Drain and flush a Rinnai water heater by shutting off the electricity and water, opening the purge valves, hooking up a pump and hoses, and circulating food grade white vinegar through the system. When finished, flush fresh water through the system to clean out the vinegar.
Shut off the electrical power supply to the heater, and then use the handles to shut off the hot and cold water supplies. With the purge port valves closed, open the drain caps on both the hot and cold water sides. Run one hose from the purge port valve on the hot water side to a bucket of 2 to 4 gallons of white vinegar, run a second hose from the pump inlet to the bucket, and run a third hose from the pump to the cold water purge port valve. Open the valves, turn on the pump, and allow the vinegar cleaning solution to flow through the system for 45 minutes to two hours, depending on the hardness of the water and the length of time since the last cleaning.
Once the vinegar flush is complete, flush the system again with fresh water. Remove the hoses, replace the purge port valve drain caps, and turn on the water supply and then the electricity. Periodically flushing a Rinnai water heater prevents lime, mineral deposits and sediments from building up in the passages and damaging the heat exchanger.