Rheem Tankless Water Heater Code 29

You wake up from the bed, go to your washroom, and turn on the hot water faucet. But you don’t get any hot water like other days. Needless to say; how uncomfortable it is if you get no hot water heater on an ice-cold day in winter

That’s why you go to your Rheem tankless water heater, check the control board, and see it flashes the error code 29. Unluckily, you don’t know the meaning of this error message and what causes the water heater to throw the error message 29. 

Don’t worry! This guide will help you to clear the error code from the display and allow your water heater to operate smoothly. So, let’s dive in. 

Table of Contents

Rheem Tankless Water Heater Code 29: Why Does It Pop Up?

Error code 29 on Rheem Tankless Water Heater means the heat exchanger outlet temperature is too low. Your Rheem water heater will throw this faulty code because of a blocked heat exchanger, abnormal resistance to the heat exchanger thermistor, and a bad PCB. 

Below, I will break down the reasons behind this water heater issue.  

01. Blocked Heat Exchanger

The heat exchanger should be free from scale build-up for normal water heater operation. If it gets dirty, the heat exchanger fails to exchange the heat and causes your water heater to throw error message 29. 

Now, you can ask- how can I determine if the heat exchanger gets dirty? Well, your water heater will display maintenance information ‘4y’ on the remote control. Now, verify the heat exchanger temperature to see whether the measured temperature matches that indicated by 4y or not. 

If yes, understand the heat exchange gets blocked or dirty. 

How To Fix: 

Start with inspecting the heat exchanger for restriction. If there is any blockage, remove them. I suggest you watch this tutorial if you don’t know how to clean the heat exchanger: 

Also, make sure the air inlet is clear and free of debris. 

02. Abnormal Resistance To Heat Exchanger Thermistor

A healthy heat exchanger thermistor should have a resistance between 4k to 10k Ohms. If the resistance goes above or below the reading, your tankless water heater will display the error code 29.  

How To Fix: 

Without testing the heat exchanger thermistor electrically, you can’t say anything about the resistance of this component. 

Grab a multimeter and set it to the Ohm setting. Now, attach the probes of your multimeter to the connector R between Black#3 and Yellow#5 wires. It should give a reading between 4k to 10K Ohms. 

If you don’t get this reading, I bet the heat exchanger thermistor is at fault. In other words, you should replace this water heater component to clear the flashing code 29. 

03. Faulty PCB

A defective PCB or Printer Circuit Board can also be the culprit behind this water heater issue. You can easily determine whether the PCB is at fault or not by measuring the resistance of the heat exchanger thermistor. 

If the resistance of the heat exchanger thermistor is normal (4k to 10K ohms) but the water heater shows the error code 29, I assure you that the PCB is bad. 

How To Fix: 

Replacing the PCB is the only option to solve this problem. Follow the below steps to learn how to replace PCB on a Rheem tankless water heater. 

  • Turn off the power, gas, and water supply. 
  • Pull out the front panel by unscrewing the four screws from the top and the bottom of the front panel. 
  • Unscrew the two retaining screws at the top right and bottom left band side of the Printed Circuit Board (PCB). 
  • Detach the multi-pin connectors from the PCB assembly. 
  • Remove the PCB assembly by gently lifting the right-hand side of the PCB. 
  • Now, install a new PCB assembly and put everything back into its place to complete the installation. 


Your Rheem water heater will display the error code 29 due to a blocked heat exchanger, a bad PCH, and when the resistance to the heat exchanger goes out of range. 

Once you fix those problems, you can easily solve the water heater issue. Fortunately, I mentioned every troubleshooting step you should follow to fix those culprits. So, follow the steps we preferred. 

Of course, don’t forget to reset your Rheem water heater once you deal with those issues. It will help you eliminate the error code and let your water heater run smoothly. 

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