Rheem Rtex-18 Troubleshooting [Complete Guide]

Rheem Rtex-18 Troubleshooting [Complete Guide]

This guide on Rheem RTEX-18 Troubleshooting will break down: 

  • 3 common Water Heater Problems and Their Solutions
  • Rheem RTEX-18 Error Codes
  • Troubleshooting steps for Rheem RTEX-18
  • How to Reset Rheem Electric Tankless Water Heater
  • Maintenance Tips for Rheem RTEX-18
  • FAQs

Note: This is an electric tankless water heater. If you have a gas unit, read this article on the Rheem Tankless Water Heater Troubleshooting Manual.

Table of Contents

Rheem Rtex-18 Troubleshooting: Common Problems & Solutions

Here are the common problems you might experience: 

1. Water Heater Isn’t Heating At All

Your electric tankless water heater won’t heat the water due to one of these reasons: 

  • The breakers at the main electric panel are off
  • Defective breaker
  • Wire the water heater incorrectly 
  • Internal part failure 

How To Fix: 

1. First, go to the main electric panel and ensure the circuit breakers for the water heater are on. If flipping the breakers to the On position doesn’t bring any luck, replace the breaker. 

2. Second, ensure you wire your water heater according to the wiring diagram in the manufacturer’s manual: 

3. If you aren’t electrically inclined, don’t hesitate to call a certified electrician to inspect the wiring and correct it. 

4. Third, a failure of the internal water part can also cause the unit to produce no hot water. I recommend you call a certified technician to resolve this problem for you. 

2. Insufficient Water Temperature

Several reasons are responsible for the tankless water heater to produce insufficient hot water, including: 

  • The set temperature is too low
  • The flow rate is too high
  • Crossed wires
  • Voltage less than 240 volts
  • Mixing too much cold water

How To Fix: 

To get sufficient hot water from your electric tankless water heater, follow these troubleshooting steps: 

1. Raise the temperature setting by pressing the Up button on the controller. Or, turn the control knob clockwise to increase the set output water temperature.

2. If the flow rate exceeds the physical heating capacity of the unit, the water heater will heat the water insufficiently. Install a flow meter and reduce the water flow by adjusting it. Check the following pictures, which will help you choose the flow regulator that will be best for your water heater. 

3. Hire a certified electrician to double-check the wiring.

4. The heating elements of your water heater require 240 volts to heat the water. If you use those elements with a lower voltage, the heating elements will produce less heating power. In this case, upgrade the heater to a larger input. 

5. Don’t mix as much cold water with your tankless water heater compared to your conventional tank water heater. 

3. Water Temperature At The Faucet Is Less Or Greater Than The Set Temperature

You will experience this issue with your water heater if: 

  • The unit may receive insufficient electric supply
  • Anti-scald feature gets activated
  • Thermal loss
  • The water temperature at the faucet is too hot

How To Fix: 

Follow these troubleshooting steps to solve this issue: 

  • Ensure you get sufficient voltage at the unit to let the heating elements heat the water. Or, you can increase the water temperature to compensate for this. 
  • Some faucets have an anti-scald feature that automatically mixes cold water even when you pull the control knob to full hot. These devices are adjustable. So, turn off the cold mixes completely to avoid this problem. 
  • If the water travels through the pipe a long way, you will get less hot water than the set temperature. It’s normal. Just set the temperature a bit high. 
  • The water temperature will be too high if the water flow is low, the set point temperature is too high, or the internal part is at fault. To resolve this issue, reduce the set-point temperature and increase the water flow. Lastly, call a certified technician if there is an issue with an internal part. 

Rheem RTEX-18 Error Codes

Here is the list of error codes, their meanings, and steps you can follow to reset them.

Error CodeMeanings/ProblemSolutions
E1Problem with inlet water temperature sensorCheck the wiring to the sensor for loose connections and tighten them up if needed. Replace the sensor 
E2Issue With Outlet Water Temperature SensorInspect the wiring connections to the heating element and secure the wire connections if needed. Replace the heating element if it gets cracked or corrodedEnsure the voltage your water heater receives is within the recommended range. 
E3Defective Heating ElementEnsure you ventilate the water heater adequately and there is no obstruction around the unreleased blockage from the exhaust or air inletsDon’t set the temperature too high
E5High Temperature Cut-offEnsure you ventilate the water heater adequately and there is no obstruction around the unreleased blockage from the exhaust or air inletsDon’t set the temperature too high
E7Communication ErrorSecure the loose connections on the control boardEnsure all the wire connections are tight and secureIf the problem still persists, call a certified service personnel. 

Troubleshooting Steps for Rheem RTEX-18

Go along with these steps to troubleshoot your RTEX-18 Electric Tankless Water Heater like a pro: 

  • Take your eyes on the controller display for error codes. When the tankless unit goes through a problem, it will throw an error code on the display. Fortunately, I already described the common error messages of Rheem RTEX18 along with their solutions. 
  • Verify the unit is receiving sufficient electric supply. Also, turn on the circuit breaker for the water heater to the On position. 
  • Verify the water supply to the unit is consistent and that the water flow or pressure is within specifications. 
  • Ensure the inlet and outlet hoses are free of obstructions. 
  • Inspect every piping connection and fitting for leaks.
  • Clean the inlet filter if it gets dirty or replace it to maintain optimal water flow.
  • Adjust the temperature setting so that it doesn’t exceed the safety level. 
  • Maintain proper ventilation around the unit. Remove any blockages from the air intake and exhaust vents. 
  • Reset the unit because it will help you resolve minor issues. You will learn the resetting procedure in the next chapter. 
  • Contact Rheem’s Customer Support for additional troubleshooting guidance. 

How To Reset Rheem (RTEX-18) Electric Tankless Water Heater?

If you experience a minor problem with your tankless water heater, resetting the unit might bring luck to you. The following steps will help you reset the unit successfully: 

  • Turn off the unit at the circuit breaker.
  • Leave the unit for several minutes. This waiting period will reset the internal components and clear the minor glitches. 
  • Restore the power to the unit by turning on the circuit breaker for the water heater. 

Maintenance Tips for Rheem RTEX-18

I recommend you follow the maintenance tips below to maximize the performance and efficiency of your Rheem RTEX-18. 

Check For Water Leaks

Inspect the connections on the inlet and outlet of the unit annually for any signs of damage, cracks, and leakage. Address those issues and resolve them. 

Flush The Unit

Flush your tankless water heater once a year or every six months if the water contains excessive minerals. Flushing the unit will help remove sediment and mineral buildups and maintain optimal performance for years. 

Clean the Inlet Filter 

Inspect the inlet filter periodically and clean the inlet filter by removing dirt and debris. 


1. What should I do if my RTEX-18 is displaying an error code?

Error codes on the controller display indicate a specific problem with the water heater. This electric tankless water heater will throw faulty codes like E1, E2, and E3. Each error code has a different meaning and should follow specific troubleshooting steps to resolve them. Above, I mentioned each error code’s meaning and how you reset the faulty message. 

2. Why is my RTEX-18 not producing hot water?

Your electric tankless water heater won’t produce hot water if there is a problem with the power supply, water supply, or blockage in the inlet filter. Resolve these issues to get warm water from your water heater. 

3. What do I do if there is a water leak from my RTEX-18?

Locate the leakage point on your water heater first. Generally, loose connections and damaged pipes are the most suspicious water-leaking areas. So, check every connection and tighten them up. Apart from this, replace the damaged pipes. 

4. How often should I clean the inlet filter on Rheem RTEX?

The frequency of cleaning the inlet filter will depend on the water quality of your area. If you live in a hard water area, you should clean the filter every six months. Otherwise, you should clean the inlet filter annually to maintain the optimal water flow. 

When You Should Contact Rheem Support Team

Rheem RTEX-18 is a reliable and efficient water heater. But you will experience issues like insufficient hot water over time. 

Fortunately,  I broke down every common problem. Follow the troubleshooting steps I mentioned and get your water heater to function like a champ. 

However, don’t hesitate to seek professional help by contacting a certified service guy or Rheem Customer Support. 

Good Luck!

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