Rheem Tankless Water Heater Error Code 45 Reset & Fix

Rheem tankless water heater error code 45 indicates water overheating or a tripped limit switch, which usually requires clearing water flow blockages before performing a reset.

Table of Contents

What does error code 45 mean on a Rheem tankless water heater?
Why is my Rheem tankless water heater displaying error 45?
How do you reset a Rheem tankless water heater error code 45?
What secondary steps should you take if error 45 returns?
Which troubleshooting action matches your specific error 45 symptom?
When should you stop and call a professional technician?
Before You Call
How can you prevent error 45 from coming back?

What does error code 45 mean on a Rheem tankless water heater?

Error code 45 indicates that the unit has detected an overheating condition or a tripped high-limit safety switch.

When the heat exchanger temperature exceeds safe operating thresholds, the system shuts down combustion to prevent thermal damage. On most Rheem tankless models, code 45 signals that water inside the unit became excessively hot or water flow was severely restricted while heating.

Signals an overheating temperature condition inside the main heat exchanger
Trips the thermal safety cut-off switch to stop gas burner operation
Protects internal plumbing components from warping or cracking due to heat
Triggers automatically when water flow drops below safe operational minimum levels
May vary slightly by model year so consult your manual to confirm

Why is my Rheem tankless water heater displaying error 45?

Error 45 is most frequently caused by mineral scale buildup, clogged inlet water filters, or faulty temperature sensors.

Heat exchanger tubing requires steady water flow to absorb and carry away heat efficiently. Restricted water movement causes local water temperatures to spike rapidly, tripping the high-limit control switch.

Heavy mineral scale accumulation inside the copper or stainless heat exchanger coils
Debris clogging the cold water inlet filter screen restricting incoming water flow
Partially closed cold water supply valve or low household water pressure issues
Faulty thermistor sensor sending incorrect high-temperature readings to control board
Extremely high gas pressure or improper venting causing excessive heat retention

How do you reset a Rheem tankless water heater error code 45?

You can clear error 45 by cycling system power after allowing the heat exchanger to cool completely.

Performing a hard power cycle resets the electronic control board and clears historical error codes. Never attempt a reset while the unit is still excessively hot to prevent immediate re-tripping.

Turn off hot water faucets and allow the water heater to cool for 20 minutes.
Unplug the water heater power cord or switch off the dedicated circuit breaker.
Wait at least two minutes to fully discharge residual electrical power from control circuits.
Plug the unit back into the outlet or flip the circuit breaker back on.
Turn on a hot water fixture to test for normal ignition and operation.

What secondary steps should you take if error 45 returns?

If error 45 reappears after resetting, you must clean the water filter and flush mineral scale deposits.

Simply clearing the digital code without addressing restricted water flow will cause the thermal limit to trip repeatedly. Routine maintenance restores proper water circulation through the heating chamber.

Shut off cold water inlet valve and main gas supply line valve completely.
Unplug the power cord to prevent electric shock before servicing internal components.
Remove and thoroughly rinse the cold water inlet filter screen under clean tap water.
Perform a full vinegar flush using a submersible pump to remove heat exchanger scale.
Reinstall cleaned filter, open water valves, plug in unit, and test hot water flow.

Which troubleshooting action matches your specific error 45 symptom?

Comparing your unit’s symptoms helps pinpoint whether you face a simple filter clog or scale accumulation.

Different operational behaviors indicate specific root causes for code 45. Use this quick reference matrix to select the most appropriate repair approach.

Symptom	Likely cause / fix
Error appears instantly on hot water request	Clean cold water inlet filter screen
Error appears after running water several minutes	Flush heat exchanger to remove scale buildup
Code stays on display after power cycle	Check limit switch wire connections or thermistors
Water feels lukewarm before code triggers	Check incoming water flow rate and pressure

When should you stop and call a professional technician?

Stop troubleshooting immediately if you smell gas, spot water leaks, or error 45 recurs after flushing.

Gas supply systems and internal electrical wiring carry severe safety risks when handled without proper training. Recognizing when DIY repair is unsafe protects your home and keeps your warranty intact.

Smell of natural gas or propane near the water heater unit
Persistent error code 45 immediately following a thorough heat exchanger flush
Visible water leakage coming from internal piping or heat exchanger casing welds
Burnt wiring odor or visible scorching on the main electronic control board
Unusual banging or rumbling sounds coming from the burner combustion chamber

Before You Call

Gathering key equipment information beforehand helps the service technician diagnose and source parts faster.

Having details ready saves time and ensures a smooth service call. Write down the specifics off the rating plate attached to the side or bottom of your tankless unit.

Exact model number and full serial number located on side rating label
Age of installation and current warranty coverage documentation if available
List of specific troubleshooting steps and flushing attempts you already completed
Whether error code 45 occurs continuously or intermittently during water use
Local water hardness levels or recent plumbing changes in your home

How can you prevent error 45 from coming back?

Prevent future overheat errors by maintaining clean water filters and performing yearly descaling flushes.

Consistent preventive care prevents hard water minerals from insulating heat exchanger walls. Proper maintenance preserves water heater efficiency and significantly extends the lifespan of internal heating elements.

Flush heat exchanger with food-grade white vinegar every 12 months regularly
Inspect and clean the cold water inlet filter mesh every six months
Install a whole-home water softener if local water hardness exceeds moderate levels
Maintain adequate clearance around outside intake vents and exhaust piping terminals
Schedule annual professional inspection of gas burner assembly and vent system

Frequently Asked Questions

Can I bypass the thermal switch to clear error code 45 temporarily?

No, you should never bypass or jumper safety switches on a gas appliance. The thermal limit switch prevents heat exchanger melting and catastrophic fire hazards. Bypassing safety components can lead to severe equipment damage or unsafe operating conditions. Always resolve the underlying overheating cause or replace a faulty switch with an OEM component.

Will resetting error 45 damage my Rheem water heater?

Resetting the unit once or twice to clear a transient code will not cause damage. However, repeatedly clearing code 45 without fixing restricted water flow causes continuous heat stress on the heat exchanger. This severe thermal cycling can crack internal copper lines over time. Always check flow filters and descale the system if the error returns.

Why does Rheem error 45 happen only when running a bath?

Filling a bathtub demands high, sustained water flow over an extended period. If your heat exchanger has moderate mineral scaling, sustained operation causes heat to build up faster than water can carry it away. Shorter uses like handwashing may not generate enough continuous heat to trip the limit switch. A thorough vinegar flush usually resolves this issue.

Is Rheem tankless error code 45 covered under manufacturer warranty?

Warranty coverage depends on the root cause of the error. Component failures like a defective thermistor or limit switch are generally covered during the parts warranty period. However, error 45 caused by lime scale accumulation or neglected water filter maintenance is considered a maintenance issue and is typically not covered by factory warranties.

What if code 45 remains on the display while water stays completely cold?

If error 45 displays immediately when power is connected without the burner firing, the high-limit switch or wiring circuit may be locked in an open state. This happens after extreme overheat events where manual-reset limit switches trip. A qualified technician must inspect internal wiring, test switch continuity with a multimeter, and replace damaged safety sensors.

Bottom Line

To resolve Rheem tankless error 45, start by disconnecting power and inspecting your cold water inlet filter for debris. If clearing the filter and cycling power does not resolve the code, perform a full descaling flush on the heat exchanger.

Sources

Rheem Tankless Gas Water Heater Owner’s Manual
Rheem Commercial & Residential Service & Technical Support
Rheem Tankless Troubleshooting & Error Code Reference Guide