Rinnai Code 55: Combustion Abnormality Detected

Quick Answer

Rinnai Code 55 indicates the unit detected abnormal combustion conditions. The combustion process was outside expected parameters: too lean, too rich, or unstable. Vent restriction is the most common cause because it prevents exhaust from clearing the combustion chamber properly, causing recirculation of combustion gases. Low or high gas pressure shifts the air-to-fuel ratio outside the acceptable range. Burner fouling from scale, oils, or combustion deposits can change the flame characteristics. A combustion sensor fault is less common but can trigger false Code 55 readings. The vent termination cap and full vent run should be inspected before any internal components.

RinnaiWaterheaterSeverity: highDifficulty: intermediate68% DIY Success

Time to Fix

30–120 min

Difficulty

intermediate

Parts Cost

$15 – $70

Tools Needed

Flashlight, Multimeter

Wrong code?

Not seeing 55 on your display?

Select what your appliance is actually doing and we'll take you to the right guide.

combustion airflow abnormality

NavienE110

What Does the 55 Code Mean?

Combustion was detected as abnormal by the unit's sensors. Check vent restriction first, then gas pressure, then the burner assembly for fouling.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Vent restriction partially blocking exhaust gas clearance40%

Gas supply pressure too low or too high, causing improper air-fuel ratio24%

Burner surface fouled with oil residue or combustion deposits14%

Combustion quality sensor fault generating false abnormality readings12%

Combustion air supply blocked or insufficient for indoor installation10%

Symptoms You May Notice

Code 55 on controller display
Unit starts but shuts down during operation
Yellow or orange flame visible through sight glass if equipped
Slight odor of unburned gas or combustion products near the unit
Unit operates normally on some calls but shows Code 55 on others

Can you reset a Rinnai waterheater to clear the 55 code?

After addressing the identified cause, press the power button off and on to reset Code 55. If the code was caused by a vent blockage that has been cleared, the unit should operate normally on the next heating demand. If Code 55 returns within 24 hours, the root cause has not been fully resolved.

Tools Required for Diagnosis

FlashlightMultimeterManometer (for gas pressure)Soft brush (for burner cleaning)Screwdriver set

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

Go to the exterior of the building and visually inspect the vent

Go to the exterior of the building and visually inspect the vent termination cap. Look for physical blockages: bird nests, ice formation, debris, crushed pipe, or a termination cap that has been damaged or faces into prevailing wind without a wind guard.

Trace the entire vent pipe run from the unit to the termination

Verify gas supply pressure with a manometer at the unit's test

With power and gas off, remove the burner access panel

If all physical components check out

Replacement Parts

If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.

Part Name	OEM Number	Estimated Price
Vent Termination Wind GuardVaries by vent diameter · $15–$40	Varies by vent diameter	$15 – $40
Combustion Quality SensorContact Rinnai with model number · $30–$70	Contact Rinnai with model number	$30 – $70

Verify part fitment for your Rinnai model

Repair vs. Replace Calculator

Appliance Age (Years)7 Yrs

Labor Approach

Our Verdict

REPAIR

Estimated Cost: $43 (Parts) vs $1250 (New Machine)

Frequently Asked Questions

Is Rinnai Code 55 dangerous?

Code 55 warrants prompt attention because abnormal combustion can generate higher-than-normal carbon monoxide levels. The unit has shut down as a safety measure, which is correct behavior. Do not attempt to bypass or repeatedly reset the unit without investigating the cause. Ensure the installation area has working carbon monoxide detectors and adequate ventilation while troubleshooting.

Can paint or household chemicals cause Rinnai Code 55?

Yes. Rinnai units installed in utility rooms, garages, or near laundry areas can draw combustion air that contains paint fumes, aerosol propellants, cleaning chemicals, or fabric softener vapors. These compounds can disrupt the air-fuel ratio, cause incomplete combustion, and trigger Code 55. Always provide clean, uncontaminated air to the combustion air inlet. Direct-vent models that draw air from outside are not affected by indoor air quality.

My Rinnai shows Code 55 only when the wind is blowing hard. What should I do?

Wind-induced backpressure at the vent termination is a known trigger for combustion abnormality codes on installations where the vent cap faces prevailing winds. Install a wind guard on the termination cap to reduce backpressure. If the vent terminates on a wall that consistently faces strong wind, relocating the termination or extending it to a less exposed location may be necessary.

How do I clean a fouled Rinnai burner?

With gas off, power off, and the burner panel removed, use a soft-bristle brush to gently clean deposit buildup from the burner surface. Do not use metal brushes, abrasive pads, or compressed air directly at the burner ports as this can damage the perforations. If scale flakes from the heat exchanger are landing on the burner, the heat exchanger descaling should happen first to stop the source of the contamination.

Code 55 appeared once and has not returned. Do I need to do anything?

A single isolated occurrence of Code 55 is worth monitoring but may have been a transient condition: a momentary pressure fluctuation, a brief vent cap obstruction from a leaf or plastic bag that cleared itself, or unusual weather conditions. Inspect the vent termination and confirm gas pressure is normal. If Code 55 does not return after 1 to 2 weeks of normal use, the trigger was likely transient.

Related Rinnai Waterheater Error Codes

Code: HUBRinnai Tankless Water Heater Error Codes: Complete Guide Code: 63Rinnai Code 63: Combustion Fan Error Code: 61Rinnai Code 61: Combustion Fan Motor Failure Code: RESETHow to Reset a Rinnai Tankless Water Heater: All Models Guide

Same Fix on Other Brands

Navien waterheaterE110: Navien E110 Error Code: Exhaust Blockage and Venting Fault Explained

Models Known to Experience 55 Errors

This repair applies to most Rinnai waterheaters with this error code. Common model numbers include:

RUR98iN, RU199iN, RU98eN, V65iN, RL75iN, RU160iP

Last verified for technical accuracy on March 14, 2026