Rheem Tankless Water Heater Error Code 33: Meaning and Fix

Rheem tankless water heater error code 33 typically indicates an exhaust temperature high limit fault, requiring you to inspect the venting system for blockages or restrictions.

Table of Contents

What does error code 33 mean on a Rheem tankless water heater?
What usually causes error code 33 to trigger?
How can I safely perform quick fixes for error code 33?
What next steps should I take if the initial fixes fail?
Which fix matches your specific error 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 Rheem tankless water heater?

Error code 33 on a Rheem tankless water heater indicates an excessive exhaust temperature, which triggers the high limit switch to protect the unit.

When combustion gases become too hot or cannot escape efficiently, the internal safety sensors trip to prevent permanent damage to the heat exchanger. This fault often points to venting issues, improper combustion air supply, or a malfunctioning sensor.

The exhaust temperature sensor has detected unsafe operating heat levels inside the venting pathway.
Exhaust gas blockages can trap high heat near the top of the tankless water heater unit.
Improper vent pipe installation or restricted air intake can cause this specific high limit fault.
Check your specific owner manual as exact error definitions can vary by production year.
Confirm the fault behavior by noting if the unit locks out immediately upon starting up.

What usually causes error code 33 to trigger?

This error is typically caused by blocked venting, improper pipe sizing, or inadequate combustion air supply in the installation area.

Tankless water heaters rely heavily on unobstructed airflow for safe and efficient operation. When airflow is restricted, internal temperatures rise rapidly, forcing the unit to shut down for safety.

Debris, nests, or ice buildup obstructing the exterior intake or exhaust vent termination cap.
Vent pipe lengths exceeding the maximum manufacturer recommendations for your specific model.
Incorrect vent material usage that fails to dissipate or withstand high exhaust temperatures properly.
Poor indoor ventilation depriving the combustion chamber of sufficient fresh air for burning.
A faulty or degraded high limit exhaust temperature sensor reporting false high readings.

How can I safely perform quick fixes for error code 33?

You can safely troubleshoot the issue by checking power supplies, clearing vent blockages, and resetting the water heater unit.

Before touching any internal components, always turn off the electrical breaker to the water heater. Never attempt to service gas valves or internal sealed parts without proper licensing.

Turn off the power switch on the water heater and shut off the main circuit breaker.
Inspect the exterior intake and exhaust vent termination caps for leaves, nests, or ice.
Verify that all indoor vent pipe connections remain securely fitted and free of separation.
Power the unit back on at the breaker and reset the system using the digital controller.
Run a hot water tap nearby to check if the error code 33 returns immediately.

What next steps should I take if the initial fixes fail?

If clearing the vents and resetting fails, inspect internal wiring harnesses and test the exhaust thermistor resistance.

Persistent high limit faults often require deeper diagnostic checks on internal electrical connections. Ensure the power remains disconnected during any inspection of internal wiring components.

Unplug the appliance or switch off the dedicated circuit breaker before opening the front cover.
Inspect the wiring harness connected to the exhaust temperature sensor for secure terminals.
Check for any signs of soot buildup or physical damage around the combustion chamber area.
Measure the sensor resistance with a multimeter if you have technical repair experience.
Reassemble the front panel securely and restore power to test system operation once more.

Which fix matches your specific error symptom?

Match your immediate symptom to the corresponding corrective action to quickly identify the necessary repair procedure.

Different manifestations of error code 33 can point toward distinct underlying causes, ranging from simple exterior vent blockages to internal component sensor failures.

Symptom	Likely cause / fix
Code appears during high hot water demand	Clear exterior vent blockage or clean air intake.
Code appears immediately upon startup	Check for disconnected wiring or faulty exhaust sensor.
Code persists after a full system reset	Verify vent pipe length and professional installation specs.
Intermittent code during windy weather	Install a certified termination hood resistant to backdrafts.

When should you stop troubleshooting and call a professional?

Stop troubleshooting immediately and call a licensed technician if you smell gas, observe vent damage, or face electrical issues.

Gas appliances carry inherent safety risks involving combustion and carbon monoxide. Never attempt repairs on gas valves, gas lines, or damaged venting systems on your own.

Any detection of a gas leak or strange odor requires shutting off the gas supply and calling a professional.
Cracked, separated, or improperly sealed exhaust venting must be repaired by a certified technician.
If internal electrical control boards show burn marks or component failure, call support.
When error code 33 persists despite clear vents and functional sensors, professional diagnosis is required.
Never modify gas combustion settings or internal gas manifold components without official certification.

What information should you have ready before calling for service?

Prepare your exact appliance model number, serial number, warranty status, and a list of troubleshooting steps already tried.

Having accurate information ready helps technical support or local technicians diagnose your Rheem tankless water heater faster and determine parts availability.

Locate the rating plate label on the side of the water heater cabinet for the model and serial numbers.
Note the exact purchase date or installation date to determine if your unit remains under active warranty.
Write down the specific error code number and the exact conditions when the fault occurs.
List all troubleshooting steps you already performed, such as checking vents and resetting power.
Have your home address and contact details ready for dispatching a qualified service technician.

How can you keep error code 33 from coming back?

Prevent future occurrences by scheduling annual maintenance, keeping outdoor vents clear, and monitoring combustion airflow.

Routine preventive care ensures your tankless water heater operates safely and efficiently throughout its lifespan, minimizing unexpected shutdowns.

Schedule an annual professional inspection and flush of your tankless water heater system.
Inspect outdoor intake and exhaust vent terminals monthly for debris, leaves, or seasonal snow buildup.
Ensure surrounding utility room areas remain free of dust, lint, and chemical vapors that block airflow.
Monitor hot water performance regularly to catch minor operational anomalies before faults trigger.
Review manufacturer guidelines periodically to stay updated on recommended maintenance schedules.

Frequently Asked Questions

Can a blocked intake vent also trigger error code 33?

Yes, a blocked intake vent restricts the necessary oxygen supply for proper combustion. This restriction causes combustion inefficiencies and elevated temperatures, which can trip the exhaust high limit sensor and display error code 33.

What should I do if error code 33 clears temporarily but returns later?

If the code returns after a reset, it usually indicates a recurring environmental or mechanical problem like partial vent blockage or a failing sensor. Turn off the unit and have a certified technician inspect the venting system and internal sensors.

Is it safe to keep using the water heater when error code 33 is active?

No, it is not safe or possible to use continuous hot water while the error is active. The water heater locks out combustion operations intentionally to prevent overheating hazards until the underlying temperature fault is fully resolved.

Does error code 33 mean my heat exchanger is damaged?

Not necessarily. Error code 33 is a protective safety feature designed to prevent heat exchanger damage by shutting down the unit before extreme temperatures occur. However, ignoring repeated faults can eventually cause long-term component wear.

Can high winds outside cause my Rheem water heater to show error code 33?

Severe wind gusts can create backdrafts or pressure imbalances in improper vent terminations, trapping hot exhaust gases inside. If this happens frequently, consult a professional regarding certified wind-resistant termination hoods.

Bottom Line

Always disconnect power before inspecting your water heater, and verify your venting system is clear. Call a licensed professional if the error persists.

Sources

Rheem owner’s manual
Rheem installation guide
Manufacturer support page