Rheem Tankless Water Heater Code P1 [Why+ How To Fix]

Rheem Tankless Water heater code P1 is mainly the error indication of low water flow through the unit. The root reasons behind this error code include:

  • Clogged aerators.
  • Blocked water inlet filters.
  • Malfunctioning of the water flow sensor.
  • Sediment deposits in the heater.

Also, a higher temperature setting and using an undersized tankless water heater will lead to the diagnostic error code P1. 

The problem can be solved by cleaning the aerators and by checking the water pressure of the water line. Also, cleaning the clogged water inlet filter and aerators is the potential solution to erase the error code.

rheem tankless water heater code p1

If that doesn’t work well, be sure sediment clogged up the heating element which needs to flush out. Read our comprehensive guide to learn the details.

Quick Solutions For Rheem Tankless Water Heater Code P1

Before moving to the detailing parts, let’s have a short glimpse at the quick fixes for the error code P1 on the tankless water heater:

Probable Reasons Possible Solutions
Clogged aerators> Disassemble the aerators on the faucets and clean out the stuck debris with any sharp objects. 
Blocked water inlet filter> Clean the water filter screen to remove the particles.
Malfunctioning of the water flow sensor> Replace the bad flow sensor.
Sediments deposited inside the water> Drain and flush out your water heater. 
Low water pressure from the water line. > Look for corrosion in the pipeline.
> Clean or replace the clogged cartridge.
> Increase the water pressure from the pressure regulator.
> Lower the heater temperature from the preset one. 
Undersized water heater> Upgrade your water heater to the right size. 

How To Fix Error Code P1 On Rheem Tankless Water Heater [7 Easy Solutions]

Here are all responsive factors for error code P1 on Rheem tankless water heater with the effective solutions:

Note: You can also read our guide on why Rheem tankless water heater code E5 and how to fix it.

1. Clogged Aerators

If there is not sufficient water pressure in your tankless system, higher chances are the aerators on the faucets are clogged.  An aerator is basically a flow-restricting device that is attached to the tip of faucets.

It reduces the amount of water coming out from the faucet and creates a consistent, straight stream of water.

As the aerator is always in contact with the hard water, thus it will get clogged from the deposits of minerals, leading to low water flow. In such cases, you will need to clean the clogged aerators to restore the optimum flow rate. 

The cleaning processes are quite straightforward to do. Let’s move to the how-to section.

How To Clean Clogged Faucet Aerators

Follow the below simple steps to clean a faucet aerator:

Step 1: In the first step, unthread the faucet aerator counterclockwise and remove it from the end of the faucet spout. If needed, use pliers.

 faucet aerator counterclockwise

Step 2: Now take a small screwdriver and pry out any stuck parts inside the aerator.

Step 3: In this step, disassemble your faucet to clean the aerator thoroughly using a knife tip or toothpick. Now rinse off large pieces of sediment that are stuck in the aerator screen.

sediment stuck in the aerator screen

Step 4: You may need to use a degreaser to clean the grease-based. For this instance, use a dish detergent mixed with hot water in a container.

Then soak the aerator in the solution overnight to dissolve the mineral deposits. Instead, you can use vinegar but avoid using any corrosive chemicals.

Step 5: Scrub the aerator parts with a toothbrush and rinse under running water. When done, reassemble the aerator parts and attach them back onto the spout of the faucet. 

2. Blocked Water Inlet Filter

All tankless water heaters including the Rheem model equips a water inlet filter that is used to protect the heater from the damage of scale accumulation.

Plus it plays a great role in regulating the water flow of the system to a minimum rate that is essential for a continuous heating process. 

blocked water inlet filter

But after long use, scale builds up on the filter screen leading to reduce the water flow through the heater. Consequently, you will get the error code P1 on the display. 

So, in this troubleshooting step, you need to disassemble the water filter and clean it. Also, experts recommend cleaning out the filter once every six months to maintain the heater’s efficiency. 

How To Clean Rheem Tankless Water Heater Filter

Go through the step-by-step process to clean the water filter of your tankless water heater:

Step #1: Start by turning off the power to the unit. Unplug the unit from the power connection from the circuit breaker or turn off the power button on the front panel. 

power button of the rheem tankless water heater

Step #2: Now, turn off all the cold water shut-off valves from the bottom of the unit. 

Step #3: Loosen the filter from the connection by unscrewing it counterclockwise. First, try with your hand or use oilers if needed, Place a little container or jar while pulling out the filter. 

Step #4: Detach the plastic that holds the filter in place. It will look like the below picture. 

 detach the plastic holds from the filter

Step #5: Now clean out all the grim and particles that caught up in the screen under running cold water. Use a Q-tip or toothpick for any stubborn particles from inside the screen.

Step #6: Once done, tap it and confirm there is no remaining grim. Finally, reassemble the filter back to its place.

3. Malfunctioning From The Water Flow Sensor

If there is low water flow in the tankless water heater system, malfunctioning of the water flow sensor is another root culprit to blame.

After cleaning out the filter screen and still having the same water flow issue, be sure it is time to inspect the flow sensor. 

faulty water flow sensor

The water flow sensor is basically a reed sensor within the tankless water heater that is used to detect the presence of water flow and initiate the heating process by sending the rate to PCB. 

If somehow, the flow sensor stops working or is broken, it will not monitor the water flow and also fail to send the signal to the controller. Due to malfunctioning from the water flow, the PCB will show the P1 error code on the screen. 

How To Fix:

First up, turn off the power supply to the heater and also turn off the water shut-off valve. Unscrew the front access to access the flow sensor. Locate the sensor from the lower part of the unit that is attached just above the cold water connection. Confused? See the below picture.

locate the sensor from the lower part

Pull out the sensor and inspect it for any stuck grim and particles. Higher chances are it gets clogged up and due to that it fails to spin freely, leading to low water flow. So, clean out the dust from the sensor by blowing air into it. 

Then, monitor the sensor to see whether it is broken. If so, replace it with the new one. Contact a professional to replace the bad flow sensor. 

4. Sediment Deposits In The Heating System

While the heating process, your tankless water heater always comes in contact with hard water. Minerals from the water like calcium and magnesium get hard and settle down in the heating elements.  

When sediment builds up, you will encounter several malfunctions in the system including low water flow.

Basically, mineral deposits and sediment block the water flow and prevent your water heater from running at its peak efficiency. The good news is that that problem can be solved by just flushing the heater.

How To Flush A Rheem Tankless Water Heater

Go through the below few phases to drain and flush out the tankless water heater:

Phase #1: Switch Off The Power Supply & Isolation Valves

As for the first step, turn off the power connection to your water heater and close the gas isolation valve. Then, turn off the blue inlet valve & red outlet valves connected at the bottom of the heater. Let the heater cool. Never drain the scalding hot water.

Phase #2: Attach The Garden Hoses

Take two large hoses that you will fit into the inlet and outlet valves. Now detach the valve port cap and attach the hoses there. Ensure a water-tight seal with pliers.

attach the garden hoses

Phase #3: Connect A Circulation Pump With The Hoses

This time you will need a circulation pump that will help to discharge the sediment deposits from the heater. If you are using a flushing kit, you will get a pump, bucket, de-scaler, and hoses including everything in the kit. 

connect a circulation pump with the hoses

Simply connect the other end of the hose to the pump and place it in a large bucket.

Phase #4: Add The Cleaning Agent

In this step, you will have to use a cleaning agent. Take a cleaning agent and discharge it into a five-gallon bucket. 

add the cleaning agent 

Turn on both valves. Right then the chemical will start circulating through the system. Wait for 45 minutes to one hour for proper cleaning. This will remove the scale and flushes out the sediment.

Phase #5: Flush Out The Cleaning Solution

  • After time passed, turn off the pump and both valves. 
  • Just disconnect the hose from the cold inlet valve and keep the red valve connected. 
  • Turn on the cold water shut-off valve to remove the remaining cleaning agent through the second hose into your bucket.

Phase #6: Reassemble The Water Heater

  • Once done, close the valve and detach the hose from the port. Reassemble everything back in place. Reconnect the electrical supply.
  • Now check the water flow of your water heater. 
  • Take note that, it’s better to flush your tankless water heater at least once per year to maintain efficiency.

5. Low Water Pressure From The Water Line

Aside from all the above factors relating to the water heater, it is quite essential to determine that there is sufficient water pressure in your house.

low water pressure from the water line

There’s a good chance that corrosion builds up inside the plumbing pipe that is restricting the water flow. This mostly happens if you are living in an old house.

Besides, if you have a single-handle faucet, the cartridge may be blocked with minerals. Or, it’s also possible that the cartridge inside the valve is misaligned. 

All that factors will greatly impact the water flow, leading to the error code P1 on your tankless water heater.

How To Fix:

Follow the below simple guide to diminishing the water pressure limitations in the plumbing in your house:

  • First off, you have to increase the water pressure of your house. Locate the water pressure regulator for the water lines. It will be nearer to your tankless water heater.
  • Just loosen the locknut and turn it clockwise to increase the pressure. Make sure the pressure of your house should be in between 45 psi-60 psi.
  • Then head to the shower faucet or shower head of your house. Disassemble the faucet and look for hard minerals build up in the cartridge. Clean out the cartridge and make sure it is aligned properly with the shower valve.
  • Most of the time, tankless water heaters restrict water flow until the water reaches the preset temperature. According to the BEC Green, the higher the temperature, the lower the flow rate. In such cases, lowering the temperature will increase the flow rate.
  • Also, you won’t get much pressure if the pipes are too narrow. check for corrosion inside the pipes of your house. You have to replace the corroded water pipes with new ones. Contact a professional to handle the best task for you.

6. Undersized Tankless Water Heater

Behind a low water flow in the tankless water heater, higher chances are that you are using an undersized tankless system. Choosing the right size water heater is quite essential to maintain your household’s hit water needs effectively. 

If you are using a too-small tankless system, the water flow rate through taps will negatively be impacted.

Basically, here the temperature of the water will constantly warm but the flow rate from the fixture diminishes. The diminished water flow causes error code P1 on the screen.

How To Fix:

As for the fix, you need to upgrade your tankless system to the right size. The following estimates provide you with a short description of the capacity when to purchase a new tankless water heater for your home: 

Number Of Plumbing Fixtures In HouseMinimum Flow Rate
1 to 2 fixtures3.5 GPM
2 to 3 fixtures 5 GPM
3 to 4 fixtures7 GPM
5 or more fixtures Higher than the before ones

Also, prefer to have a gas tankless water heater as a gas tankless water heater can ensure more hot water at higher flow rates than the electric tankless one. The below chart will give you a better idea:

electric and gas rheem tankless water heater flow chart

Also, ensure that the flow rates will meet the peak demand at your desired temperature. To figure out the peak demand, just add the number of fixtures that you want to use for your water heater with their flow rates.

7. Reset Your Tankless Water Heater

In most cases, only resetting your tankless water heater will prove well to clear the code from the PCB. So, before checking the above issues, first, try the reset. Here learning how to reset Rheem tankless water heater will help you make the job simple. 


What is the best flushing kit for flushing a water heater?

There is a wide range of flushing kits available for regular maintenance and to keep your heater clean. You can choose the Kelaro flushing kit or Chromex flush kit to eliminate deposits inside the water heaters. My Plumbing Stuff is a non-toxic, non-hazardous, and non-corrosive flush kit that you can choose. 

How do you flush the heavy sediment on a water heater?

You can use effective chemicals to flush out the heavy sediment. Let the chemicals sit inside the tank to diagnose the hard mineral. Then drain and flush your tank multiple times.

Do I need a water pressure regulator for my tankless water heater?

Yes! If there is too higher water flow on your house’s plumbing fixture, you can install a pressure regulator. It will reduce the system pressure to a minimum level. The water pressure level should not exceed  80 psi

In Closing!

While using your tankless water heater, you may notice a sudden drop in pressure that leads to the error code P1. In such cases, cleaning the essential heater parts like the water inlet filter should be your first troubleshooting step. 

If needed, flush and drain the water heater to diminish the sediment buildup that is restricting the water flow. Also, do concentrate on whether you are using the right size heater by maintaining the correct water flow. 

Well! That’s all from us here for you. Hope that there is no confusion left over on Rheem tankless water heater code P1. Do share your valuable thoughts regarding our guide in the comment section.

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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