The maximum vent length when using 2-inch approved vent pipe is 75 ft (23 m). The maximum vent length when using 3-inch approved vent pipe is 150 ft (45 m). The intake and exhaust vent pipes can be terminated either horizontally or vertically, using one of the multiple approved vent termination types. The maximum vent length is reduced according to the number of elbows used, so best results, the tankless water heater’s venting system should be kept as short and straight as possible. Exhaust and intake air pipes must be supported at least every 4 ft (1.2 m). Refer to the installation manual for additional details.