Rinnai Code 55: Combustion Abnormality Detected

Quick Answer

Rinnai Code 55 indicates abnormal combustion, usually caused by a vent restriction. To fix it, inspect the exhaust and intake pipes for debris or blockages to ensure proper airflow.

A code 55 often points to environmental factors rather than internal component failure. In my experience, this error usually triggers when the unit cannot breathe properly due to snow, debris, or improper venting installation. Addressing these external airflow issues is the first step to restoring stable combustion and preventing system lockouts.

RinnaiWaterheaterSeverity: highDifficulty: intermediate68% DIY Success

Time to Fix

30–120 min

Difficulty

intermediate

Parts Cost

$15 – $70

Tools Needed

Flashlight, Multimeter

What Does the 55 Code Mean?

Think of Code 55 as the heater telling you its lungs are congested. In the field, I find this error pops up most often after a big storm or a seasonal change when gas pressures fluctuate. It's a sensitive code designed to prevent carbon monoxide buildup, so treat it as a signal to verify the air-to-fuel mixture is perfect before swapping any parts.

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?

To clear the code, press the Power button on the controller to turn the unit off, wait about 30 seconds, and then turn it back on. But if the underlying venting or gas issue is still there, the internal sensors will trip the 55 again within minutes of the burner firing up. Don't just keep resetting it, you've got to find the blockage first.

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.

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

Frequently Asked Questions

Is Rinnai Code 55 dangerous?

Yes, it is. Code 55 is a safety shutdown because the heater knows it isn't burning fuel cleanly. Incomplete combustion creates carbon monoxide, which is odorless and deadly. If you smell a rotten egg gas odor or a sharp, pungent burnt smell, shut the gas off immediately. The unit's doing its job by locking out to protect your family, so don't attempt to bypass the sensors or keep resetting it hoping it just goes away on its own.

Can paint or household chemicals cause Rinnai Code 55?

Absolutely. I've seen Code 55 triggered by someone painting their basement or even using heavy-duty floor wax near the heater. The intake pulls those chemical vapors into the burner, where they change the chemistry of the flame enough to confuse the sensor. If you're doing renovations, pipe in fresh air from outside or keep the area extremely well ventilated. Seriously, I've had customers cause this code three times in a week before they figured out their paint fumes were the culprit.

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

This is a classic Rinnai quirk and I get calls about it every winter. If your vent terminates on the windward side of the house, a strong gust can literally blow the flame around inside the combustion chamber. If it's only happening during storms, you probably need a Rinnai-approved wind shroud for your vent cap. It's a simple fix that prevents the wind from choking the exhaust and causing a nuisance trip. Way cheaper than a service call.

How do I clean a fouled Rinnai burner?

Be very gentle here. I use a soft nylon brush or even a vacuum with a brush attachment to pull debris out. What you're looking for are those little white flakes, which is actually aluminum oxide from the heat exchanger. If the burner is packed with that stuff, you need to flush the heat exchanger too, or the debris will just fall back down and trigger the code again. Don't use anything abrasive and don't poke at the ports with a wire or anything metal.

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

Sometimes a glitch is just a gulp of air in the gas line or a bird flying too close to the intake. If it happened once and hasn't come back in a month, I honestly wouldn't lose sleep over it. But if it happens twice in a week, the sensor's telling you a story. Start by checking your vent termination for any temporary obstructions like leaves or plastic bags that might've blown away before you noticed them.

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

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 14, 2026