Rinnai · Tankless Water Heater Critical
90 — Combustion Abnormality
The unit detected abnormal combustion — usually incomplete combustion or wrong flame characteristics. Safety lockout.
- Keep using it?
- No — service first
- DIY difficulty
- Hard
- Verified against
- rinnai.us ↗
How to fix it
- STOP using unit and call a qualified gas tech — this is a combustion safety code.
- Verify gas type matches unit dataplate.
- Verify dip switches for gas and altitude.
- Inspect intake and exhaust for blockage; clear as needed.
- Have inlet gas pressure measured static and dynamic.
- Inspect burner and heat exchanger for soot; clean or replace as required.
- Replace fan motor if RPM is out of spec.
Why it happens
Typical signs: Code 90 after running · Yellow flame or sooting visible · Sometimes accompanied by burning smell
- Restricted vent or intake Partial blockage starving combustion.
- Incorrect gas type / orifice Unit set up for NG running on LP or vice versa.
- Wrong gas pressure Inlet pressure too high or too low.
- Heat exchanger fouling / sooting Soot blocking combustion path.
- Failed combustion fan Wrong air/fuel ratio.
- Dip-switch / dial set wrong Wrong altitude or gas type setting.
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.