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Rinnai · Combi boiler

681 DHW supply temperature abnormality

Medium severity I-Series Plus 150K and smallerI-Series Plus 175K and higher

What the control is telling you

The measured bypass ratio, calculated from the inlet, heat exchanger and outlet thermistors, doesn't match the expected ratio — often from a misaligned bypass valve or a bad temperature reading.

Applies to: I-Series Plus Combi boilers — 150K BTU and smaller and 175K BTU and higher

Likely causes

  • Bypass valve/servo misalignment
  • Faulty or disconnected inlet thermistor
  • Faulty or disconnected heat exchanger thermistor
  • Faulty or disconnected outlet thermistor
  • Failed PC Board

Test and repair steps

  1. Warning: Turn off electrical supply before taking resistance readings.
  2. Check the inlet thermistor in performance data 08 against a thermometer at the same spot; over 5° difference means continue, -40 means unplugged/bad connection (see code 341).
  3. Check the heat exchanger thermistor in performance data 10 the same way (see code 331).
  4. Check the outgoing water thermistor in performance data 07 the same way (see code 161).
  5. Replace any inlet, outlet or heat exchanger thermistor that reads abnormally.
  6. Inspect the bypass valve and servo for misalignment and replace as needed (see code 661).
  7. If all thermistor values are correct and the code comes back, replace the PC Board.

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Source: Rinnai Troubleshooting I-Series Plus Boiler Diagnostic Codes (800000375), p.38. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.