Rheem Tankless Water Heater No Hot Water

Rheem Tankless Water Heater No Hot Water

If your Rheem Tankless Water Heater is producing no hot water, check the following potential issues:

  • Inspect the gas supply. Ensure the gas tank is full of gas, and verify that you turn on the gas valve fully open.
  • Ensure that the plumber vents the unit according to the manufacturer’s guidelines for proper combustion. Also, check the exhaust venting for a blockage and ensure it’s free from debris buildup. 
  • Check the burner for debris buildup and clean the burner if required for proper combustion. 
  • The minimum water flow for initiating the tankless unit is 0.5 GPM. If the flow rate is lower than this, increase the water flow. Also, check the water filter for blockage. Clean the water filter to maintain the proper flow rate. 
  • Descale your tankless water heater to avoid any potential issues.
  • Check for error codes on the control panel for troubleshooting.

You can contact the Rheem Customer Support if the problem persists. To discover more troubleshooting steps, you can continue to read.

Table of Contents

Rheem Tankless Water Heater No Hot Water [9 Easy Solutions]

Now, it’s time to discover the culprits that cause your tankless water heater to deliver no hot water. 

01. Unit Isn’t On or Getting No Power

Expecting hot water from your tankless water heater without turning it on is like storing water in a bucket with hole. If you forget to plug the unit into a standard outlet, you can’t turn on the unit. 

Similarly, plugging the unit into an inappropriate or damaged electric outlet will bring no luck for you in terms of operating the unit. 

Nonetheless, faulty circuit breaker and wiring the unit incorrectly (if it’s new installation) prevents the unit from turning on. 

On the other hand, insufficient gas in the tank may cause the water heater to deliver no hot water if you are burning the gas. Besides, if you don’t open the gas valve fully, your water heater burner will receive no gas. As a result, it fails to produce hot water.

How To Fix: 

Firstly, check whether you plug the power cord into an electric outlet or not. Of course, the electric outlet you use to power the heater should be functional. You can check if the outlet is fine or  not by plugging another appliance like your phone charger. 

Secondly, ensure the circuit breaker for the water heater is in the on position. If not, just flip it up to let the electric power flow through it to the water heater. Don’t forget to check the condition of the breaker. 

If the breaker keeps tripping or gets defective, I highly recommend you call a certified electrician to fix this issue.

Therefore, make sure the installer wires the water heater correctly. If needed, contact a qualified electrician for possible cross wiring and let him resolve the wiring issue.  

Ensure you have paid the gas bill if you have a Rheem Gas Tankless Water Heater. Then, verify that you turn on the gas valve to let the gas flow continuously.

Every tankless water heater has a flow rate. It will help you determine how much water you can use simultaneously. If you run multiple appliances like a dishwasher or shower, it will exceed the capacity of your water heater. 

How To Fix: 

To overcome such an issue, run only one appliance at a time. For example, turn on the shower only when you need it. Never try to operate the dishwasher and shower or turn on multiple hot water faucets simultaneously. Instead, prioritize a specific water activity and run the water heater accordingly.

A tankless water heater has a minimum flow rate to initiate the water heater. If a certain amount of water doesn’t flow through the unit per minute, the water heater won’t heat the water. 

The minimum flow rate is 0.5 GPM. I recommend you increase the flow rate at the faucet to see if the unit starts heating up. Also, verify that the cold water inlet filter is free of blockage. 

Mineral deposits, debris buildup, and dirt cause a clog to the tankless water heater burner. Due to this restriction, improper gas is being supplied to the burner. As a result, the unit fails to produce hot water as it receives an insufficient gas supply for burning. 

Cleaning the burner will be an effective solution to this issue. Check out this video to learn how to clean the burner on a Rheem Tankless Water Heater: 

However, if you are not handy, you can hire a certified plumber to do this task for you. 

05. Set the Temperature Too Low

You will get not enough or completely no hot water out of your tankless water heater if you set the temperature too low. Fortunately, this issue is pretty straightforward to fix. Your child can even solve it by turning up the water temperature. 

How To Fix: 

To turn up the temperature, follow these steps: 

  • Disable the display lock by pressing down the two buttons in the center of the front panel. 
  • Press the Setting buttons and select Set Point by scrolling through the menu. 
  • Now, press the Up button on the controller to increase water temperature. You can turn up the temperature in increments of 1°F. 
  • Once you set the temperature according to your desired value, just press the ENTER button to store this temperature setting. 

06. Water Shut-off Valve Is Not Fully Opened

Never expect your tankless water heater to produce hot water if the cold water shut-off valve is Off. Similarly, if you don’t open this valve fully, no water will come into the unit. No incoming water means no outcoming hot water. 

How To Fix:

Begin with checking the cold water shut-off valve to ensure it’s in the On position. If not, just turn this water shut-off valve counter-clockwise to let the water flow through it. Of course, you should open this valve fully- not partially. 

07. Forget To Open The Hot Water Faucet Fully

The main burner of the unit will go off if the incoming water volume is inadequate. In short, you must open the hot water faucet fully if you want to get hot water out of your water heater. 

How To Fix: 

Just inspect the hot water faucets. Then, open all the hot water faucets fully to let the burner on properly. 

08. Defective Mixing Valve

A mixing valve on a water heater controls the temperature of hot water by mixing (as the name suggests) the hot and the cold water. This valve helps to prevent burn and deliver continuous hot water supply. 

If the mixing valve is malfunctioning, it fails to mix the hot and the cold water properly. As a result, you will never get a continuous flow of hot water from the unit. 

Water leaking out of the mixing valve and abnormal valve operation are the common signs of a defective mixing valve. 

How To Fix: 

Just locate the mixing valve on your Rheem water heater and unplug it. Inspect it for dirt and debris buildup and release the blockage. If cleaning the mixing valve doesn’t work for you, you must replace it with a new one.

09. Water Piping Gets Frozen

Forget about getting hot water out of the unit if the water piping gets frozen. Basically, every tankless water heater, including your Rheem, comes with a freeze protection feature. 

However, this protection feature will not guarantee to protect the outside piping from freezing weather. 

How To Fix: 

First off, allow the piping to thaw. I recommend you complete the following steps if the water piping gets frozen: 

  • Turn the unit and unplug it from the electric outlet.
  • Shut down the gas shut-off valve. 
  • Now, turn on a hot water faucet slightly until you get a water steam of at least ⅛ in. 

The best suggestion I can give you is- wrap insulation around the water piping and valves. 

10. Water Filter Gets Clogged

A dirty water filter can also be the culprit that prevents the unit from delivering enough hot water. Over time, the filter gets restricted by dirt, debris, and mineral deposits. As a result, water can’t flow through it. To get rid of this issue, you must clean the water filter. 

How To Fix: 

Follow the below steps to clean the water filter properly: 

  • Turn your tankless water heater off and detach it from the electric outlet.
  • Turn the water supply off to the unit by turning the water shut-off valve clockwise.
  • Drain your water heater. If you don’t know how to drain the unit, you can check out this guide on Rheem Tankless Water Heater Maintenance to learn. 
  • Remove the water filter from the base of the cold water inlet line and slide it out gently. 
  • Wash the water filter by running cold water. If needed, scrub this filter with a soft brush. 
  • Reinstall the water filter in the cold water inlet line and secure it with tightening up the nut. 
  • Plug the unit into an electric outlet and turn on  the water heater.

11. Scale Buildup In The Heat Exchanger

The Rheem tankless water heater will stop heating the water if there is scale or mineral formation in the heat exchanger. If you want the unit to function trouble-freely, you must flush the heat exchanger. 

How To Fix: 

Flushing the heat exchanger is not rocket science. You can use vinegar solution to descale the unit. I recommend you check this article on Will Vinegar Damage A Hot Water Heater. In that article, I broke down how to flush the tankless unit like a pro. 

12. Error Code Displayed On The Controller

Your tankless water heater will not work at all if it throws an error code on the display. I highly recommend you check this guide on Rheem Tankless Water Heater Error Codes. This guide broke down the meaning of the fault codes and what action you need to take to erase those codes. 

Why Does My Tankless Hot Water Heater Go Cold?

Your tankless hot water heater may go cold due to one of the following reasons:

Broken flow sensor

The flow sensor measures how much water enters the system and sends this data to the control board. If this sensor is at fault, the control board won’t receive accurate info from the flow sensor. As a result, the water heater will cycle between hot and cold.

Plumbing System Crossover

It typically occurs when you connect the hot and cold water lines. Due to this, you will get no hot water from your tankless water heater. To avoid this issue, check the hot and cold water lines and verify that the connection is correct.

Undersized Unit

If you get a water heater that fails to keep up with your household’s hot water demand, you will get lukewarm water as output. Consult with a professional when choosing a tankless water heater for your home. Or, you can install separate water heaters for appliances like washing machines or dishwashers.

How Do I Reset My Rheem Tankless Water Heater?

Follow these simple steps to reset your Rheem Tankless Water Heater:

  • Turn off the power to the water heater.
  • Shut off the gas supply to the water heater.
  • Leave the unit unplugged from the electricity and gas supply for 30 seconds.
  • Thirty (30) seconds later, plug the unit and turn on the gas supply. It will help you reset your Rheem Tankless Water Heater.


Getting not enough or no hot water is a common water heater problem on a tankless water heater. You can easily fix this issue by yourself if you follow the troubleshooting steps I described. Just follow my guideline and it will solve the no hot water issue on your Rheem tankless water heater. Good Luck!

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