Instructions on how to clean/flush a tankless water heater (Rinnai)
1) Unplug Water Heater
2) Fill a bucket with vinegar (34 gallons) and put a submersible pump in bucket
3) Shut off cold & hot water into water heater
4) unscrew service port covers (with valves closed)
5) run cold water line from cold water service port into pump
6) run hot water line from hot water service port into bucket
7) open hot & cold water service port valves
8) plug in water heater, turn down temp if you want
9) turn pump on
10) run for 45 minutes or so
11) Never call it a "hot water heater", becuase you don't heat hot water. You are simply heating water.

EDIT 1/11/2021: A few things I realized I left out. I guess I had a few too many of those beers and forgot to make a video showing the rest of the process.
1) You also need to clean the filters, I didn't include how to do this; I will try to make a supplemental video showing how to do that the next time I do this.
2) Unless you want to shower in vinegar, or drink vinegar, flush your water heater with fresh water after the vinegarflush is complete. This can be done a couple of ways. You could follow the same process with the submersible pump and flush clean water thru the water heater for awhile (best option), or a backup option is to turn the hot water on from a sink and run it for 15 minutes or so. The latter is the more lazy option and will waste water and allow vinegar to run thru your inhome plumbing, but would still work. My advice, take option (a) and flush fresh water thru the water heater via the submersible pump. Don't be lazy like me.

EDIT 2/7/2023: A few more things;
1) Rinse your tubes/connections, inside and out, after flushing the water heater. The vinegar will not be nice to them in time.
2) No need to turn the water heater on while pumping vinegar thru it.
3) Some have asked about turning off the gas to it, I'm not sure why this would impact anything one way or the other
4) This has been confirmed to work with vodka/lemonade as your drink of choice when doing this in summer months.