The short answer
Rinnai's guide calls for flushing at least once a year: close the valves, connect hoses to the purge ports, and circulate 4 gallons of undiluted white vinegar through the heater, which may take up to 45 minutes. Its SENSEI bulletin specifies at least an hour at 3 to 4 GPM, then a five-minute fresh-water rinse. Follow your own heater's manual: makers set their own method and interval.
What you need
- Isolation (service) valves on the hot and cold lines, such as BRIGHTDECK A01 3/4" Tankless Isolation Valve Kit and CMI 3005IPS Tankless Isolation Valve Kit
- A small submersible pump, two hoses and a bucket, as in Chromex CXTE-1694 Tankless Flush Kit and Chromex CXTE-1976 Tankless Flush Kit
- About 4 gallons of white vinegar, or the descaler the maker allows
The steps
- Turn off power (and gas) to the heater.
- Close the hot, cold and main valves; open the purge ports slowly.
- Connect the pump hose to the cold-side port and the return hose to the hot side.
- Circulate the vinegar for the time the manual gives.
- Rinse with fresh water for about five minutes; clean the inlet filter; reopen the valves and purge the air.
Use what your maker allows
SafetyRinnai's consumer guide says never to use chemical solutions in its heaters, because drinking and bathing water runs through them. Use what your maker allows.
Products mentioned
Isolation valve kit · BRIGHTDECK
BRIGHTDECK A01 3/4" Tankless Isolation Valve Kit
Isolation valve kit · most American residential tank
Descaling kit · Chromex
Chromex CXTE-1694 Tankless Flush Kit
Descaling kit · most all tankless water heater
Descaling kit · Chromex
Chromex CXTE-1976 Tankless Flush Kit
Descaling kit · most all tankless water heater
Questions readers ask
How often should a tankless water heater be flushed?
Rinnai says at least once a year; hard water calls for more often. Check your model's manual.
Vinegar or descaler?
Rinnai's procedures use undiluted white vinegar and its consumer guide warns against chemical solutions. Other makers allow specific descalers; follow the manual.