90 Critical Hard — consider a tech

Rinnai Tankless Water Heater 90 — Combustion Abnormality

Rinnai Tankless Water Heater · Error code 90

What 90 Means

The unit detected abnormal combustion — usually incomplete combustion or wrong flame characteristics. Safety lockout.

Symptoms

  • Code 90 after running
  • Yellow flame or sooting visible
  • Sometimes accompanied by burning smell

Common Causes

1
Restricted vent or intake

Partial blockage starving combustion.

2
Incorrect gas type / orifice

Unit set up for NG running on LP or vice versa.

3
Wrong gas pressure

Inlet pressure too high or too low.

4
Heat exchanger fouling / sooting

Soot blocking combustion path.

5
Failed combustion fan

Wrong air/fuel ratio.

6
Dip-switch / dial set wrong

Wrong altitude or gas type setting.

How to Fix It

  1. STOP using unit and call a qualified gas tech — this is a combustion safety code.
  2. Verify gas type matches unit dataplate.
  3. Verify dip switches for gas and altitude.
  4. Inspect intake and exhaust for blockage; clear as needed.
  5. Have inlet gas pressure measured static and dynamic.
  6. Inspect burner and heat exchanger for soot; clean or replace as required.
  7. Replace fan motor if RPM is out of spec.

More Rinnai Tankless Water Heater Codes

02 02 — Power Interruption / Memory Loss 03 03 — Loss of Communication / EZConnect Issue 10 10 — Air Supply or Exhaust Blockage 11 11 — No Ignition 12 12 — Flame Failure 14 14 — Thermal Fuse / Overheat Cutoff

All Rinnai codes → All tankless water heater codes →

Verified against manufacturer documentation: rinnai.us

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician.