Rinnai Error Code 33 — What It Means
Rinnai error code 33 indicates exhaust overheat — the flue exhaust temperature exceeded safe limits as measured by the exhaust thermistor. Rinnai tankless water heaters monitor outgoing flue gas temperature to ensure the venting system isn’t overheating. Code 33 trips when the exhaust temperature rises above approximately 230°F (110°C), which can indicate a combustion problem, blocked exhaust, or heat exchanger scale buildup that’s concentrating heat in the exhaust rather than transferring it to the water.
Common Causes
- Scale buildup in the heat exchanger — Calcium deposits reduce water-side heat transfer, so combustion heat passes into the flue rather than the water, raising exhaust temperature.
- Blocked exhaust vent — A partially blocked flue pipe restricts gas flow, causing exhaust heat to back up.
- Combustion air restriction — Insufficient fresh air for combustion forces the unit to run rich, generating hotter-than-normal exhaust gases.
- Faulty exhaust thermistor — The sensor drifts high or develops an intermittent fault, reporting falsely elevated temperatures.
Step-by-Step Fix
- Inspect the exhaust vent termination — Go to where the vent exits the building. Confirm the terminal cap is unobstructed (no bird nests, ice, or debris). For concentric vents, check both the exhaust and air intake openings.
- Check combustion air supply — If the unit is in a closet or equipment room, confirm there is adequate makeup air per the installation manual. A small, sealed room can starve combustion air.
- Descale the heat exchanger — Connect a descaling pump and circulate a citric acid or white vinegar solution through the heat exchanger service ports for 30–60 minutes. Flush thoroughly with water after descaling.
- Clean the burner assembly — A dirty burner produces uneven combustion. Inspect and clean the burner per the Rinnai service manual; this typically requires removing the burner assembly.
- Test the exhaust thermistor — With power off, disconnect the exhaust sensor and measure resistance. Compare to Rinnai’s sensor specification. Out-of-spec = replace the sensor.
- Reset the unit — Press the on/off button twice or cut power at the disconnect for 30 seconds. Restore and run a hot water draw to verify code 33 is cleared.
Parts Often Needed
| Part | Notes |
|---|---|
| Exhaust temperature thermistor | Rinnai model-specific; verify part number for your series (V, RU, or RL) |
| Descaling kit | Pump + citric acid; available from Rinnai dealers |
| Vent terminal cap | Replace if damaged or blocked by animal activity |
Need Replacement Parts?
Finding the right replacement parts for your equipment is critical to getting it running again. Check current prices on Amazon — prices and availability change frequently.
When to Call a Pro
If descaling and vent inspection don’t resolve code 33, have a technician measure actual combustion air/fuel ratio (CO₂ or O₂ levels) with a combustion analyzer. Persistent exhaust overtemperature with clean heat exchanger can indicate a gas pressure or valve modulation problem requiring licensed service.