Rinnai code 58: secondary heat exchanger sensor

Condensing tankless heaters have a second heat exchanger that captures extra heat from the exhaust. Rinnai fits bi-metal safety switches where the air box meets it. Code 58 means one of them opened.

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

Rinnai lists this code for its non-SENSEI series.

What it usually means

Rinnai's guide is direct: code 58 indicates scale building up inside the secondary heat exchanger. Scale keeps heat from moving into the water, the area overheats and the switch opens.

The fix

1. A technician checks the wiring and confirms the switch circuit reads correctly.
2. The heat exchanger is flushed following the owner's manual.
3. Power is switched off and back on to clear the code.

Why a normal reset may not clear it

Rinnai explains that this code stays on even when the heater is idle. It clears only after the cause is fixed and the power to the unit is switched off and back on, which Rinnai calls a hard reset.

Stopping it from coming back

Rinnai warns that hard water must be treated to prevent scale and heat exchanger damage. Flush on schedule, and in hard water areas consider a scale-reduction filter or water softener. Our guide to descale a tankless water heater walks through the job.

Before you call

Have the model and serial number from the rating label ready, along with the exact code and when it appears: at startup, during use, or only at high demand. Mention any recent work on the gas, water or electrical supply and when the heater was last descaled. This helps the technician bring the right parts and often saves a second visit.

See every code on our Rinnai error codes page.