Rinnai Tankless Error Code 16: What an Over Temperature Warning Means for East Cooper Homes
The unit got too hot and threw a warning. Here's what Code 16 means for Charleston, Mount Pleasant, and Daniel Island homes — and what you can safely check.
If your Rinnai tankless flashes Code 16 and hot water just quit in Mount Pleasant, Daniel Island, or somewhere else in East Cooper — here's the short version.
Code 16 means over temperature warning. The heater sensed outlet water or heat-exchanger temperature climbing higher than it should. Rinnai shuts down on purpose so the unit doesn't keep cooking itself. Not a random glitch. Different from Code 14 (thermal fuse / overheat lockout) — 14 is a high-limit safety that opened. Code 16 is the board saying "I'm running too hot." Same neighborhood of heat problems. Different code.
I'm Christian with Avery's Plumbing. We work these units all over Charleston, and Code 16 shows up often enough — scaled exchangers, tight recirculation loops, blocked vents — that a plain-English walkthrough helps.
What the unit is doing
When a Rinnai runs, combustion heat transfers through the heat exchanger into the water. Sensors watch how hot that path gets. On Code 16, those readings say temperatures are past the safe window. The board stops the cycle.
Rinnai's own troubleshooting for Code 16 points at: restricted airflow around the unit or vent terminal, low circulating-system flow (short-cycling), foreign material in the combustion chamber or exhaust, and a clogged heat exchanger. Fair. It's a real heat warning — not something to reset forever and ignore.
If you're comparing codes: we already covered Code 10 (air supply / exhaust blockage), Code 11 (no ignition), Code 12 (flame failure), and Code 14 (thermal fuse / overheat) for East Cooper homes. Venting and scale can feed into several of those. Code 16 names over-temperature directly.
Common causes we see around here
Clogged heat exchanger / scale. Hard water and skipped flushes let minerals build up. Scale acts like insulation — heat stays in the metal, outlet temps spike, and you get an over-temp warning. Charleston-area water is tough on units that never get descaled.
Restricted airflow or venting. Leaves, nests, crushed pipe, poor clearances, or exhaust recirculating into the intake hold heat in the cabinet. Same story we see with Code 10 around Daniel Island and Mount Pleasant after storms — and it can show up as Code 16 when the unit simply runs too hot.
Low flow / short-cycling on recirculation. Tiny continuous flow keeps the burner bouncing. Temperatures climb the wrong way. A clogged inlet filter or an aggressive recirculation pump plays into this. Recirc loops are common in newer Mount Pleasant and Daniel Island builds — great for comfort, fussy when flow is wrong.
Debris in the combustion path. Foreign material in the chamber or exhaust messes with combustion and heat transfer. Coastal humidity around Charleston and East Cooper doesn't help outdoor or garage installs either. That's inspection territory — not weekend DIY.
Safe checks you can do yourself
Keep it simple. If you smell gas, leave the area and call your gas utility first — don't troubleshoot.
Power cycle once after it cools. Off at the controller (or breaker/unplug for a few minutes), let it cool, back on, open a hot tap. If Code 16 returns under load, the over-temp condition is still real.
Check the outdoor vent termination. Clear leaves, nests, bags, or obvious blockage. Don't shove tools deep into the pipe. Don't open the burner box.
If you have recirculation, note what it's doing. Was the loop running? Did the code hit on a tiny trickle of flow? That pattern matters.
Note the bigger pattern. Only after long showers? Years with no flush? Right after landscaping near the vent? Write it down for the tech.
Do not bypass temperature sensors or open the heat exchanger yourself. Gas appliances aren't where you get curious.
When to call a pro
Call if Code 16 comes back after a cool-down reset, if hot water keeps cutting out, if you have a recirculation system you don't want to guess on, if you're under warranty, or if anything smells or sounds off. A real diagnosis checks scale, vent path, circulating flow, inlet filter, combustion/exhaust condition, and sensor readings against Rinnai's specs — not a reset-button guess.
Around Charleston, Daniel Island, Mount Pleasant, and the rest of East Cooper, hard-water scale and recirculation/flow issues are the two we see most on Code 16. Storm-related vent junk is a close third. Better to fix the root cause once than keep cooking the same exchanger.
Soft next step
If your Rinnai is throwing Code 16 and you'd rather not guess, Avery's Plumbing can take a look. Call (843) 305-5555 or visit averysplumbing.com. We're local, we work these units every week, and we'll tell you straight what it needs.
Stay safe — and don't ignore a code that exists because the unit got too hot.
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