Rheem Tankless Water Heater Troubleshooting [A Complete Guide]

The most common Rheem Tankless electric water heater problems include:

  • The heater is not turning on.
  • Water is not getting hot enough or no hot water.
  • Too hot water.
  • The water is going cold after a few minutes.
  • The heater keeps turning off.
  • And how to troubleshoot the most common Rheem tankless water heater error codes.

Plus, leaking water and strange sounds coming out from the system are the leading problems that often Rheem tankless users encounter.

rheem tankless electric water heater troubleshooting

While troubleshooting, you need to look for major signs to identify the reasons behind the issue. Then, you have to fix the troubles based on the reasons. But to do so precisely, the need for a well-proven solving guide is obvious. 

Go on reading our extensive Rheem tankless water heater troubleshooting guide to break down each of the problems efficiently.

Rheem Tankless Water Heater Troubleshooting Manual [A Complete Guide]

Here are the details of all major Rheem tankless heater problems, explained with their quick solutions. Let’s start:

1. Heater Won’t Turn On

It is a common phenomenon that you may experience a restricted-energy supply to your water heater. There are a few reasons why your Rheem tankless water heater won’t power on

water heater won't turn on

Either the energy supply is cut off or the circuit breaker has tripped. At times, it could be a simple issue like a blown fuse on the outlet or having any loose wire in the connections. 

If all these are not related in this case, we bet that the inside water flow sensor is not working properly, causing the circuit board not to turn on. Here is the below picture indication of the flow sensor:

electric water heater flow sensor

How To Fix:

Here is all how you will deal when your Rheem tankless electric water heater won’t turn on:

  • First, check the main electrical panel of your house. Look for any obstruction, causing the circuit breaker to trip. Remove it. Then reset the breaker to get back your heater in operation.
  • In case it is a blown fuse, be quicker to replace it. Then check and find if there is any loose wire. Tighten or replace the worn-out one.
  • If all these fail to restart the heater, you need to inspect the heater inside and check the flow sensor. Contact a professional to replace the defective sensor.

2. Water Is Not Getting Hot Enough Or No Hot Water

If your Rheem tankless water heater isn’t not getting the hot water you desire, there’s a good chance that you have set the temperature too low. If not that so, the heater might be reaching the limit.

Basically, there is a limit switch in the heater, which helps to keep the temperature at an appropriate level. 

water is not getting hot enough

Now in case the high limit switch is tripping or wearing out at some point, it will switch off the heater function. Other major reasons why there is a problem in heating the water include:

  • Bad heating element.
  • Tripped circuit breaker.
  • Running too many appliances at once.
  • High water flow rate.
  • Incorrect or loosened wiring.
  • Internal part failure or mixing too much cold water.

No worry! Fix the below well-proven procedures to solve the matter quite easily.

How To Fix:

First, do the easy fix by turning up the temperature on the thermostat. If you already do so, figure out the reason for tripping the high-limit switch. Do a quick reset in this case by turning off the breaker to the unit. 

Besides, make sure the breakers at the main electrical panel are ON and in working order. Then look for any damages or loose wiring going through the unit. Fix that fast.

To ensure the optimal temperature output, there should be a maximum flow rate on your heater.

A tankless water heater activation flow is approximately 0.3 GPM (1.1 LPM). Check the flow rate on your heater through a flow regulator and reduce the level if it is too high. 

Still, there is no hot water, it is time to check the heating elements. De-assemble your heater and find the heating elements as indicated below.

These elements are designed for 240 volts. In case of a lower voltage, they produce less heating power. Replace the bad ones.

Make sure that too much cold water is not mixing with hot water. You may install an anti-scald guide on your faucet to reduce the amount of cold water mixed.

3. Water Is Getting Too Hot

If the outcome water from the water heater is too hot, setting the thermostat too high is the most probable fact that users often make mistakes. But in case that is not so, the water flow rate from the heater is bet too low. 

 water is getting too hot

In most cases, due to a faulty thermostat, or failing heating elements, the heater fails to maintain the temperature at an adequate level. 

Plus, the loosened temperature at the connection point or at times the defective sensor is the culprit. 

How To Fix:

There is nothing to worry about! By following the few quicker steps, you can minimize the high-temperature issue of your water heater.

First off, check the thermostat setting and set the level low if it is too high. Set the thermostat to around 120 F. 

Then, check for any loose and damaged wiring from the heater. Do a quick replacement of the worn-out wiring. Monitor the thermostat as well and ensure it is in well-working order. 

Don’t miss to check the water flow. If it is not at the recommended level, open the valve a bit more to increase the pressure.

If all the above methods have been ruled out, disconnect the front panel and inspect the heating elements. We are pretty much sure that the elements are defective and you need to replace them as soon as possible. 

Look through the temperature sensor as well and ensure it is tightened at its connection. Replace the faulty sensor to stop the heater from overheating. We suggest you contact a professional to perform this task.

4. Heater Goes Cold After A Few Minutes

It is the most often happening issue that your heater water suddenly gets cold after a few minutes.

The fact is known as a cold water sandwich and a few major raps might be counted as the culprit for this issue. A broken flow sensor is the major one of them.  

heater goes cold after a few minutes

A flow sensor is used to measure the water entering your system and through which it sends signals to the unit’s control board.

In the case of a bad sensor, fluctuation in the temperature is obvious as it is failing to sense the water flow. 

Plus plumbing crossover and low Water Pressure are also the most probable reasons why your tankless water heater is running hot and cold.

How To Fix:

First, check if your system is running through low water pressure. If the flow rate is lower than the activation flow rate required, it results in the control board stopping heating the water.

In this case, check if the valve is partially closed, or not open. Head to the valve and open it up all the way. 

Next, remove the front panel and inspect the flow sensor to see whether it is broken. Replace the faulty flow sensor with the assistance of any professional.

Thirdly, plumbing crossover can also cause the issue which happens due to a faulty mixing valve or damaged plumbing. To fix this situation, you need to inspect the mixing valve and change the cartridge. 

5. Heater Keeps Shutting Off

If your electric water heater is failing to stay on, be sure you are encountering a power supply interruption. Either it is a simple loose wire or might be a circuit breaker problem. 

rheem heater keeps shutting off

Plus, the low water flow rate through the unit and the faulty thermostat are the major reasons in this case. Most of the time, it happens due to internal mechanical defects or clogged filter screens. 

The greater chance is the heating element is failing but continuing heating the water even if the thermostat does not direct. This increases the heater temperature which trips the high limit switch, leading to turning off the power.

How To Fix:

Here are some effective ways to diminish the heater trouble that is turning off constantly:

  • First off, check all the wiring connections going through your heater and fix the loose ones. If it is a circuit breaker issue, reset the breaker.
  • Then monitor that your heater is ensuring the minimum water flow requirements. If not, fix the water flow issue.
  • Examine the thermostat function to see whether it is in good operation or not. Replace the bad thermostat. 
  • There is a built-in filter screen in your heater at the inlet connection which needs to be cleaned.
  • Inspect the heating elements. Call a professional if you are suspecting bad elements. 

6. Water Heater Leaking

Your tankless water heater may leak for any of the following reasons:

water heater leaking
  • Sediment or mineral buildup inside the system from hard water.
  • Blocked pipes.
  • Weak pipe connections.
  • Too-high water flow.
  • Bad water conditions.
  • Improper installation of the heater.

How To Fix:

First off, check for any bad pipe connection. All tankless water heaters are equipped with a relief valve that is more prone to leakages. Inspect the pipes jointing, fix the loosened one, and ensure that the valve is well tightened. 

Secondly, examine the inlet and outlet pipes for any blockages. Clean the pipes properly.

Next, check the water pH whether it is too much or in level. Because more acidic water creates pinholes in your piping and causes leaks. As a solution, you can install an acid neutralizer on your household plumbing system. 

Then, ensure that the flow water rate of your unit is at the recommended level. If it is too high, reduce the flow by adjusting the valve. Must drain or flush the system if there is any sedimentation build-up. 

7. Heater Is Making Noise

If you are hearing a strange noise while starting and heating the water, be sure your heater is going through some major malfunctions. Most often, it is likely related to heater overheating due to sediment build-up on the heating elements. 

 rheem tankless water heater is making noise

While heating, the heater pipes come in contact with hard water minerals like calcium and magnesium. After a long time, these minerals form into hard particles and clog the pipes.

Whatever the fact is, you need to descale or flush your tankless water heater to reduce the annoying sound.

How To Flush Rheem Tankless Water Heater

Follow the below step-by-step procedures to flush out your tankless water heater:

Step 1: First off, shut off the electrical power of your heater and close the water shut-off valve as well. Open the faucet to drain the remaining water from the heater. Then close the hot water shut-off valve.

Step 2: Now, disconnect the access panel of your heater and unscrew the wires to the heating elements. This time, you have to disassemble the heating elements. Just turn on the hexagonal brass top in a counter-clockwise direction. 

Step 3: Place the heating elements in the copper chambers. Now, fill the chamber with the solution of vinegar and water to remove the scale. Avoid chemical solutions of any kind. Let it sit for about 1-2 hours to break up the scale on the elements.

Step 4: Drain the cleaning solution and install the heating elements back in place. Secure all the wires of the elements. 

Step 5: Now remove the cold water inlet filter from the inlet connection. Clean the filter as well under a cold tap and replace it securely.

Step 6: Once all are done, turn on the hot & cold water shut-off valves. Place the access panel and turn on the power supply.

Scale Build-Up Prevention Tips

  • We recommend flushing your water heater once or twice a year if you are living in hard water areas. 
  • It will be better to install a water softener or water treatment device to maintain the optimal performance of your water heater. 
  • You should always be aware of the water condition that is running through your plumbing system. It should meet the below recommendation: 
TDSUp to 500 mg/L
CO2Up to 500 mg/L
Total hardnessUp to 200 mg/L
Aluminum0.05-0.2 mg/L
ChlorideUp to 250 mg/L
CopperUp to 1.0 mg/L
IronUp to 0.3 mg/L

Most Common Rheem Tankless Water Heater Error Codes Troubleshooting

All Rheem tankless water heaters have a built-in electronic diagnostic system. When the heater goes through various malfunctions, it displays an error in the LED display or on the control panel in the form of a code.

Resetting the heater is an effective rebooting process of the system to clear the code. It will help to get back the heater into normal operation after rectifying the errors.

If any case, you are unable to fix your error code even after resetting the system, you have to figure out the exact indication of the error. Then, you have to find out the possible reasons behind the error along with the solution.

Let’s track down some major error codes of the Rheem tankless water heater and how to troubleshoot the problem efficiently:

1. Error Code 00

Rheem tankless water heater “00 error code” on the display indicates that your tankless water heater has been running continuously for over an hour. 

How To Fix:

Reset the unit following any of the above two methods. Also, if there have a circulation pump installed with the heater, remove or turn it off as well.  

2. Error Code 10

Error code 10 on the front panel signifies that the flame has been lost about eight times.

How To Fix:

First off, reset the unit to clear the code. If still the error persists, here are there few things that you have to fix:

  • Monitor the gas pressure of your tankless water heater while in operation. 
  • Ensure a secure connection of the gas valve wire and flame detection sensor. Also, check for proper connection of the igniter transformer.
  • Clean the spark igniter with steel wool or replace it if required.
  • If still there has the same error, replace the PCB.

3. Error Code 11

If you see Rheem tankless water heater with an error code 11, be sure your water heater has failed the ignition process about 10 times.

Simply resetting the heater will clear the error. If not, follow the below-fixing steps:

How To Fix:

First off, ensure the gas pressure of the system is s between 5.0″ W.C and 13.0″ W.C (1.25 to 3.24 kPa). Also, make sure that the gas valve connection is secured enough.

ensure the gas pressure of the system

Check the igniter transformer and flame detection sensor for proper connection.

Sand the spark igniter using steel wool or sandpaper to remove oxides. Replace the damaged igniter.

4. Error Code 20

Error code 20 on the front panel is a clear indication that the high limit overheat switch is closed at normal and open at fault.

How To Fix:

  • Reset the water heater to clear the error.
  • If that fails, inspect the high-limit switch and ensure the proper connection. 
  • Measure the resistance of the switch. Replace the switch if the resistance reading shows zero.

5. Error Code 21

Error code 21 on the Rheem Tankless water heater is the error indicating that the heat exchanger outlet temperature is too low. This is mainly the result of a blocked heat exchanger. Other major reasons include:

  • Blocked air intake filter.
  • Clog in the neutralizer.
  • Blocked condensate drainage system.
  • Defective PCB

How To Fix:

As for the first step, you need to descale the carbon buildup from the heat exchanger. To do so, you can flush and drain your water heater. Besides, check the continuity of the heat exchanger thermistor. It should give a reading between 0.8 to 20k ohm.

Clean the clogged air intake filter. Also, inspect the neutralizer for any corrosion. Replace it if needed. In most cases, the error appears due to the blockage in the condensate drainage system that needs cleaning. Wondering how to do so? Head to our detailed guide on how to deal with Rheem tankless water heater error code 29

6. Error Code 29

Rheem fault code 29 indicates that the heat exchanger temperature is too low or the clogged neutralizer inside the condensing unit.

How To Fix:

Perform the below simple steps to resolve this issue:

Step 1: Clean out the heat exchanger fins. Unclog the air inlet screen as well.

Step 2: Ensure the secured connection of the APS/Condensate and the main controller. Inspect the hose connection to the APS/Condensate.

ensure the secured connection of the APS or condensate

Step 3: Clean out the blockages from the condensate line and termination.

Step 4: Replace the condensate neutralizer.

7. Error Code 41

Error code signifies to users that the fan speed is too high with the flame on. Unlike others, try to clear the code by resetting the unit. In case it’s not proving well, go through the below solution procedure:

How To Fix:

  • Clean the blockages from the vent connections.
  • Inspect the burner assembly and ensure it is in good condition.
  • Next, check fan operation. If you find too low or too high RPM from the fan, you need to replace it. 

8. Error Code 43

When the burner overheat switch opens, the heater shows the error code 43 on the front display. The error indicates that the system needs to be reset. If still there is the same code, follow the below section to find further troubleshooting:

How To Fix:

  • Inspect the burner overheat switch connections and ensure all are secured.
  • Besides, measure the resistance of the switch as well. Replace the switch if it shows the reading at zero. 

9. Error Code P1

Rheem tankless water heater error code P1 mainly signs that the water flow of the unit is too low. Clogged air inlet filters, bad flow sensors, sediment deposits in the heater, and clogged aerators are some of the leading reasons for this code. Also, if you are using an undersized water heater, it leads to the P1 error code.

How To Fix:

  • As for the first step, inspect and clean out the aerators of the faucets. Also, clean the water filter screen.
  • Inspect the water flow sensor for any defection. Replace if needed.
  • Look for corrosion in your house’s pipeline.
  • If you’re suspecting sediment deposits inside the heater, you’ll need to flush your water heater.
  • Also, make sure that you’re not using an undersized tankless water heater. If you are confused about what is the right size water heater based on your household hot water demands, you can take a look at our detailed discussion through this guide on how to fix the Rheem tankless water P1 error code

How You Can Reset Your Rheem Electric Tankless Water Heater

As you already know, resetting your Rheem tankless water will fix most of the error codes popping up in the heater display. Here is the straightforward method and steps to reset your Rheem tankless water heater:

Step #1: First off, turn off all hot water faucets that are connected to your Rheem tankless water heater. 

Step #2: Now shut off the power supply to your tankless system. Switch off the unit from the control panel. 

Step #3: Then, disconnect the cord from the power connection. 

Step #4: Leave the unit unplugged for about 10-15 minutes and then again plug it back into the power connection. The reset is done.

Alternatively, there is another simple method. To learn that method, you can read our comprehensive guide on why and how to reset your Rheem tankless water heater.


What is system overload in a tankless water heater?

When too many appliances are running simultaneously through a single heater, it exceeds the heater’s capacity. If this happens, the heater stops heating water and leads to system failure. This situation is known as system overload.

Do electric tankless water heaters need to be vented?

In a short answer: No! The electric tankless heater uses electric burners to heat water. As there is no chance of fuel burning, that is why it doesn’t require venting, unlike a gas or propane heater.

Wrapping Up!

Tankless electric heaters are highly efficient in heating water on demand. It’s also quite common that several problems may arise while operating that stop the heater from functioning in a mannered way. 

That’s the point users get in trouble and start looking for reasons for that issue. The case is also the same for the Rheem model. That’s why we have covered the Rheem tankless electric water heater troubleshooting guide. 

Hope that now you can fix any of your Rheem tankless heater issues with ease. Plus, don’t neglect the regular maintenance of your Rheem tankless water. It will help you to face fewer issues. 

Last but not least. Don’t hesitate to share if you have any more queries in the comment box. We will be happy to help you.

About William

William is the founder of Fireplacehubs.com. He has real life practical skills in fixing smoker & heating appliance issues. He loves to share his knowledge & helps others in fixing their appliances & saving their money. William firmly believes that anyone can repair his or her unit with the correct guidance & knowledge. See more at about us.

2 thoughts on “Rheem Tankless Water Heater Troubleshooting [A Complete Guide]”

  1. I have a Rheem tankless water heater model Rtex -27 I have had it for a couple years working great but now it turns on and immediately turns off would this be a flow sensor problem or can it be simply be reset


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