Rheem Tankless Water Heater Code E5 [7 Easy Fixes]

The Rheem tankless water heater code means that incomplete combustion is taking place in your heater or it may be that the water flow is too high.

The causes of the E5 code appearing on your heater’s display include too much water flow, incorrect gas pressure, blocked air intake, clogged exhaust, dirty air filter, lack of airflow in the room, and corroded heat exchanger.

Keep reading our Rheem tankless water heater troubleshooting manual to get rid of the error code ASAP.

Table Of Contents

Rheem Tankless Water Heater Code E5 [7 Easy Fixes]

In this section, we will take an in-depth look at the causes behind Rheem tankless water heater code E5 and explore simple solutions.

Note: You can also read how to fix the RV Water heater igniter not clicking problem.

1. Excess Water Flow

Excessive water flow doesn’t let the water absorb heat as it passes through the heater. Instead, it reaches a lukewarm state that is below the set temperature. This is why the E5 code on the electric tankless water heater is seen.


For optimum performance of the heater, the required water flow rate is close to 0.66 GPM. Moreover, you need to ensure that water pressure of 14 psi (97 kPa) to 40 psi (276 kPa) is maintained.

At 40 psi water flow will be maximum. To correct the overflow of water you can do the following. 

Before you begin, check the pressure by inserting a meter into an outside faucet and turning it on. Then, we adjust the pressure regulator.

The pressure regulator can be found on the incoming water line, which is normally not far from the water heater. Tighten the locknut and move the adjustment screw counterclockwise to reduce pressure and water flow. 

2. Incorrect Gas Pressure

The required minimum gas pressure may be different for different models. You will find yours on the water heater rating plate. However, the pressure for every model has to be within  10.5” w.c. or 2.6 KP for natural and 14” w.c./3.5 kPa for LP gas. 

If the gas pressure is inadequate, incomplete combustion will take place resulting in incomplete combustion.


Your heater has a piece of equipment called a gas pressure regulator. As you can tell, it controls inlet gas pressure. You will find it at your gas supply line.

Now, grab a flathead screwdriver and look behind at the gas regulator. Unthread the cap at the very center of the regulator.

Once you have that cap off you’ll notice that there’s a plastic adjustment inside that increases or decreases the pressure going to that equipment. So stick your flathead screwdriver in to increase the gas pressure, you’re gonna go clockwise.

Go two turns and check to see if the flame is strong enough. If necessary, turn in more. Press the reset button on Rheem tankless water heater. The code should clear on start-up.

3. Dirty Air Filter

Through the air filter, combustible air is fed to the heater. With dust blocking the heater, there will be a lack of necessary airflow resulting in incomplete combustion.

Thus, Rheem electric tankless water heater error code E5 will appear on the screen.


Remove the air filter and clean it with a rag to get all the loose dirt out. Use a garden hose to rinse the filter down and scrub it with a brush.

After it is squeaky clean, let it dry. Well, if cleaning doesn’t work, replace the air filter.

Watching the video will be helpful for you in the cleaning process.

4. Blocked Air Intake

The air intake tube of your heater can be blocked with debris, spider web, dirt, etc. This results in a lack of combustible air and results in incomplete combustion.


Take off the filter and check the air intake line to see if it is dirty. Clean it up with a thin wire brush. Keep going at it till the wire brush comes out clean.

5. Clogged Exhaust

The exhaust of your heater lets out heat and carbon dioxide from the flame. This allows the flame to breathe and the heater to not overheat. But with a blocked exhaust, this system is jeopardized.

The heater gets overheated and carbon dioxide snuffs the burner flame causing incomplete combustion. Thus, Rheem electric tankless water heater E5 code appears.


Cleaning your water heater vent is easy if you follow the steps below.

  • Find the vent pipe for your water heater. Wear protective gear if possible to cover your eyes, hands, and body.
  • Use a metal wire or plastic hook to retrieve any dirt that has become lodged.
  • Remove any remaining dirt using the portable vacuum. Replace the cap after cleaning it clean with a sponge or soft cloth.
  • Relocate any hard objects that could be impeding the pipe and causing debris to build up inside. Relocate the heater if necessary.

6. Lack of Ventilation In The Room

It could be that the room your water heater is placed in has no air circulation. This is not ideal for a flame.

For an electric tankless heater, this shouldn’t be much of an issue. But if you have a gas water heater, you may be getting the E5 code because of this.


Open up windows and install ventilators if necessary. Clean the ventilators if already in the room in case they are dirty.

7. Corrosion or Scale Deposit On Heat Exchanger

Rheem tankless water heater E5 error code may also appear because of a corroded heat exchanger that won’t produce enough heat and cause incomplete combustion. Calcium Carbonate deposits also impede the performance of heat exchangers.


You need to replace your heat exchanger if it is corroded. A heat exchanger with white deposits of Calcium Carbonate can be dissolved chemically.

After that, you can reinstall your heat exchanger. To clean your heat exchanger, we recommend a hydrochloric acid-based solvent, like Scalzo. Read more about how to clean scales from heat exchangers


What are the most common problems with tankless water heaters?

Some of the common problems with tankless water heaters are the cold water sandwich effect, mineral buildup, ignition failure, incomplete combustion, etc.

Why do tankless water heaters burst?

Gas leaks. Due to a gas leak close to the gas valve or gas piping, gas tankless water heaters can create a catastrophic explosion. An electrical spark from any nearby source could be enough to produce an explosion.

Can you run out of hot water with a tankless water heater?

It will be very hard to run out of hot water if you are using a tankless water heater. Running every faucet at once may overwhelm your heater and fail to heat. However, your heater may also be big enough to take that load as well.

Why does my tankless water heater go cold sometimes?

Your tankless water heater may run hot and cold due to a broken flow sensor. The flow sensor measures the amount of water entering your system and communicates the data to the control board of your unit. 

How often should a tankless water heater be flushed?

Once a year is good enough. A tankless water heater can acquire mineral buildup over time, which can damage the walls inside the heating chamber of your tank.


Hopefully, now you know how to get rid of your Rheem tankless water heater code E5. In case you have more queries about your Rheem heater, you can leave us a comment below.

About David

David is a man who loves doing research and finding out the best solution to any type of heating appliance issues like heaters and fireplaces. Whenever he finds the perfect solutions to any issues of heating appliances after doing in-depth research, he loves to share them with his readers. In fact, his aim is to help his readers to fix the issues with their heating appliances easily.More about us .

4 thoughts on “Rheem Tankless Water Heater Code E5 [7 Easy Fixes]”

    • Hi Frieda

      To fix the E5 issue, ensure the correct gas pressure, adequate water flows, blockage-free air intake & exhaust, and proper ventilation in the room where you place your water heater.

      Hopefully, your water heater issue will be solved.

      Best of luck!

    • Hi Thomas

      The E5 code appearing on your heater’s display due to one or more reasons like too much water flow, incorrect gas pressure, blocked air intake, clogged exhaust, dirty air filter, lack of airflow in the room, and corroded heat exchanger.

      Well, as your water heater is a new one, it is worth contacting the Rheem customer support center before doing anything by yourself as your heater must cover the warranty period.

      Best of luck!


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