Rheem Tankless Water Heater Troubleshooting Manual [Complete Guide]

This is a complete guide on Rheem Tankless Water Heater Troubleshooting Manual. In this article, we will break down the most common problems every homeowner encounters with their Rheem water heaters. They are:

  • Rheem Water Heater Not Turning On
  • Rheem Tankless Water Heater Blows Fuses
  • Rheem Tankless Water Heater Shuts Off During Shower
  • Tankless Water Heater Low Flow Problem
  • Rheem Tankless Water Heater Not Getting Hot
  • Rheem Tankless Water Heater Not Working After Power Outage
  • Rheem Tankless Water Heater No Power To Remote

Without further ado, let’s dive deeper and explore the solutions to those problems. 

Table of Contents

Rheem Tankless Water Heater Troubleshooting Manual [Here’s What To Check]

In this chapter, we will disclose every problem you may encounter over time with your Rheem Tankless Water Heater. 

01. Rheem Water Heater Not Turning On

Your Rheem water heater will not turn on if there is no power. If you forget to plug the power cord into the 3-prong wall plug, there will be no power. In plain words, you can’t turn on the water heater without any electrical power. 

Secondly, you will also fail to power your water heater if there is no voltage at the wall plug. Keep in mind; it’s not a water heater problem, it’s an electrical issue. 

You can’t turn on the water heater if it goes into maintenance mode. Therefore, no voltage or power at the 3 amp fuses, transformer, and PCB fuses are some culprits that prevent the water heater from turning on. 

How To Fix:

Start with plugging the power cord into a proper and grounded 3-prong wall plug. Make sure there is a voltage at the wall plug and the circuit breaker doesn’t get tripped. If needed, repair the circuit breaker. 

Then, ensure your unit doesn’t go into maintenance mode. If yes, there should be no power at 1Y. You can easily access the maintenance mode. 

Just turn the unit off and hold down both the Up and Down arrow keys at once for a few seconds. After a couple of seconds, you can hear an audible beep and the control panel will pop up the 0E message. Now, press the Up or Down arrow keys on the remote display to access various information about the machine. 

Next, inspect the fused power next to the junction box. Ensure the leads next to the junction box on Black and White wires are intact. If they get damaged, you must replace those wiring with 3 amp fuses. Don’t forget to measure the voltage on the wiring. The voltage should be 108 to 132 VAC. 

Also, don’t forget to check the voltage on the transformer and the PCB fuses. You can find the transformer wiring on the bottom right of the PCB at 90-110 VAC. And the PCB fuse should be on the board as well. If needed, you must replace both the transformer and the 5 amp fuse. 

02. Rheem Tankless Water Heater Blows Fuses

A short in the thermostat wiring is the main reason that causes the 3 amp fuses to blow out continuously. If the wires going through the thermostat are touching each other, the fuses will surely get blown. 

A bad electrical connection or loose wiring can also blow out fuses or circuit breakers. If there are a lot of junction boxes, fuse connections, and breaker connections, there will be more risks for the fuses and breakers to blow out. 

Turns out, undersize wiring can cause the fuses or breakers of the water to trip.

How To Fix:

First off, take the thermostat cover off and make sure no wires are touching each other. To figure out which wire is at fault, you need to take a multimeter. Then, check every wire that goes to the thermostat and the furnace cabinet ground to detect when a dead short occurs. Then, carefully inspect the wire to figure out where they are being pinched and cut the wire with sheet metal. If needed, you must also replace the fuses.

Secondly, ensure there is no loose or poor wiring. We recommend you hire a professional electrician to wire your water heater. Tell the electrician to minimize the electrical connections. 

Lastly, we recommend you wire your water heater with the right size wiring. For example, if you have a 40-gallon water heater with 3kW units, you should need a piece of 12 gauge wire rated at 20A. If you get smaller or bigger size wiring than this, it will trip the fuses and the breakers. 

Apart from these, there are a lot of reasons that cause the fuses to blow out. You can check out this article that breaks down everything that causes the water heater to keep blowing fuses. 

03. Rheem Tankless Water Heater Shuts Off During Shower

Your Rheem tankless water heater won’t stay on or keep shutting off (especially during shower) due to several reasons. 

The fancy term ‘cold water sandwich’ is the main culprit behind this issue. If the distance between the tankless water heater and shower is decent or quite long, you will get cold water for a minute and then hot water. Indeed, your water heater fluctuates between putting out hot and cold water.

How long you will experience this problem depends on the length of the pipe the water has to travel. 

Secondly, your water heater may reach the limit that causes it to shut off during the shower. Every tankless water heater has a certain flow rate. If you cross the limit, it will no longer heat your water. Running multiple hot water fixtures at once when taking shower can easily exceed the limit of your water heater. It’s a common problem with households where you don’t install the correct size unit according to your needs. 

Thirdly, a tripped circuit breaker is also responsible for a sudden shut-off unit. If the circuit breaker gets tripped, it will provide no power to your water heater. Consequently, your water heater will get turned off.  

Nonetheless, a defective flow sensor can also be the culprit behind this issue. The flow sensor actually detects the incoming cold water and sends a message to the control board to heat the water.  

If this sensor is at fault, it fails to detect the cold water that causes your water heater to shut off during shower. 

How To Fix:

Let us start with solving the ‘cold water sandwich’ issue. Just turn on hot water for residual hot water and wait for a couple of minutes. It will push the cold water and bring the hot water. Of course, it will waste a lot of water but it is kinda a simple remedy. 

Alternatively, you can connect a circulator pump system to your tankless water heater to get instant hot water. 

On the other hand, you should ask your family members to not use hot fixtures while you are going to take a shower to avoid the hot water heater limit issue. 

When it comes to fixing a tripped circuit breaker, we recommend you inspect the main service panel. Then, locate the circuit breaker that is labeled as a water heater. What you need to do is- just flip it back to the on position. If needed, replace the breaker with a new one. 

And lastly, we recommend you hire a certified plumber to replace the flow sensor if you find it defective. 

04. Tankless Water Heater Low Flow Problem

A slow or low flow rate on a tankless water heater is another common problem every homeowner encounters. The following culprits are responsible for a low or slow flow rate: 

  • A damaged flow sensor could cause a flow problem.
  • If the inlet filters get clogged with dirt, debris, and dust, water won’t flow through those filters freely. 
  • An improper or wrong cold water and hot water lines installation can also cause this water heater flow issue. 
  • Sediment build-ups in the faucet can also restrict the flow of your water heater. 
  • Defective water control valve

How To Fix:

Try out the following troubleshooting steps to fix the low flow issue. 

  • Replace the damaged flow sensor with a new one. 
  • Release the blockage from the inlet filters. 
  • Professionally install your water heater. Most importantly, install the cold water and hot water lines in sequence, not reverse. 
  • Check the faucets for sediment build-ups and remove them. 
  • Change the water control valve to your desired water flow. 

05. Rheem Tankless Water Heater Not Getting Hot

Your Rheem Tankless Water Heater won’t deliver not enough hot water or no hot water at all due to the following reasons: 

  • Forget to turn on the unit
  • You don’t completely open the water shut-off valve
  • The hot water faucet is not completely opened
  • Water piping gets frozen
  • Electrical power is disconnected
  • Set the temperature too low
  • Defective mixing valve
  • Error codes pop up on the digital display
  • Insufficient water demand
  • Connections get reversed
  • Clogged fixture aerator
  • The water filter gets dirty
  • Scale buildup in the heat exchanger

How To Fix:

  • Start with checking that you turn on the unit. If you don’t, just press the power button and turn the water heater on. 
  • Inspect the water shut-off valve to ensure whether you turn it fully on or not. If not, fully open the water shut-off valve. 
  • You will not get any hot water output until you open the hot water faucet completely.
  • Check the water piping and ensure it doesn’t get frozen. If needed, thaw the water piping or insulate it. 
  • If you accidentally disconnect the water heater from the electrical power source, restore the power to the unit by plugging the power cord. Otherwise, you won’t turn on the unit- let alone getting hot water. 
  • Raise the temperature to a higher setting by pressing the Up button from the remote controller.
  • If the hot and cold water lines get reversed, correct them. 
  • Clean the fixture aerator.
  • Clean the water filter. In this case, you check out this article on Rheem Tankless Water Heater Maintenance to learn how to rinse the dirty water filter. 
  • Call a professional to inspect the heat exchanger and let him release that blockage from there.

06. Insufficient Hot Water

A number of reasons are to blame if you don’t get enough hot water out of the unit. 

For example, you will never get hot water according to your expectation if the temperature setpoint is low. 

Similarly, the water heater won’t heat the water sufficiently if the gas valve is partially opened. Besides, low gas supply pressure is another issue that can throw a wrench into work.

How To Fix:

To begin with, ensure you set the temperature to a higher setting. Specifically, I will recommend you to set the temperature between 120 to 125 degrees F. In this case, just press the Up button from the remote controller. 

In addition, take a look at the gas supply valve on the gas supply line and ensure you fully open the valve. 

And last not the least, a low gas pressure issue can also cause this issue. I highly recommend you to contact the gas utility company to verify the gas meter and the gas piping. 

07. Water Too Hot

If you get scalding hot water from the unit, one of the following reasons are to blame: 

  • Set the temperature too high
  • Water shut-off valve is not completely opened
  • Small amount of water has been heated

How To Fix:

Firstly, ensure you don’t set the temperature to a higher setting. To decrease the set temperature, press the Down button from the remote controller. 

Afterward, inspect the water shut-off valve and ensure you open this valve completely. 

And lastly, increase the water flow at the hot water faucet to let more water flow through the water heater. 

08. Rheem Tankless Water Heater Not Working After Power Outage

After a power outage, there may be partial power, not complete because of some defects. Probably, it affects your water heater. There are a couple of reasons that could cause your tankless water heater to not work. They are: 

May Be The Circuit Breaker Gets Tripped

The circuit breaker may get tripped due to a surge in electricity when the power comes back. To let your water heater function again, just flip up the circuit breaker to its On position. 

On the other hand, you should call your local HVAC expert if the circuit breaker keeps tripping. Generally, the circuit breaker gets tripped because of overload. In this case, your electrician may need to re-access the electrical & wiring. 

Damaged Wiring

During power outages, some animals like critters or mice may chew the wiring leads to your water heater. Alternatively, the wire gets burnt due to overload. So, when the power comes, your water heater won’t work due to the damaged wiring. 

What you need to do now- is just find out the damaged wiring and replace it. We recommend you let your electrician deal with this task as it’s a bit of a risky workflow. 

Defective Switchboard

The switchboard (from where your water heater gets powered) may be the culprit. Maybe either the improper earthing or a loose wire connection caused the switchboard to fail. Having ABC knowledge in electrical, you could easily check it out & fix this issue.

Keep in mind; that after a power outage, if your water heater doesn’t function properly, chances are it happens due to an electrical issue. Power outages could rarely malfunction the water heater. If you check all the electricals and they are fine, don’t hesitate to check out your tankless water heater. 

Defective thermostat, burn-out heating elements, and tripped reset buttons are some results you may get when the power comes back after power outages. Fix these problems to let your water heater function again. 

09. Rheem Tankless Water Heater No Power To Remote

Hooking up the 2 remote control wires wrongly is the main culprit behind this issue. You will get no power to your main remote control if you attach the 2 wires of your main remote control incorrectly. 

Some DIY installers hook up the remote control wires to the 2 screws on the lower left of the control board inside the unit. Basically, those two screws are for 120V AC (the main power to the unit). But the heck is- your remote control runs off of 12V DC. So, when you hook up the remote control wires to those 2 screws, it will just fry your remote control. 

How To Fix:

The only solution left to fix this problem is- just to replace the remote control. Then, hire a certified HVAC guy to install the remote control professionally. Also, don’t forget to inspect other heater components like the control board. 

How Do I Reset My Rheem Tankless Water Heater?

Resetting your tankless water heater will fix issues like if your Rheem heater displays error codes. Follow the below steps to reset your Rheem tankless water heater. 

Just remove the power cord from the actual power source like the electrical outlet. Then, leave your water heater for 30 seconds in state of disconnected from the power source.

Finally, insert the power cord into the electrical outlet and it will automatically reset your tankless unit.

On the other hand, if you have a direct connect, go to the circuit breaker and flip it down. Again, leave the unit for 30 seconds and flip the breaker to the On position. Thus, it will help you reset the unit automatically.


Where is the reset button on a Rheem electric water heater?

You can find the reset button close to the thermostat on a Rheem electric water heater. It’s actually a red button. If you want to access it, you must take the panel cover out from the unit. And the reset button is exactly behind the insulation.

How do you reset the pilot light on a water heater?

First off, remove the access panel and lock the temperature control valve. Turn this control valve to the off position. Wait for 10 minutes. Then, move the dial to the Pilot position. You can find a red button with a fire logo. Keep pushing the button until you hear a click. Hold down the button and push the igniter switch at the same time. Repeat this several times which will light up the pilot. Wait for a minute while holding down the red button. After a minute, release the button and the flame should stay on. Finally, turn the temperature control valve to the On position and set the temperature.

What causes the gas water heater pilot to go out?

A defective, bent, or dirty thermocouple is the main culprit that causes your gas heater pilot to go out. Replacing or cleaning the thermocouple will be an easy fix for the pilot problem.

How do you stop a water heater pilot from blowing?

You can stop a water heater pilot from blowing out by keeping the thermocouple clean. Also, make sure the gas release valve (if it’s a gas unit) is free of dirt or carbon build-ups. On the other hand, ensure the heating element is intact and no dirt buildup on it if it’s an electric unit.

What causes a water heater to backdraft?

A clogged vent pipe or poor vent installation is the main culprit that causes a water heater to backdraft. To avoid this issue, you must install the vent system professionally. And of course, you need to check the vent pipe occasionally for blockage. If you find any blockage, just release it. 


Every appliance is prone to malfunction. And your Rheem tankless water heater is not an exception to this. 

We broke down every defect or problem you may go through with your water heater over time. So, we recommend you bookmark this troubleshooting guideline. It will surely come in handy whenever you encounter a problem with your Rheem Tankless Water Heater. 

If you have any questions related to water heaters, leave a comment below. We will respond to you right off the bat. 

8 thoughts on “Rheem Tankless Water Heater Troubleshooting Manual [Complete Guide]”

  1. I want to flush two units together. I was able to flush one but not able to flush the second one. Can you help?

  2. I need a picture showing where the reset button is at on my rheem performance platinum tankless wh . Im trying to make sure it even powers on before installing it but the controller doesn’t even come on.

    • Thanks, Taylan Lowe for leaving a comment.

      To get access to the reset button on Rheem Performance Platinum Tankless Water Heater, you should remove the upper thermostat cover. Once you remove the upper cover, you can see the red reset button.

      You ask for a picture, but what if I give you a video link? Check out this video to locate the reset button on your Rheem Performance Platinum Tankless Water Heater:

        • Sorry Met…

          Rheem tankless water heater has no reset button. To reset the unit what you need to do is- just unplug the unit from the power source and leave the water heater for 30 seconds. Then, restore the power to the unit which will automatically reset the unit. Hope, it will help you.

          And thank you for pointing out this.

  3. 10 year old Rheem tankless WH (RTG-74PVN) that won’t stay on. When I turn on the hot water, I can hear it light for 5 or 10 seconds but it then turns off. No hot water… I had a new Rheem unit installed today and it is doing the same thing. HELP. Water pressure seems to be good… Gas is on and flowing.

    • The problem seems a bit complex. Do you install it professionally? I mean you should hire a professional to install the unit for you. Sometimes, reversed connections can cause this issue.

      The best suggestion I can offer is to call a certified plumber to have the unit checked professionally.

      Best of luck!


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