Rinnai · Tankless Water Heater Critical
16 — Outgoing Water Over-Temperature
Outlet water temperature exceeded the safety limit. Unit shuts down to prevent scalding and exchanger damage.
- Keep using it?
- No — service first
- DIY difficulty
- Moderate
- Verified against
- rinnai.us ↗
How to fix it
- Clean the inlet water filter and confirm full flow.
- Descale the heat exchanger with manufacturer-approved solution.
- Test outlet thermistor resistance per service manual.
- Check for hot/cold crossover at single-handle faucets and the recirc loop check valve.
- Verify gas valve modulates — if stuck, replace valve assembly.
- Replace outlet thermistor if out of spec.
Why it happens
Typical signs: Hot water briefly very hot then cold · Code 16 after high-demand draw · Scale visible at fixtures
- Scale in heat exchanger Insulating scale layer causes localized superheat.
- Low water flow Clogged filter or partly closed valve — heat per unit volume is too high.
- Faulty outlet thermistor Sensor reading high or unstable.
- Gas valve stuck open / over-firing Modulating valve failing to throttle.
- Recirculation crossover Hot/cold crossover causing temperature creep.
Parts you may need
If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.
More Rinnai tankless water heater codes
Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: rinnai.us.