How to Flush a Rinnai Tankless Water Heater
Flushing a Rinnai tankless water heater circulates a compatible solution through the heat exchanger to remove mineral scale; it is not the same as draining household plumbing or wiping an inlet filter. Rinnai publishes a 13-step procedure using the hot and cold isolation valves, purge ports, service hoses, a submersible pump, and four gallons of undiluted white vinegar. The exact sequence matters because the appliance uses electricity or gas and serves potable water. This guide explains that official workflow, but the installation and model manual control. If service valves are absent, confusing, leaking, or inaccessible—or if you are not confident isolating utilities—use a licensed plumber.
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Primary reference: Rinnai official tankless water heater flushing procedure. Always check the manual for your exact model.
Quick reference
- Confirm
- Match the exact Rinnai model, manual, and isolation-valve layout.
- Isolate
- Shut down energy and close household hot and cold water paths.
- Circulate
- Use the official hose, pump, bucket, and white-vinegar procedure.
- Rinse
- Flush with fresh water and dispose of solution responsibly.
- Restore
- Cap ports, reopen valves, purge air, check leaks, and verify operation.
What you will need
- The installation and operation manual for the exact Rinnai model
- Two compatible service hoses
- A clean five-gallon bucket
- A clean submersible circulation pump
- Four gallons of undiluted white vinegar, following Rinnai's U.S. procedure
- Adjustable pliers only if the valve kit requires them
- Towels and eye protection
- A licensed professional when valves or utilities are unclear
Step-by-step instructions
Step 1: Verify the model, valves, and service path
Read the exact manual and locate hot and cold isolation valves, hot and cold purge ports, power control, gas shutoff or breaker, and a safe discharge point. Photograph the original valve positions. Do not improvise through ordinary plumbing fittings or flush a combi boiler with a domestic-water-only procedure. Stop if a valve is missing, seized, corroded, or leaking.
Step 2: Shut down energy safely
Turn the appliance off using the specified control and disconnect its power source. Follow the model manual for gas isolation; Rinnai's procedure calls for shutting the gas supply or appropriate breaker before service. Do not open a sealed gas compartment. Let hot components and water cool enough to handle, and keep electrical connections dry throughout the flush.
Step 3: Isolate the heater from the house
Close the hot and cold isolation valves in the sequence shown by Rinnai so the flush loop serves the heater rather than the home's fixtures. Confirm that each handle position matches the valve labeling instead of relying only on color. Relieve pressure exactly as the manual directs before removing purge caps. Hot pressurized water can cause burns.
Step 4: Connect the service hoses and pump
Attach one hose from the cold purge port to the submersible pump and a second hose from the hot purge port back to the bucket, matching Rinnai's diagram. Tighten only enough to prevent leaks and keep hose ends secure. Put the clean pump in the bucket, then open the purge port valves by turning them to the service position specified by the valve kit.
Step 5: Circulate the approved solution
Rinnai's U.S. instructions specify four gallons of undiluted white vinegar and warn against chemical solutions. Add the vinegar to the bucket, energize only the pump, and circulate for the time in the exact procedure. Monitor the hoses, bucket level, temperature, and connections continuously. Stop the pump immediately if anything leaks, collapses, runs dry, or behaves unexpectedly.
Step 6: End circulation and rinse the loop
Switch off and unplug the pump before moving hoses. Close the appropriate purge port, remove the cold-side hose as directed, and use the cold-water supply to rinse the heat exchanger into the bucket through the hot-side hose for the manual's specified time. Do not discharge hot acidic liquid onto plants, reactive metal, or an unsafe drain; follow local disposal rules.
Step 7: Restore valves in the exact sequence
Close both purge valves, remove the remaining service hose, inspect the cap seals, and reinstall purge caps without cross-threading. Open the cold and hot isolation valves to return the heater to the plumbing system. Restore gas and electrical power only after all fittings are secure and dry, using the Rinnai sequence for your model.
Step 8: Purge air, inspect, and document
Run a hot-water fixture as the manual directs to purge air and confirm stable flow. Check every service connection for seepage, then verify normal ignition, temperature, and error status. Record the date, solution, and observations for maintenance planning. Persistent codes, scale, restricted flow, combustion odor, or leakage requires qualified Rinnai service rather than another stronger flush.
Specific cleaning questions
Open a question for a concise answer about stains, odors, safety, frequency, and troubleshooting.