Rinnai 16: Over Temperature Warning

Code 16 is Rinnai's early-warning version of an overheat fault — the unit isn't shutting down yet, but the control board has detected conditions that, left alone, could get there.

Applies to: Rinnai tankless and condensing water heaters.

What it means

Over Temperature Warning — the unit detected conditions that could lead to overheating. Think of it as one step before the more serious 14 (Thermal Fuse) fault: the control board is flagging the same underlying stress before an actual safety device has to intervene.

Most common causes

The pattern behind this warning looks a lot like what causes an overheat trip:

Restricted air flow around the unit or vent terminal
Low water flow in a circulating system causing short-cycling
Foreign material in the combustion chamber or exhaust piping
Clogged heat exchanger

What you can safely check yourself

The same two checks that apply to the thermal fuse fault are worth doing here, ideally before it escalates:

Check for anything blocking air flow near the unit or its vent
If a recirculation system is installed, verify it isn't causing very low flow through the unit

When to call a professional

The warning is frequent or the unit's performance has noticeably dropped — the heat exchanger likely needs professional cleaning.

Heat exchanger cleaning and any venting adjustments should be handled by a licensed HVAC technician or plumber.

What the fix costs

Because this is a warning rather than a lockout, many owners let a first occurrence pass and just keep an eye on it — there's no repair cost if it doesn't return. If it keeps showing up, a professional heat exchanger cleaning or flush generally runs $80–$200, and a full diagnostic visit if something more is going on typically lands in the $150–$600 range (Angi, Fixr cost guides). Letting the underlying cause go unaddressed risks it progressing to the 14 fault, where the fix — or a full unit replacement — costs considerably more.

Related codes

All Rinnai codes

Every Rinnai water heater fault code — the full list on one page.

Safety. An occasional 16 warning isn't cause for panic, but don't ignore a pattern. If it keeps showing up, that's the unit telling you it's heading toward an actual overheat lockout — get the heat exchanger and venting checked before it gets there rather than after.

Sources

Rinnai tankless water heater fault code reference and troubleshooting guide (manufacturer documentation)

This site is an independent reference, not a substitute for a licensed plumber or electrician. Gas water heaters can leak carbon monoxide, leak gas, or catch fire if serviced incorrectly. Anything involving the gas line, gas control valve, venting, or main electrical wiring should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.