Rinnai code 16: water too hot, causes and fixes

Code 16 means the water leaving your Rinnai got far hotter than the temperature you set. Rinnai's diagnostic guide describes it as a high outgoing water temperature shutdown, triggered when outlet water passes 203°F (95°C).

That is a scald risk, so the unit stops heating. It is not a code to keep resetting.

Rinnai makes several generations of tankless heaters (SENSEI, SENSEI RX/CX and older non-SENSEI models). The meaning below comes from Rinnai's own diagnostic code guide; the exact service steps differ slightly between generations.

What causes code 16

• Restricted airflow. A blocked vent terminal or a problem with the combustion fan upsets the burn and the temperature control.
• A stuck check valve behind the fan that restricts air on newer SENSEI models.
• Debris in the combustion chamber or exhaust piping.
• A blocked heat exchanger. Scale inside or buildup between the fins keeps heat from transferring evenly.
• Wrong gas type setting. A unit set for natural gas but running on propane, or the reverse, will not burn correctly.
• A failing gas valve or fan motor, which Rinnai lists as the parts to replace once everything else checks out.

What you can check

1. Look at the vent terminal outside and clear any leaves, nests, snow or ice.
2. Think back to the last time the unit was descaled. If it has been more than a year in a hard water area, scale is a likely suspect.
3. Check the cold water inlet filter; a clogged filter reduces flow through the heat exchanger.

When to call a technician

Fan tests, gas type settings, the gas valve and the heat exchanger all sit inside the sealed unit. If code 16 returns after you clear the vent, have a licensed installer inspect it. If scale is the cause, the fix may be as simple as learning to descale a tankless water heater and doing it on schedule.

See the full list on our Rinnai error codes page.