Rinnai code 54: High exhaust gas temperature
Continuous flow hot water system (Australia), tankless water heater (United States)
Exhaust gas leaving the heater is hotter than its limit.
This needs a licensed gas technician. There are a few safe checks you can do first, marked below. Anything inside the unit is a job for a professional.
This involves the flue or exhaust. A blocked or leaking flue can let carbon monoxide into the home.
What causes it
Australian models
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The flue system or the heat exchanger fins are blocked.
Check the outside flue or vent terminal is clearYou can do this
- Find where the flue or vent pipe ends outside the building. High-efficiency furnaces and some water heaters also have a combustion air intake pipe beside it: check that one too. On some outdoor water heaters the flue outlet is part of the front cover.
- Look for anything blocking it, such as leaves, a bird or insect nest, snow, plants, or items stored against the wall.
- Cut back plants and move stored items away from it. Do not touch, poke into or hose the flue outlet. If something is inside it, book a service. The terminal can be hot. Do not climb onto the roof or use a ladder to reach it; if it is out of reach, book a technician.
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Scale build-up inside the heat exchanger causing hot spots.
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The incoming water is already too hot, or the heater was not converted properly after a gas type change.
US models
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Restrictions in the vent intake or exhaust, or vent terminal clearances not met.
Same check as above: Check the outside flue or vent terminal is clear
-
Scale build-up in the heat exchanger causing hot spots.
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Inlet water that is already too hot, the wrong gas type, or fan air leakage.
How to read this code on your unit
Booking a licensed gas technician
Book a licensed gas technicianlicensed gas technician
- Turn the unit off and leave the covers on. Gas components must only be serviced by a licensed technician.
- Note the code, how often it appears, and the model and serial numbers from the rating label.
- Book a licensed gas technician and give them those details.
Have the heat exchanger flushed and descaledplumber
- Scale from hard water builds up inside the heat exchanger and restricts flow and heat transfer.
- Book a licensed plumber or the manufacturer's service agent to flush and descale the heat exchanger using the manufacturer's flushing procedure.
- Ask about water treatment if you have hard water, so the scale does not come back.
Models that show this code
73 models in 15 ranges
| Range | Models |
|---|---|
| Heavy Duty (commercial) | REU-AM3237WC (HD32e), REU-VCM2837FFC (HD28i), REU-VCM2837WC (HD28e), REU-NB3237WC-AK (Heavy Duty 32C), REU-NB3237FFC-AK, REU-NB3237WC-50-AK, REU-NB3237FFC-50-AK |
| Infinity | REU-AM3237W (Infinity 32), REU-AM2626WDL (Infinity 26 Touch), REU-AM2626WD (Infinity 26), REU-AM2024WD (Infinity 20), REU-AM1620WD (Infinity 16), REU-AM1220WD (Infinity 12), REU-A2626WB (B26), REU-A2024WB (B20), REU-A1620WB (B16) |
| Infinity B | REU-BM2626W (Infinity B26), REU-BM2024W (Infinity B20), REU-BM1620W (Infinity B16), REU-BM1220W (Infinity B12) |
| Infinity Enviro | REU-E2626W (Infinity Enviro 26), REU-E1620W (Infinity Enviro 16), REU-NB3237W-AK (Infinity Enviro 32), REU-NB3237W-50-AK |
| Infinity internal | REU-VCM2837FF (Infinity 28i) |
| Commercial (KB) | C199i (REU-KBD3237FFUDC-US), C199e (REU-KB3237WDC-US) |
| KB | RU80i (REU-KB2530FFUD-US), RU80e (REU-KB2530WD-US), RU90e (REU-KBD2934WD-US), RU98i (REU-KB3237FFUD-US), RU98e (REU-KB3237WD-US) |
| KBD | RUC80i (REU-KBD2530FFUD-US), RUC90i (REU-KBD2934FFUD-US), RUC98i (REU-KBD3237FFUD-US) |
| KBP | RUR98i (REU-KBP3237FFUD-US), RUR98e (REU-KBP3237WD-US) |
| KCM | RUCS65i (REU-KCM2025FFU-US), RUCS75i (REU-KCM2528FFU-US), RUS65e (REU-KCM2025W-US), RUS75e (REU-KCM2528W-US) |
| Luxury (VC) | RL75i (REU-VC2528FFUD-US), RL75i(A) (REU-VC2528FFUD-US(A)), RL75e (REU-VC2528WD-US), RL75e(A) (REU-VC2528WD-US(A)), RL94i (REU-VC2837FFUD-US), RLX94i (REU-VC2737FFUD-US), RL94e (REU-VC2837WD-US) |
| SENSEI commercial | CU199i (REU-N3237FFC-US), CU160i (REU-N2530FFC-US), CU199e (REU-N3237WC-US), CU160e (REU-N2530WC-US) |
| SENSEI residential | RU199i (REU-N3237FF-US), RU180i (REU-N2934FF-US), RU160i (REU-N2530FF-US), RU130i (REU-N2024FF-US), RU199e (REU-N3237W-US), RU180e (REU-N2934W-US), RU160e (REU-N2530W-US), RU130e (REU-N2024W-US) |
| SENSEI residential with pump | RUR199i (REU-NP3237FF-US), RUR160i (REU-NP2530FF-US), RUR199e (REU-NP3237W-US), RUR160e (REU-NP2530W-US) |
| Value (VC) | V53e (REU-VAM1620W-US), V53De (REU-AM1620WD-US), V65i (REU-VC2025FFU-US), V65e (REU-VC2025W-US), V75i (REU-VC2528FFU-US), V75e (REU-VC2528W-US), V94i (REU-VC2837FFU-US), V94Xi (REU-VC2737FFU-US), V94e (REU-VC2837W-US) |
Where this comes from
- Error codes for gas continuous flow water heaters (HW Gas CF ERR) , page 4
- Infinity Enviro 32 / HDC32 operation and installation manual , page 12
- Troubleshooting tankless water heater diagnostic codes (100000652), non-SENSEI and SENSEI series , page 42
- N Series residential condensing installation and operation manual (100000467), RU199e/RU180e/RU160e/RU130e , page 60
Checked against the manufacturer's manual on 9 October 2026.
Other Rinnai tankless water heater codes
| Code | Meaning | What you can do |
|---|---|---|
| 34 | Temperature sensor fault (depends on model) | Call a pro |
| 36 | Lower tank thermistor fault | Call a pro |
| 38 | Exhaust temperature thermistor fault | Call a pro |
| 41 | Frost protection (ambient) sensor fault | Call a pro |
| 51 | Inlet water thermistor fault | Call a pro |
| 52 | Gas modulating valve fault | Call a pro |
| 55 | Service reminder | Safe checks first, then a pro |
| 57 | Burner overheat switch open | Call a pro |
| 58 | Secondary heat exchanger overheat | Call a pro |
| 61 | Combustion fan failure | Call a pro |
| 63 | Recirculation low flow | Call a pro |
| 65 | Water flow control failure | Safe checks first, then a pro |
See all exhaust overheat codes on tankless water heaters across brands.