Rinnai REU-VA2019FFUD-UC Operation And Installation Manual page 33

Flue Installation (R98LSi, R98LSi-ASME)
Horizontal with Concentric Termination
SLOPE INTAKE AND EXHAUST
TOWARD OUTSIDE TERMINATION
MINIMUM 1/4" PER FOOT
Vertical Terminations
The air intake requires a return bend or two 90º elbows to prevent entry of rain.
The vent termination and air intake must be in the same pressure zone.
VA Series Indoor LS Manual 33