How to Flush & Descale a Rinnai Tankless Water Heater

Flushing a Rinnai tankless is the single most important maintenance task. Scale forms inside the heat exchanger from hard water and reduces GPM output, increases gas consumption, and eventually triggers Code 14 (thermal fuse). Plan 90 minutes once a year.

Tools needed

• Submersible pump with ½" or ¾" outlet — $40-60 kit from Amazon
• Two 5-gallon buckets
• 4 gallons of white vinegar OR 1 gallon of commercial descaler (Rinnai sells branded; CLR PRO and Hercules Sizzle also work)
• Two ¾" garden hoses
• Crescent wrench

Prep

1. Turn off the gas supply at the unit's gas valve
2. Turn off the cold-water inlet isolation valve (the blue/red T-handle valves below the unit — these are required by code on every install)
3. Open a hot-water faucet inside the house to release pressure
4. Cool the unit for 10 minutes

Connect the pump

1. Attach one hose to the inlet isolation valve service port (typically a ½" threaded drain valve)
2. Drop that hose into the bucket with descaler
3. Attach the second hose to the outlet isolation valve service port
4. Drop that hose into the same bucket (closed loop)
5. Drop the submersible pump into the bucket; connect to the inlet hose

Run the descale

1. Open both isolation valve service ports (small lever, perpendicular = open)
2. Close the inlet and outlet main valves (blue and red handles parallel to pipe = closed)
3. Plug in the pump
4. Circulate vinegar/descaler through the heat exchanger for 45-60 minutes. The fluid will discolor and may foam — this is scale dissolving.

Rinse

1. Unplug the pump
2. Close the inlet service port
3. Disconnect the inlet hose
4. Open the inlet main valve briefly to flush fresh water through and out the outlet hose into a second bucket — flush 5-10 gallons
5. Close the outlet service port and disconnect the outlet hose
6. Open both main valves

Restart

1. Open a hot tap in the house and let it run until water is steady (no air)
2. Turn the gas back on
3. Power-cycle the unit if any error code appears
4. Verify normal operation

Vinegar vs commercial descaler

Vinegar is cheaper but slower and weaker. Commercial descaler (Rinnai-branded, CLR PRO, Hercules Sizzle) is faster, stronger, food-safe rinse, and Rinnai-approved. For routine annual maintenance: vinegar is fine. For first-time descale on a scaled unit: commercial descaler.

How often?

Hard water (>11 gpg) — annually. Soft water (<7 gpg) — every 2 years. With a softener — every 3 years.

Bottom line

This is the maintenance task Rinnai support most often blames warranty denials on. Document it (date + photo of the bucket water) for warranty defense. Full annual checklist on our Rinnai maintenance hub.