Why Rheem Tankless Water Heater Won’t Turn On [Solved]

Your Rheem tankless heater won’t turn on because of the following leading reasons:

  • Power supply Issue
  • Overloaded Problem 
  • Ignition failure, 
  • A tripped circuit breaker, 
  • Or, crossed its limitations.

Whatever the issues are, you can fix them by repairing the power supply and resetting your unit.

rheem tankless water heater won't turn on

Fortunately, we have figured out the common reasons which prevent your water heater from turning on and outlined their easy solutions.

So, follow our troubleshooting guide, and you will definitely be capable of solving your problems by yourself. So, let’s go ahead…

Why Rheem Tankless Water Heater Won’t Turn On [Reasons + Solutions]

In this section, we will tell you the possible causes of your Rheem tankless water heater not turning on and show you their easy solutions. So, go through the lines.

Note: You can also read how to fix the Rheem water heater beeping problem.

1. Power Supply & Switch Issues

The issues with the power supply and the switch are one of the major culprits which prevent your Rheem tankless water heater from starting up. In fact, you can’t turn on the water heater without electricity. So, it’s not a problem with your water heater, but an issue with the system you’re operating.

power supply & switch issues

Firstly, you won’t turn on the water heater if the electric switch stumbles or it goes into repair mode. Therefore, the heater won’t get any power because of insufficient voltage with the transformer, 3 amp fuses, and PCB fuses, which prevent your water heater from turning on. Hopefully, you will get a code of C8 because of the PC board failure.

Secondly, your water heater will remain shut off if there is an issue with the power switch. So, if the power cord is unplugged, normally your heater won’t turn on until you turn it back on.

Manually, a tankless water heater requires a minimum of time (1-2 minutes) to ignite. Whenever you turn on the power, you may not get the water heater in running mode immediately. In most cases, this isn’t likely an issue with your water heater but it is a problem with the system you are using.

However, you can simply solve the issue.


  • First, don’t forget to plug in the 3-prong wall plug. Make sure the power switch is in the On position. Just wait for 1-2 minutes to ignite the pilot. Basically, the heat exchanger takes a while to heat up. Then the problem will be fixed easily.
  • Next, ensure there is enough voltage and the circuit breaker at the service panel is not tripping. If needed, repair the circuit breaker. Make sure it doesn’t go into maintenance mode. If it goes to repair mode, you can easily go into repair mode with the manual instructions.
  • Just turn off the unit and press down on both the Up and Down arrow keys for a few seconds. Then, you will hear an audible sound like “click”. Again, press the arrow keys up and down on the remote display to get various important messages on what you have to do next to fix the issues.
  • Ensure the black and white wire connections are intact to the electric board. If it is needed, repair the wiring with 3 amp fuses. Don’t forget to measure the voltage on the wiring, transformer, and PCB fuses. It should be 180 to 132 VAC. If it gets damaged, you have to replace the transformer and the blown fuses.
  • Then, check if the electrical switch has stumbled. You can first reset the switch to the OFF position and then keep it in the ON position. Finally, the problems will be solved.

2. Tripped Circuit Breaker

A tripped circuit breaker is another culprit which prevents your Rheem heater from turning on. It may get tripped because of a surge in electricity when the power comes back on. Besides, your heater’s circuit may be damaged due to an overload on the electric metal panel.

tripped circuit breaker

Typically, the electric water heater has a circuit-style safety switch that is located near the thermostat. If it trips, your heater will get too hot, and the high-temperature safety switch will shut off all electric power to the water heater.


First, ensure that the water heater’s circuit breaker in the service panel hasn’t tripped. If it gets tripped, you can simply reset the unit. In order to let your water heater run again, just switch it off at the breaker and then back on at the On position.

3. Heater Crossed Limit

Your Rheem tankless water heater has crossed its limit, which is also another reason for your heater not turning on. The tankless water heater’s mode will be altered frequently whenever it crosses its limitation or capacity.

Then, it will show the error code 00, which means your water heater has been running continuously for 1 or more hours. 

Normally, an electric water heater takes a few minutes to run on as usual. This problem happens whenever multiple people are trying to get either their shower, cleaning dish, or towel on at the same time. This system is also called the “overloaded issue.”

As a result, your water heater can’t turn on if the heater doesn’t get a chance to pump out properly. And, even an overloaded issue can prevent all the systems altogether from turning on the heater.

This is not a serious issue with your Rheem water heater, but it is an issue with the system you are operating. Here are the tips to fix overloaded issues.


  • First of all, check how many people are using the water fixtures. Try using fewer appliances at once. Because your heater might have shut off because of overloading the system.
  • Shut off the hot water taps and remove the circulation pump. Because this unit is designed to run simultaneously with a circulation process.
  • Then, to fix overloading problems, you should reset the unit. If the problem is not solved yet, it will reset itself. Otherwise, you need to manually restart the system. Keep in mind, it doesn’t indicate anything is broken, just that you need to reset your water heater.
  • Again, you may consider replacing the existing water heater with a higher capacity unit as per your demand. Hopefully, the overloaded issue will be solved and your tankless water heater will turn on.

4. Defective Thermocouple

A defective or damaged thermocouple is mostly responsible for your Rheem tankless water heater’s burning issue. This problem may happen when the thermocouple is dirty, bent, or damaged.

Unfortunately, your heater’s thermal switch may trip if the water heater doesn’t get sufficient air. Besides, it may misbehave due to being dirty or clogged by any debris. If it is tripped, you will hear an audible ‘click’.


  • First, find the thermocouple next to the pilot light. Verify whether it has been tripped. If you can’t hear the sound from the thermocouple, then try to light the pilot according to the instructions in the user’s manual for the heater.
  •  Inspect if it has become dirty or defective in any way. After finding any debris, turn off the gas to cool down the unit and then clean it completely.
  • You can now restart your water heater to fix the thermocouple troubles. If your water heater doesn’t start yet, then there is an issue with the heating elements, and you can easily test this issue with an electrical tester.

5. Malfunctioning Control Board

The Rheem tankless water heater’s control board plays a vital role in turning on the heater functionally. If all the connections don’t go to the main burner, it may fail to start your water heater.

Because the board allows power to be supplied to the water heater to turn on as usual. But if the power unit doesn’t go to the main burner, it may prevent you from turning on your water heater. This warns you that the power board may be bad.

Don’t be so frustrated. This is not your heater’s trouble, but the issue is either with your remote control, a bad or faulty wire to the remote control, or the power supply board.

Luckily, whenever you encounter the issue, the display will show you an error code -76 to solve the issue.


  • To fix the issue, disconnect all the wire connections from the remote control and reconnect them directly to the remote terminal of the power circuit board. Then try to restart the unit. If the tips don’t work, replace the remote control of your heater.
  • Next, inspect all the wire connections to the control board if the unit is properly connected to the control board/electrical board.
  • Make sure the grounded power supply and circuit breakers are on.
  • Check the wire connection between the red and black. If it gets loose, reconnect it.
  •  Plus, ensure there is at least a minimum flow rate to fire the unit. Remember, the unit won’t fire off if water is running backward.
  •  If you have a blown fire and don’t have ABC knowledge of repairing it by wiring, call a qualified electrician to fix the problems.

6. Ignition & Flame Failure

The failure of the flame is another common cause of not running your Rheem tankless water heater. This might indicate there are issues with your gas valves, electric wires, and venting system.

Besides, there are also several reasons behind the failure of flame and ignition with the heater. Namely, either the gas supply is too low or the valves for gas or water are closed. Consequently, both faults are mostly responsible for not igniting the flame at all.

And then, your water heater will show an Error Code-10, which signals “flame failure,” meaning that there is a defective flame in the water heater.

Plus, you will have encountered it if the electric components, wiring, or sensors are burned out. Because the burner can’t provide a flame to turn on the water heater. Luckily, the Rheem tankless water heater will show an error code-11 which also indicates your heater has troubles with ignition.

Here, both the error codes hint at or warn you about what to do next to fix the problems.


  • First of all, ensure that there is enough gas in the tank before you turn it on, and don’t forget to pay the gas bill.
  • Inspect that the ignition operation is working well. If it is not a problem, check and open every gas or water valve fully to ignite the flame. Then, make sure nothing is clogging the vents both inside and outside of the heater.
  • Hopefully, ignition and flame failure issues won’t be a major cause for turning on your Rheem tankless water heater.
  • Lastly, unless you are good at working with gas or electric appliances, it is wise to call a professional. He will figure out the problems and solve them quickly and safely.

How To Reset Rheem Tankless Water Heater?

Your Rheem tankless water heater may have some common issues. This doesn’t mean that your water heater has been damaged or defective.

reset the rheem tankless water heater

In this case, at first, you should try to solve the issue by resetting your water heater before taking any steps to change any parts or spending a lot of money. It will help to come back your water heater to the default factory setting. Let’s know how to reset it.

  • First of all, take the electric panel out and locate the water heater circuit.
  •  Next, remove the panel cover and replace it.
  • If the gas valves are disconnected, reconnect them and keep the gas valves in the OFF position.
  • Now, find the reset button behind the insulation or near the thermocouple. (This may be hidden on the side of the metal panel on the unit.)
  • After finding the red reset button, just press and hold down the “Reset” button until it clicks, and then just after 3 seconds, release it. Now, switch on to start.

Fortunately, the most common problems with your Rheem tankless water heater that are not turning on issues will be fixed.


What would be the ideal setting for my Rheem tankless water heater?

Typically, the ideal setting for your Rheem tankless water heater is 120 degrees. Whatever it fully rests on your preference. You can set 125-130 D for heating quickly. The lowest setting is 90 degrees Fahrenheit.

How long does a Rheem tankless water heater take to heat up?

The Rheem tankless water heaters normally take 30–40 minutes to thoroughly heat the whole water. Here, how quickly it will heat up depends on the size.

How do I know whether my tankless water heater is bad?

There are some signs that will tell you your tankless water heater has been faulty. For example, if you get lukewarm water, a sudden drop in pressure, and the heater remains off, leaks and the water heater starts misbehavior, etc.

Final Thoughts

You have already realized why your Rheem tankless water heater won’t turn on and got their simple solutions.

So, follow our troubleshooting guide on Rheem tankless water heater turning-on issues and fix them by yourself.

Yet, if you face any other problems starting up your tankless water heater, let us know by your comments. We will reach out to you ASAP with a probable solution.

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 “Why Rheem Tankless Water Heater Won’t Turn On [Solved]”

    • Hi Dustin

      Your Rheem tankless heater won’t turn on may be due to one or more reasons like the power supply issue, being overloaded, ignition failure, a tripped circuit breaker, or having crossed its limit.

      However, as you smell like burnt electronics, it would be wise to take help from a professional to inspect your unit to fix it.

      Best of luck!


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