Rheem Tankless Water Heater Error 16 — Outlet Overheat

What Does This Error Mean?

Rheem tankless Error 16 fires when the hot water outlet thermistor reads above 207°F (97°C).
The unit cuts the gas valve to keep scalding water out of the lines.
The thermistor itself is rarely wrong — Error 16 almost always means real overheating, usually from low flow combined with a high temperature setpoint, scale on the heat exchanger, or recirculation crossover.

Affected Models

Rheem RTGH (Prestige condensing) tankless
Rheem RTG mid-tier tankless
Rheem Performance Platinum tankless

Common Causes

Flow rate just above minimum — unit cycles long enough to spike outlet temp
Scale on the heat exchanger holding heat after burner shuts off
Recirculation loop crossover (cold water mixing into hot)
Bad outlet thermistor (genuinely faulty — uncommon)
Setpoint too high (above 140°F / 60°C) on a low-flow tap

How to Fix It

Lower the setpoint and retest.

On the front panel, drop the temperature to 120°F (49°C).
Run hot water at a high-flow tap (shower or tub).
If Error 16 stops appearing, the previous setpoint was too aggressive for the current flow conditions.
120°F is plenty for most household uses and avoids scald risk.
Check the recirculation loop.

If you have a recirculation system, it can mix cold water back into the hot side via a stuck or backwards check valve.
The unit fires, then sees a high outlet temp because hot water is being held in the line.
Turn off the recirc pump for a day and see if Error 16 goes away.
Flush the heat exchanger.

Same procedure as Error 14 — vinegar flush with isolation valves and a pump.
Scale insulates the inside of the exchanger.
The burner cuts off when the thermistor reads correctly, but residual heat in the scale layer keeps cooking the water for several seconds, spiking the outlet temperature.
Confirm flow at the failing tap.

Some bathroom faucets have aerators that drop flow below the unit's stable range.
Remove the aerator and run the tap full open — if Error 16 stops, flow was the problem.
Replace with an aerator that flows at least 1.5 GPM.
Have a plumber test the thermistor.

If steps above don't fix it, the outlet thermistor itself can be measured with a multimeter — its resistance should drop as temperature rises.
A thermistor stuck at high resistance reports high temperature even when water is normal.
Replacement is straightforward but should be done by someone who can also confirm the heat exchanger isn't the real issue.

When to Call a Professional

If a flush and flow check don't clear Error 16, the heat exchanger may have internal scaling that requires service.
Don't keep cycling power — repeated overheats damage the exchanger.

Frequently Asked Questions

Why does Error 16 happen only when I run a single tap?

Single-tap flow is often near the unit's minimum activation rate.
The burner cycles on, off, on, off — and during the off phase, residual heat in the heat exchanger and pipes keeps raising water temperature.
The thermistor sees the spike and trips Error 16.
A higher constant flow (shower) keeps temperature stable.