-wall termination. An 87° elbow is equivalent to 5 ft. (1.5 m) of vent length. The condensate collector must be installed on the heater's flue. The condensate trap and collector are required on all vertical installations.
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.
Drain damage due to condensate from a tankless water heater. Even if the drain is plastic and is impervious, local building code may require use of a neutralizer.