Well here is the video you all have been asking fore LOL No its the number one question I get asked what vent to use and what vent not to use.
When I train plumbers on Rinnai heaters I tell the the first rule of thumb with venting if its metal its wrong period.
An easy way to remember what vent to use it if its non condensing like a V or a RL series its metal inside.
Condensing like a RU or a RUR its plastic inside.
NEVER USE THE METAL b VENT OR FLUE PIPE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
The advantage with the RU and The RUR series unites (Sensei) is you could also vent it with schedule 40 ( Red letter) PVC piping.
I hope this will answer a lot of questions on what to use and what not to use when venting a Rinnai tankless heater.

Check out my two Instagram pages for more Rinnai tips and pictures of my installs
michaellatronico
The Rinnai guy Mike
I hope you enjoy this video.
Mike
PLEASE USE A LICENSED PLUMBER TRAINED IN LEVEL THREE RINNAI TRAINING. FOLLOW ALL LOCAL AND NATIONAL PLUMBING AND GAS CODES AND TO FILE A PERMIT WITH YOUR LOCAL BUILDING DEPARTMENT.