Rinnai Tankless Water Heater Error Code 33: Meaning and Fix

Rinnai tankless water heater error code 33 typically points to an abnormal exhaust temperature or a high limit switch trip, often caused by improper venting or insufficient airflow.

Table of Contents

What does error code 33 mean on a Rinnai tankless water heater?
What usually causes error code 33 to appear?
What are the quick fixes to try first?
What should you do if the initial fixes fail?
Which fix matches your specific symptom?
When should you stop troubleshooting and call a professional?
What information should you have ready before calling for service?
How can you keep error code 33 from coming back?

What does error code 33 mean on a Rinnai tankless water heater?

Error code 33 on a Rinnai tankless water heater indicates an abnormal temperature reading from the heat exchanger thermistor or a high limit switch activation.

This alert is designed to protect your unit from overheating and potential component damage. It triggers when internal temperatures exceed safe operating limits, signaling that heat is not escaping properly through the exhaust system or that the water flow is restricted.

The heat exchanger thermistor measures excessively high exhaust temperatures during operation.
The high limit switch automatically opens to cut power and prevent severe internal damage.
Blocked intake or exhaust vent pipes trap hot combustion gases inside the unit.
Mineral scale buildup inside the heat exchanger reduces heat transfer efficiency significantly.
Gas pressure issues can create an over-firing condition that overheats the internal components.

What usually causes error code 33 to appear?

This error is most commonly caused by blocked intake or exhaust venting, heavy scale accumulation, or failing sensors.

Understanding the root cause helps you determine whether you can resolve the issue safely at home or if you need specialized service. Because this involves combustion and exhaust, environmental factors around your home play a major role.

Debris, nests, or ice blocking the outdoor vent termination cap and restricting airflow.
Venting runs that exceed maximum allowable lengths or feature too many sharp elbows.
Severe hard water scaling inside the heat exchanger restricting proper water flow.
A faulty thermistor or high limit switch sending incorrect high-temperature readings.
Incorrect gas pressure settings causing the burner to run too hot.

What are the quick fixes to try first?

Start by power cycling the unit, inspecting the outdoor vents for blockages, and ensuring adequate water flow.

Before performing any checks, always unplug the water heater or turn off the dedicated circuit breaker to prevent electrical shock. Carefully review your owner manual to locate components safely before attempting any basic checks.

Turn off the power switch on the unit and unplug the power cord from the wall outlet.
Go outside and inspect the intake and exhaust vent terminals for leaves, nests, or ice blockages.
Clear any visible obstructions away from the vent pipes to restore proper airflow immediately.
Open a nearby hot water faucet fully to verify that water is flowing without significant restriction.
Plug the unit back in, turn the power switch on, and test operation by running hot water.

What should you do if the initial fixes fail?

If the error persists, check for internal scale buildup and verify that your venting installation complies with manufacturer specifications.

Persistent error code 33 instances usually require a more thorough technical inspection. Mineral deposits inside the heat exchanger can trap heat, while improper vent sizing or loose connections can cause dangerous recirculation of exhaust gases.

Disconnect power to the water heater before opening the front cabinet panel.
Inspect the internal wiring harnesses connected to the thermistor and high limit switch for loose plugs.
Flush the heat exchanger using a compatible descaling kit if hard water scaling is suspected.
Inspect all interior vent pipe joints to ensure they are securely connected and properly sealed.
Measure the total length and configuration of your venting system against manual guidelines.

Which fix matches your specific symptom?

Use this reference table to match specific operational symptoms with their most likely causes and recommended corrections.

Different triggers require distinct corrective actions to clear error code 33 safely. Review the symptoms below to determine the appropriate next step for your water heater installation.

Symptom	Likely cause / fix
Error appears immediately on startup	Faulty thermistor sensor / Replace sensor if resistance fails
Error appears during high hot water demand	Restricted water flow or scaling / Flush heat exchanger
Error appears on windy or stormy days	Vent cap obstruction or downdraft / Clear cap and check termination
Code returns after resetting power	High limit switch tripped / Verify airflow and call professional

When should you stop troubleshooting and call a professional?

Stop immediately and call a licensed technician if you smell gas, notice damaged venting, or if resetting the unit fails.

Gas appliances require specialized training and combustion analyzers to service safely. Tampering with gas valves, sealed combustion chambers, or venting systems can create serious carbon monoxide hazards in your home.

Stop working immediately if you detect any smell of natural gas or propane near the unit.
Call a professional if the exhaust venting pipe is cracked, separated, or improperly installed.
Contact a technician if the error code returns immediately after a complete power reset.
Engage certified service if internal gas pressure adjustments or burner repairs are required.

What information should you have ready before calling for service?

Gather your exact model number, serial number, purchase date, and a summary of troubleshooting steps you already tried.

Having these details ready helps customer support or service technicians diagnose error code 33 much faster. You can typically find this required information on the metal rating label attached to the side of your water heater cabinet.

Locate the complete model number and serial number printed on the cabinet rating plate.
Check your warranty status and purchase documentation for coverage verification.
Write down the exact sequence of troubleshooting steps and resets you attempted.
Note whether the error occurs randomly or during specific household water usage.

How can you keep error code 33 from coming back?

Prevent future occurrences by scheduling annual maintenance, flushing the heat exchanger, and keeping vent terminals clear.

Routine preventive care protects your tankless water heater from premature component failure and efficiency loss. Maintaining clear airflow and clean internal waterways ensures safe, reliable hot water delivery year-round.

Schedule an annual professional service check to inspect burners, venting, and gas connections.
Flush the heat exchanger every twelve to twenty-four months, especially in hard water regions.
Inspect outdoor intake and exhaust vent terminals monthly for leaves, snow, or spider webs.
Monitor water flow rates and clean the cold water inlet filter screen regularly.

Frequently Asked Questions

Can I simply press the power button to clear error code 33 permanently?

Resetting the unit via the power button will only clear the code temporarily if the underlying overheating condition remains unresolved. If the heat exchanger is still too hot or venting is blocked, the safety switch will trip again and the error will return.

Is it safe to operate my Rinnai tankless water heater with error code 33 active?

No, you should not continue operating the appliance while this error is active. Error code 33 indicates a thermal protection shutdown, meaning the unit is locking out burner operation to prevent dangerous overheating and potential damage.

What tools do I need to inspect the venting for error code 33?

Basic visual inspection requires only a flashlight to check outdoor termination caps and interior pipe joints. Never attempt to disassemble rigid vent piping without proper training, as improper reassembly can cause dangerous exhaust leaks.

How often should I flush my tankless water heater to prevent temperature errors?

You should flush your tankless water heater at least once a year, or every six months if your home has hard water with high mineral content. Regular descaling prevents mineral buildup from insulating the heat exchanger and triggering high-limit errors.

What should I do if error code 33 appears alongside other error codes?

When multiple error codes appear together, note down every code displayed on the digital controller before cutting power. Multiple codes often point to a broader supply issue, such as combined gas pressure and airflow restrictions, requiring professional diagnosis.

Bottom Line

Always turn off the power breaker before inspecting your unit, and verify outdoor vent terminals are clear of any obstructions.

Sources

Rinnai Operation Manual
Rinnai Installation Guide
Manufacturer Support Page