Rheem Tankless Water Heater No Hot Water

Rheem Tankless Water Heater will deliver no hot water due to the following reasons: 

  • Unit is off or not getting any power
  • Set the temperature too low
  • Not fully open the water shut-off valve
  • Open the hot water faucet partially
  • Bad mixing valve
  • Frozen water piping
  • Clogged water filter
  • Dirty heat exchanger
  • Error code displayed on the controller

To get hot water out of the water heater, you must troubleshoot those issues and fix them. Fortunately, this guide will help you in troubleshooting and solving those problems. 

So, let’s get started. 

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. 

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.

Lastly, 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.  

02. 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. 

03. 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. 

04. 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. 

05. 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.

06. 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. 

07. 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.

08. 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. 

09. 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. 


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!

Read Also:

Leave a Comment