Rheem Tankless Water Heater No Hot Water [Solved]

You are experiencing Rheem tankless water heater has no hot water to use because of the following reasons:

  • Turned off the power source
  • Too low-temperature setting
  • Water shut-off valves and hot water fixtures aren’t opened fully
  • Frozen water pipes
  • Interruption in gas supply
  • Clogged water filter
  • Plugged heat exchanger
  • Bad mixing valve
  • Error code is flashing on the controller. 

Plus, dirty burners/igniters or blocked air intake filters and exhaust venting are some of the major reasons why your water heater will deliver no hot water. 

rheem tankless water heater no hot water

However, at times, only adjusting the water temperature is enough to resolve the issue. If the effort doesn’t bring any luck, go through our article to troubleshoot those issues by yourself.

Quick Solutions For Rheem Tankless Water Heater No Hot Water

Before moving to the detailing part, let’s take a look at the below quick table highlighting all the probable reasons for this issue with their precise solutions:

Probable ReasonsPossible Solutions
Faulty power supply > Plug the water heater into a well-working electrical outlet.
> Make sure the circuit breaker isn’t tripped and all electrical connections are secured. 
Low-temperature setting> Turn up the temperature on the remote controller. 
Dirty igniters or burners > Clean out the burners and igniters. 
Corroded heat exchanger> Descale your water heater. 
Gas supply issue> Ensure the use of the right fuel type and manifold gas pressure. 
Blockage in the air supply> Clean out exhaust venting and air intake filter. 
Problems in water supply> Open the water shut-off valve and hot water faucets all the way. 
> Check for frozen water piping and thaw them with heating equipment. 
Defective mixing valve> Replace the faulty mixing valve. 
System overload > Close the extra hot water faucets to balance the hot water demands. 
Clogged water inlet filter> Clean the clogged filter
Error code is displaying > Figure out the error indication and fix them. 

Rheem Tankless Water Heater Not Heating [11 Easy Solutions]

Here, we will discuss the most common reasons why your Rheem tankless water heater won’t deliver hot water in detail with their effective solutions. Let’s read between the lines…

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

1. The Power Source Is Off

If your Rheme tankless water is not providing hot water, the problem is simply related to the faulty power supply to the unit. A higher possibility is you have forgotten to plug the unit into a functional electric outlet. 

Or, you have plugged the unit into a faulty electric outlet that is not permitting the system to operate the unit fluently.

On the other hand, a tripped circuit board or defective wiring is also a major cause that prevents your water heater from turning on.

How To Fix:

First of all, make sure you have plugged the power cord into a functional outlet. If needed, check the continuity of the electrical outlet using a voltage meter as you can see in the picture below.

check electrical outlet using a voltage meter

Next, verify that the circuit breaker is in the ON position. If not, just flip it up to let an uninterrupted power supply to the unit.

Lastly, if you identify the breaker has tripped or the wiring gets damaged, we highly recommend you call a certified electrician to resolve the electric issues.

2. Too Low-Temperature Setting

If there is all well set from the power supply, the incorrect temperature setting is the second suspect behind the malfunctions in the heating process. Higher chances are you’ve set the temperature too low. 

If this is the case, the problem can easily be resolved by just turning up the temperature on the remote controller. 

How To Fix:

You will find 120 -Degrees F as a default setting on the controller which can be set up to 140 degrees F. To do so, you need to disable the lock. Follow the below instructions:

Step #1: Press the up arrow button for 4-5 seconds until it flashes 140 degrees F. 

Step #2: Right then, press the down arrow altogether at the same time (shown in the picture below) and keep holding the buttons until you will hear a beep.

keep holding the buttons until you will hear a beep

Step #3: Now you can set your desired temperature setting and enjoy the hot water. If still you face any problems with adjusting the temperature, watch the tutorial video given below to set it correctly.

3. Dirty Igniter/Burners

If you have a gas-operated tankless water heater, there might be an issue with the flame that is preventing in heating of the water. Basically, when the heater gets activated, the igniter on the combustion chamber creates a spark that ignites the gas on the burners.

But if the combustion components get clogged by shoot build-up, they fail to continue the flame and ignition process. Consequently, due to flame and ignition failure, the system can’t heat the water.

How To Fix:

Before inspecting the burner assembly, make sure you’ve turned off the electric and gas supply to your water heater. Disassemble the front cover. Now, disconnect the screws that are holding the burner assembly to the combustion chamber in place.

Detach the igniter and flame sensor wiring. Then, pull out the burner toward you. You will find grim or foreign materials stuck on the burner ribbons.

grim or foreign materials stuck on the burner ribbons of rheem tankless water heater

Take an air compressor and blow out the dirt and debris from the ribbon. When done, wipe down the element with a microfiber cloth, along with the sand igniter and flame rods surround the assembly with a piece of fine sandpaper. 

Ensure a snug electrical contact of the igniter and flame rods with the end of the ceramic probes. After cleaning, reassemble the burner assembly and secure the front cover.

4. Plugged Heat Exchanger

A blocked heat exchanger is another big reason behind your Rheem tankless water heater not providing hot water.

During the heating process, the inside heating elements including the heat exchanger come in contact with hard water that highly dissolves minerals like calcium and magnesium.

When the hard water passes through the exchanger fins, the minerals get plugged up and form in scale after a long time. In such cases, the heat exchanger loses its efficiency which affects the complete heating procedure. 

How To Fix:

The only way to deal with this issue is to descale and flush your water heater. The flushing procedure is quite straightforward and you can do it by yourself if you have an effective flushing kit and the needed elements. 

flushing kit and the needed elements

Alternatively, you can use undiluted white vinegar to clean your water heater. For this instance, go through our comprehensive guide to learn the detailed process of how you will flush your water heater like a pro. 

5. Gas Supply Issue

In the case of gas operated tankless water heater, interruption in the gas supply is another leading factor if you are suffering insufficient hot water from the system. Here either you are using the wrong type of fuel or there might be incorrect manifold gas pressure that is causing the unit not to ignite.

Here are the simple fixes to follow: 

i). Incorrect Gas Type

First up you need to check whether you are using the correct fuel as per the recommendation. For this instance, go through the manual for your water heater. If there is any mistake, change the fuel type. 

ii). Low Manifold Gas Pressure

Secondly, you need to ensure the proper inlet gas pressure to your water heater. If there is natural gas, the minimum static gas pressure is 4.0″ W.C., and  8.0″ W.C. is recommended for propane.

Now take a manometer and measure the high and low manifold gas pressure for your tankless water heater. If needed, calibrate them to the standard pressure following the manual. 

iii). Air In The Gas Line

Thirdly, check for air in the gas line.  If there is air in the gas lines, your unit will not get the required gas for ignition. Simply purging the gas lines will get resolve this difficulty.

6. Blocked Exhaust Venting Or Air Intake Filter

All tankless water heater system requires the proper amount of fresh air to continue the combustion process. When the heating process starts, combustion air enters through the venting that ignites the burner. Also, the combustion gases safely exit through the exhaust venting at the same time. 

But due to a lack of regular maintenance, the venting gets blocked by shit, dirt, debris, and foreign materials, creating air blockage in the system. 

Along with the exhaust venting, there is an air filter used in water heaters that help to draw fresh air from outside for combustion and keep the bugs and dirt out of your system. 

 But in case it gets clogged by dirt, debris, and lint, there will be insufficient free air in the combustion chamber. 

All the above terms interrupt the ignition process which is why you will get hot water from your water heater.

How To Fix:

First, turn off your tankless unit from the remote controller or main circuit panel. Turn off the water and gas shut-off valves as well. All these are must-to-do safety measures that you need to apply before cleaning.

Anyway, unscrew the front access panel and set it aside. Now locate the air intake filter from the top inside. It will be screwed with the top panel. Unscrew it and gently pull it toward you.

blocked exhaust venting or air intake filter

You will find stubborn dirt particles stuck in the filter screen. Rub the screen with a soft brush and remove all the debris. Rinse it under running water. Let it dry and reassemble it back in place.

When done, inspect the exhaust venting of your tankless unit. Take the vacuum cleaner and insert its extension hose through the venting mouth. Vacuum out all the inside areas as much as you can. 

Use a duct cleaner to clean the unreachable area. While cleaning, don’t forget to check for any holes in the pipes. Also, ensure that the venting system is installed correctly.

7. Clogged Water Inlet Filter

Almost all tankless water heater features a water inlet filter that preserves the unit against mineral build-up and sediment. It works as a scale inhibitor that is retaining minerals in a solution to protect the water heater from damage. 

rheem tankless water heater water inlet flter

As for always being in contact with the hard water, getting blocked by debris is quite obvious.

When the filter fills with debris and clogs, water fails to pass through the blocked line. Thus there will be no chance to heat the water. 

How To Fix:

As a fix, you need to inspect the water filter and clean it properly. Locate the water filter at the bottom just next to the gas inlet valve.

Unscrew the filter by turning it to the left counterclockwise.  If needed, take pliers and pull out the filter from the connection.

You will find clogged debris on the screen.  Simply take a Q-tip or toothpick to clean the stuck dirt particles from the screen. Scrub the filter with a soft brush and then rinse it under water. 

After proper cleaning, install the filter back in place and ensure a secured connection. Now, check for hot water.

8. Problems In The Water Supply

In most cases, the no hot water issue on the tankless water heater lies in the improper water supply. There are various factors that can be responsible here. Like a closed water shut-off valve, the hot water faucet is not opened fully or the water pipeline gets frozen due to outside freezing weather. 

i). Water Shut-off Valve Is Not Opened All The Way

You will find two water shut-off valves at the bottom of the unit labeled red and blue colored handle. Basically, when you turn on the heater, the cold water inlet valve (blue handle) feeds water cold into the water heater from the main supply line and starts the heating process.

But the best chances are you haven’t opened the shut-off valves fully or it remains in the OFF position. That results in no hot water from the system.

Inspect the water shut–off valve to check whether it is in the “ON” position. If not, turn the valve lever counterclockwise to open the valve fully like in the picture below.

turn the valve lever counterclockwise to open the valve fully

ii). Closed Hot Water Faucet

Regarding water supply issues, the closed hot water faucet is another major reason to have insufficient hot water from your Rheem water heater. The heating process like (the main burner) will remain off while incoming water pressure is insufficient.

Or the faucet or aerator head may get clogged due to the hard water supply for a long time as in the picture below. A rusty or dirty aerator will certainly prevent a continuous hot water supply.

rheem tankless water heater rusty or dirty aerator

How To Fix:

  • First off, check the hot water faucets and open them all the way to let the burner work properly. 
  • Then inspect the faucet aerator for any blockages. If so, simply disassemble the faucet parts and clean out the stuck debris with any sharp objects.

iii). Frozen Water lines

Frozen water pipelines are another culprit that prevents your tankless water heater from delivering hot water. This mostly happens due to cold weather outside. Normally every tankless water heater has a freeze protection feature that prevents the outside piping from freezing weather.

But when the outside temperature goes down 4°F C,  the protective feature fails and the piping system gets frozen. Consequentially, your heater can’t provide sufficient hot water to use.

How To Fix:

First off, locate the frozen area of the water supply line. Now you need to thaw the frozen pipes. Here is how to do it:

Step #1: Turn off the heater and unplug it from the power connection.

Step #2: Now turn on all the water faucets at full speed. Keep the faucet open until the frozen area begins to melt and water will begin to flow through the frozen area.

Step #3: If that doesn’t work, apply heat to the freeze section by wrapping an electric heating pad around the pipe. This will help to restore water pressure.

  • Preventive Tips: Throughout the cold climate, insulate all the susceptible water pipes and water shut-off valves with proper insulation.

9. Defective Mixing Valve

Apart from all the above reasons, the defective mixing valve is another responsive factor to blame. Unlike other tankless water heaters, the Rheem model has a thermostatic mixing valve that features to regulate the water temperature by mixing hot and cold water together.

rheem tankless water heater defective mixing valve

In such ways, it ensures the hot water temperature in a safe level to prevent scalding and disease. 

But a non-operational mixing will fail to blend the cold water with the hot water in the proper ratio. Thus, getting cold water from the faucet is quite obvious. You will notice drips or leakage around the mixing valve or flow reduction from the hot water tap. 

How To Fix:

Locate the mixing valve from underneath the inside unit as shown in the picture. 

locate the reem tankless water heater mixing valve

If it is clogged with dirt and debris, clean it and the problem will solve. But in case of the defective part, you need to take it off and install a new mixing valve back in place. 

Well, you can do it yourself by following the below effective video. Or, contact a professional for the replacement tasks. 

10. System Overload

If there is too cold water from your tankless unit, the best chances are the water gets reduced due to higher household hot water demand. The problem mainly happens when too many fixtures in your house run at a time that overloads the system. 

When the system has reached its limit, the unit shut down immediately, leaving you with cold or lukewarm water.

How To Fix:

To overcome this problem, here are the simple fixes that you need to follow:

  • First, turn off the extra faucets and taps to make the balance of the hot water needs.
  • If your heater has shut down due to system overload, simply reset the Rheem tankless water unit
  • In case still, your water heater is not heating, another possibility is that your unit is not big enough to deliver enough hot water for your household. In such cases, consider upgrading your water heater with the bigger one. We recommend learning the flow rate capacity of the heater to get a clear good idea of how much hot water can be used at the same time. 
  • Or, you may install separate tankless water heaters with a parallel connection to your recent unit. It will help to fulfill your household hot water demand simultaneously. 

11. Error Code Is Displaying On The Controller

Last but not least, you may encounter a diagnostic error code from the controller that is restricting to increase the water temperature as it should. Several times, the tankless water heater throws a variety of error codes on the display indicating a specific malfunction going through the system. 

For this instance, we suggest going through our comprehensive Rheem tankless water heater troubleshooting guide. Here we have discussed the major error codes with their indication and precious solutions processes.


Why is my Rheem tankless water heater producing too much hot water?

Your tankless heater is producing too much hot water due to the setting temperature being so high, pressure release valves getting blocked or the thermostat having malfunctions. To fix it, simply set the temperature lower or replace your thermostat.

Why does my Rheem tankless water heater keep shutting off?

Your tankless water heater is shutting off because there is not enough water running through the unit. This is also a critical issue of overheating. So, check it regularly for your safety.

Which temperature setting would be ideal for my Rheem water heater?

We recommend you set your Rheem tankless water heater temperature at 120-124 degrees Fahrenheit. You can increase it if needed. But a sudden temperature change may be dangerous for your water heater.


Already, we have sorted out all the most common reasons behind your Rheem tankless water heater no hot water issue, and explained their easy solutions.

So, whenever you experience your Rheem tankless water heater not heating issue, simply follow our troubleshooting guide and fix them yourself. 

Yet, if you face any other issues with your Rheem tankless water heater hot water supply issue, don’t hesitate to reach out to us through comments. We will respond to you ASAP with a possible 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.

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