Rinnai Tankless Error Code 58: What a Secondary Heat Exchanger Overheat Fault Means for East Cooper Homes
If your Rinnai tankless flashes Code 58 in Mount Pleasant, Daniel Island, or somewhere else in East Cooper, the heater is telling you a safety switch on the secondary heat exchanger has opened — and around Charleston that almost always points back to scale.
Code 58 means one of the two bi-metal switches on the secondary heat exchanger has opened. Those switches sit on the rear of the secondary exchanger where the air box meets the heat exchanger. Rinnai treats this as a hard fault: the code can show up whether the unit is firing or sitting idle, and it will not clear until the underlying problem is fixed and you do a full power cycle. This is not a soft warning you can ignore while showers still run.
I'm Christian with Avery & Sons Plumbing + Tankless. Here's a plain walkthrough for neighbors — what Code 58 means, what you can safely check, and when to call.
What the unit is doing
Condensing Rinnai units pull extra heat from the exhaust through a secondary heat exchanger for better efficiency and cooler vent temps. That cooler secondary side is where mineral scale likes to settle in Charleston / East Cooper hard water.
When scale coats those passages, heat transfer suffers, local hot spots trip a bi-metal switch, and the board flashes Code 58. Same hard-water story as Codes LC, 14, and 16 — different sensor, same root cause in a lot of Lowcountry homes.
Common causes we see around here
Scale in the secondary heat exchanger. The big one for Daniel Island, Mount Pleasant, and the rest of East Cooper. Minerals coat the secondary passages, restrict heat transfer, and trip the bi-metal. An overdue flush is often the real fix, not just swapping a switch.
Failed or open bi-metal switch. Age, heat stress, or wiring damage can open the switch even when scale is modest. A tech ohms the circuit and replaces it if it stays open after a flush and cool-down.
Loose or damaged wiring. A cracked, nicked, or damp harness near the secondary exchanger can look like an open switch to the board.
Skipping the hard reset after a real repair. Fix the problem first, then power-cycle. Clearing the display alone just brings Code 58 back.
Safe checks you can do yourself
Keep it simple. If you smell gas, leave and call your gas utility — do not troubleshoot.
Write down what the display shows. Code 58 alone, or stacked with LC, 14, or 16? Running or idle?
Clean the inlet water filter. Shut the cold isolation valve, relieve pressure at a faucet, pull the inlet screen, and rinse debris. Not Code 58 by itself, but restricted flow makes scale worse.
Check the condensate drain on a condensing unit. A blocked drain is another sign it needs a real service look.
Try one hard reset after cool-down. Off at the controller, breaker for 30 seconds, power back on. If Code 58 returns, stop resetting.
Do not pull the secondary exchanger or bi-metal switches yourself. That means isolating water and gas, draining, and resealing correctly.
Do not keep power-cycling a Code 58 hoping it goes away. An open secondary overheat switch is the heater protecting itself from damage you do not want to finish.
When to call a pro
Call if Code 58 will not clear after one hard reset, if you also see LC, 14, or 16, if water temperature feels off, or if the unit has not had a descaling flush in a long time.
A real service call will check the bi-metal harness, confirm whether a switch is open, flush the secondary heat exchanger per Rinnai’s procedure, and treat hard water so the code does not return. Around Charleston, Daniel Island, Mount Pleasant, and East Cooper, we see Code 58 most often from scale in the secondary exchanger — the same mineral load behind so many of our tankless calls.
Soft next step
If your Rinnai is throwing Code 58, clean the inlet screen, try one hard reset after the unit cools, and get eyes on it if the code sticks. Avery & Sons Plumbing + Tankless can help. Call (843) 305-5555 or visit averysplumbing.com. We're local, we work these heaters every week, and we'll tell you what it needs without the scare tactics.
Stay comfortable — Code 58 is the heater protecting the secondary heat exchanger, not a mystery you have to live with.
Comments