Rinnai Rinnai RL75iN RL Series Tankless Water Heater 41 Error Fix: Abnormal Temperature Rise

Error code 41 on the Rinnai Rinnai RL75iN RL Series Tankless Water Heater means: Abnormal Temperature Rise. The unit has detected an abnormally rapid temperature rise in the heat exchanger, indicating restricted water flow or a heat transfer problem. The control board monitors the rate of temperature change between the inlet and outlet thermistors. If the temperature differential rises too quickly (faster than the burner output should produce), it indicates too little water is flowing through the heat exchanger for the current firing rate. This is a critical safety protection against overheating. Causes: (1) Minimum flow rate not met — a low-flow fixture (e.g., bathroom sink with aerator) may not provide the minimum 0.4-0.6 GPM required to keep the unit running. (2) Partially closed isolation valve — check both inlet and outlet isolation valves are fully open. (3) Scale buildup in the heat exchanger — mineral deposits reduce the internal cross-section of heat exchanger tubes, restricting water flow. Flush the heat exchanger with a descaling solution. (4) Clogged inlet water filter — the mesh filter at the cold water inlet catches sediment. Remove and clean it. (5) Failed flow sensor providing incorrect flow readings to the control board.

The most successful fix is "Flush and Descale the Heat Exchanger to Remove Scale Buildup" (medium difficulty, ~60 minutes). Estimated DIY cost: $15-$25. HomeMD has 2 community-ranked fixes for this error.

This error is rated critical severity. It is generally not urgent and can often be resolved with a simple DIY fix.

Yes — the top-ranked community fix ("Flush and Descale the Heat Exchanger to Remove Scale Buildup") is rated medium difficulty and takes about 60 minutes. Tools needed: Adjustable wrench, Submersible pump, Bucket, Hose clamps.

DIY cost: $15-$25. Professional repair typically costs $150-$250. Many homeowners save money by trying the top-ranked community fix first.

The Rinnai Rinnai RL75iN RL Series Tankless Water Heater has 8 other known error codes, including: 02 (Ignition Failure), 03 (Bath Fill Function Abnormality), 05 (Blocked Exhaust / Air Supply), 10 (Blocked Exhaust — Air Pressure Switch), 11 (No Ignition), and 3 more.

Error code 41 also appears on 8 other Rinnai models: Rinnai RE180iN RE Series Non-Condensing Tankless Water Heater, Rinnai RL94iN RL Series Tankless Water Heater, Rinnai RSC160iN Condensing Combi Boiler, Rinnai RU130iN Sensei Super High Efficiency Plus Tankless Water Heater, Rinnai RU199iN Sensei Super High Efficiency Plus Tankless Water Heater, Rinnai RUR199iN Sensei Recirculating Tankless Water Heater, and 2 more. The fix may vary by model, so check each model's specific page for tailored repair steps.