Rheem Tankless Water Heater Code 99 [Why+ How To Fix]

Rheem tankless water heater code 99 is the error indication of the abnormal combustion of the system which means the blower motor isn’t creating enough ventilation. 

rheem tankless water heater code 99

That means the blockages in the venting and exhaust flue are the leading reasons for this error code. Other major reasons include:

  • Blocked air inlet filter.
  • Clogged combustion fan.
  • Or, corroded heat exchanger fins.

Well, whatever the reason is, the problem can be resolved by cleaning out the vent blockages. But before that, you can do a hard reset of this code from the dip switch assembly.

If that doesn’t bring luck to you, inspect the combustion fan and inlet filter for blockages or descale the blocked heat exchanger fins.

Quick Solutions For Rheem Tankless Water Heater Code 99

Before moving to the detailed troubleshooting, let’s walk you through a quick table, highlighting all the probable reasons with the quick solutions for the error code 99.

Probable ReasonsPossible Solutions
Blocked air inlet filter> Rinse all the dirt & debris from the air filter screen. 
Clogged combustion fan> Locate and clean out the fan motor assembly.
> If needed, replace the damaged one. 
Blocked or obstructed venting or exhaust flue.> Inspect all the intake and exhaust vents of your tankless system for any blockages.
> Clean the vents. 
Disconnected the vent structure from the water heater.> Ensure the flapper valve at the top of the heat exchanger moves freely with the vent termination of the water heater. 
Corroded heat exchanger fins> Descale the scale deposits by flushing the water heater. 

How To Fix Error Code 99 On Rheem Tankless Water Heater [5 Easy Solutions]

Now get into all the possible reasons for error code 99 on Rheem tankless water heater with effective solving procedures in detail:

Note: Also read our guide on Rheem tankless water heater code 76 and how to fix it.

1. Rheem Tankless Water Heater Code 99 Reset

As for the step, you can do a hard reset of the error code 99 on the Rheem tankless water heater control panel.

rheem tankless water heater code 99 reset

This is the easiest and quite effective troubleshooting step that will help to diminish the code from the display. 

Go through the below how-to section.

How To Hard Reset Error Code 99 On Rheem Tankless Water Heater

Follow the below step-by-step instructions:

Step #1: First, take off the front cover from your tankless water heater system. Use a normal Phillips screwdriver and remove all the screws on the front cover. 

take off tankless water heater front cover

Step #2: Now move to the PCB from the bottom of the inside. Locate the dip switches at the top left-hand side of the PCB. Here will find two dip switch assemblies with three buttons underneath. 

dip switch assemblies

Step #3: First off, hold down the min and max buttons from underneath. While holding down the button, flip-up switch 2 from dip 1 using a sharp object. Confused? See the picture below:

flip-up switch 2 from dip 1

Step #4: Wait for 4-5 seconds. Then, flip down the switch back in position and take your finger off the min and max buttons. 

take your finger off the min and max buttons

Step #5: The error code from the control panel should be erased. Finally, Attach the unit’s front cover.

error code erased from the control panel

2. Blocked Air Intake Filter

After doing the hard reset, still there is the same code on the control panel, be sure it is time to move the fixing steps to prevent the abnormal combustion issue. 

In this fact, a clogged air intake filter is the leading culprit to blame when there is a lack of combustion air in the system.

 blocked Rheem tankless water heater air intake filter

Unlike other tankless systems, the Rheem model equips an air intake filter that keeps the bugs and dirt out of your system. 

As for being always in contact with dust or lint, the air filter becomes clogged. Due to that, the airflow to the system becomes slower. 

The tankless system needs fresh air to fuel the combustion engine inside and for a clogged filter, it gets stopped. Consequently, the control panel shows error code 99. 

No worry! A quick cleaning is enough to resolve this issue:

How To Clean The Air Intake Filter In Tankless Water Heater?

Follow the below simple steps to clean the air intake filter in your water heater:

clean the rheem tankless water heater air filter

Step #1: Turn Off The Water Heater’s Power

As for this step, turn off the electric supply to your water heater. Flip the circuit breaker switch to the Off position or turn off the power from the control panel.

Step #2: Remove The Front Panel

Unscrew the front panel screws. Lift the cover straight off. 

Step #3: Locate The Air Intake Filter

Locate the air intake filter from just the top right corner of the inside panel. It will be facing upwards, clipped with the upper panel.

Step #4: Clean Out Air Filter

Remove removing the filter from the assembly. Take the filter screen under running and wash off all the filths and grim stuck on it. Use a brush to clean the filter.

Step #5: Reassemble The Water Heater

Once done, dry off the filter using a towel. Re-install it back in place. Attach the front cover and restore the power to the unit. 

3. Clogged Combustion Fan

Rheem gas tankless water heater features a combustion fan that draws the fresh air through the heat exchange and out the exhaust.

rheem gas tankless water heater clogged combustion fan

Then the gas valve opens and the ignition process starts on the burner. In such a way, it plays a great role in the combustion process. 

After long use, specks of dirt and grime clog up the combustion fans which creates restrictions on free moving. Due to that, the fan fails to pull free air into or out of the unit, and the combustion process gets interrupted. 

Then The PCB (control board) shows the error code. In this troubleshooting step, we will inspect and service the combustion fans.

How To Fix:

First off, make sure you have turned the gas valve off and unplugged the unit. Remove the heater’s front cover using a Phillips head screwdriver to inspect the inside. 

Now locate the GFCI mounting bracket plate. Remove the two screws and push the wires aside to detach the plate as you can see in the picture below.

detach the GFCI mounting bracket plate of rheem tankless water heater

When done, simply press the top connector and disconnect the fan from the wiring harness. It will be attached to the three fan screws. 

Just remove the two screws with a Phillips head screwdriver and pull out the entire fan motor from the housing.

Cleaning Procedures:

  • Take a brush and clean out the clogged dirt and debris from the motor. 
  • Use compressed air to remove the particles more effectively. 
  • If there is stuck grim on the fan blade, you need to soak it in a degreaser. 
  • Well! After cleaning, reassemble the fan assembly back in place. But before that, make sure it is dried off completely.  

4. Blocked Venting Or Exhaust Flue

In the case of error code 99 on the Rheem tankless water heater, blocked venting is one of the root culprits to blame. Higher chances are the venting and exhaust flue are blocked with shoots. 

rheem tankless water heater venting

Plus, birds’ nests, rodents, and wasps can also block the venting creating an interruption in the combustion processes. Consequently, due to a lack of combustion air or insufficient ventilation, error 99 pops up on the display. 

So, in this step, you have to inspect all the venting for your tankless system and clean it properly. Follow the below section.

How To Do:

First, check out the proper installation of your water heater. Make sure there is no object too close to your water heater that is blocking the air supply. 

Disconnect the venting structure from the unit and check for code. If still the same situation, inspect the intake and exhaust vents all the way to see any blockages. 

Before that, don’t forget to turn off the gas supply. Remove the vent hood if you can. Now, take a vacuum cleaner and insert the vacuum hose and extension through the venting port.

Vacuum up the entire area as much as you can. If there is any large debris stuck in the unreachable area, use a duct cleaner and pick up all the dust particles. Once cleaning is done, replace the vent hood.

Besides cleaning you have to ensure the correct exhaust venting installation with your water heater. For this instance:

Tip #1: Look at the picture below that is an incorrect connection of exhaust. So, you need to ensure the snugly fitted exhaust connection with the vent termination on the top of the heater.

incorrect exhaust venting

Otherwise, the combustion air comes from the room not from outside the house which may lead to a critical situation.  

Tip #2: There should be a minimum of 12 inches of vertical and 12 inches of horizontal difference between the intake and exhaust venting. This eliminates the possibility of exhaust back into the air intake. 

5. Corroded Heat Exchanger Fins

The error code 99 on the tankless water indicates combustion faults. If you have a non-condensing tankless water heater, the corroded heat exchanger is another leading reason to suspect. 

While the heating process, the heating elements on the heater come in contact with hard minerals like calcium or magnesium. Scale buildup occurs from the hard minerals and settled down into the heat exchanger’s fins. 

In such case, the heat exchanger fails to produce exhaust gases with the temperature raising and exhaust gas can’t able to release from the inside ring. 

How To Fix:

So as for the solutions, you need to descale and flush your water heater to remove the scale build-up from the heat exchanger. The flushing procedures of the tankless water heater are quite straightforward and you can perform it by yourself. 

Read our ultimate solution guide on Rheem tankless water heater code 29 to fix the corroded heat exchanger issue.

Here, you can also watch the video given below on how to fix the Rheem tankless water heater ER 99.


Do tankless water heaters need combustion air?

Yes! If it is a gas-operated tankless water heater, it certainly needs a certain amount of fresh air to use for combustion. Without entering the proper combustion air, the exhaust gases fail to vent out leading to ignition interruption. 

Do water heaters need to be flushed annually?

Yes! It is highly recommended to flush your water heater at least once a year. It prevents scale build-up in the essential heating elements and increases the heater efficiency.

How many years do electric water heaters last?

The factors depend on how well you are maintaining your water heater. Generally, it will last up to 10-15 years if it is an electrically operated water heater. A gas water heater lasts 8-12 years with regular maintenance. 

In Closing

Rheem tankless error code 99 is nothing but an abnormality in the combustion process that is caused by an improper mixture of fuel and air. When this code appears on the controlled display, your heater may shut down as a safety precaution. 

To prevent this trouble code in the future, you have to be concerned about proper maintenance of your Rheem tankless system. Inspect the entire heating components like the combustion fan, air intake filter, or heat exchanger at least once or twice a year. Check for any clogs or damages and service them. 

Well! That’s all from us here. Hope that there should be no confusion left over on Rheem tankless water heater code 99. If you have any queries, let us know in the comment section below. We’ll back to you soon with a 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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