Rinnai code 21: data transfer error

Rinnai SENSEI heaters store their setup, such as gas type, altitude, model size and recirculation mode, as parameters on the control board. When a board is replaced, those settings are copied to the new one with a transfer cable. Code 21 means that did not work as intended.

The meaning and steps below come from Rinnai's own diagnostic code guide for its tankless water heaters.

Rinnai lists this code for its SENSEI series.

What a technician does

1. Confirms the data transfer from the old board to the new one actually completed.
2. Checks that all parameters are set correctly for the specific model.
3. Reviews the transfer instructions supplied with the new board.
4. If the transfer failed, programs all 14 parameters manually using the board replacement instructions.

Why the settings matter

Wrong parameters can cause other codes and unsafe operation. For example, a wrong gas type or altitude setting affects combustion, and a wrong recirculation setting can trigger code 63. That is why Rinnai does not want a heater running on a board with incomplete settings.

What homeowners can do

If code 21 appeared right after a board replacement, call the technician who did the work. It is part of finishing that repair.

Before you call

Have the model and serial number from the rating label ready, and note whether your unit is a SENSEI or an older non-SENSEI model. Tell the technician the exact code, when it appears and any recent service or parts replacement. That often saves a second visit.

See every code on our Rinnai error codes page.