Navien Tankless Water Heater No Hot Water [Reasons + Solutions]

The major reasons for Navien tankless water heater no hot water include:

  • Defective flow sensor.
  • Lack of combustion air.
  • Poor water flow.
  • Ignition failure.

Plus, plugged-up heat exchangers by the hard water mineral build-up and degradation in the power supply also interrupt the heater in heating the water.

navien tankless water heater no hot water

No worry! You can easily resolve the issue by yourself as long as you’ll follow our well-proven comprehensive guide to a T. Let’s start.

Table of Contents

Quick Solutions To Why Navien Tankless Water Heater Not Heating

Let’s walk you through a quick table, highlighting all the reasons and fixes for this issue to make your further reading easier.

Probable ReasonsQuick Solutions
Bad flow sensorReplace the defective flow sensor.
Blocked water inlet filterRinse the filter and clean the dirt. 
Plugged-up air intake filterScrub the air filter using a toothbrush.
Power supply issueEnsure the power cord of the heater is properly grounded. Reset the tripped circuit breaker.
Ignition or flame failureMake sure the gas valve is open. Clean the burner assembly. Replace the bad flame rod assembly.
Corroded heat exchanger Drain and flush your tankless water heater. 
Low water flowTurn on the water shut-off valve. 

Here are the complete details of all the major reasons and the effective solutions. Scroll down!

Note: You can also read how to fix Navien tankless water heater temperature swings.

1. Bad Flow Sensor

If there is no hot water from your Navien tankless water heater, be sure the flow sensor of the system has stopped working. You will find the sensor in the lower part of the unit, attached above the cold water connection. 

It mainly monitors the presence of water flow within the system and sends the rate to PCB.

bad flow sensor

Now in case of a broken flow regulator, it fails to send the correct signal to the controller and due to that, the PCB won’t maintain the flame. As a result, your Navien tankless system won’t heat the water.

How To Fix

First off, turn off the power supply to your water heater. Then, unscrew the front cover plate. Locate and inspect the flow sensor. Check it to see for any stuck dirt and debris. If so, remove the clamps from the sensor and clean it properly.

navien water heater flow sensor

If the flow sensor is defective or broken, you must have to replace it to get back your heater into normal operation. But there require professional skills to replace the sensor. So we recommend contacting a technician to complete the task.

Tips: It’ll be better to install a corrosion-free stainless steel flow sensor to prevent quick rust build-up and to ensure high process pressure from the turbine.

2. Blocked Water Inlet Filter

A tankless water heater equips a cold water inlet filter that is designed to ensure the water entering in your heater system. Plus, it helps to maintain the minimum water flow rate to the system to continue the heating process.

blocked water inlet filter

As it is always in contact with hard water, the building of minerals in the filter is quite obvious. A clogged filter fails to regulate the water flow and due to that, the heater won’t produce the hot water in the required amount. Only cleaning the filter will save you from this hassle.

How To Clean Tankless Water Heater Filter

Here are some simple processes to clean up your tankless water heater filter.

navien tankless water heater filter

Step 1: First off, unplug your water heater from the power unit. Or, you may simply turn off the power button on the front panel. 

Step 2:  Turn off the water supply from the shut-off valve located a the bottom.  Open the hot water faucets.

Step 3: Now pull out the water inlet filter from the bottom of the unit. You can use a pair of pliers if it’s too tight. 

Just unscrew the filter by turning it to the counterclockwise position.  Take a container underneath the filter while unscrewing and remove the filter. 

Step 4: Now remove all the grime and particles stuck in the screen under the running water pressure. You may also need a q-tip or a toothpick for stubborn particles. 

Step 5: Once done, let the filter dry off and then reinstall it back in place. Plug the unit and turn on the water supply.

3. Plugged-Up Air Intake Filter

In the case of the hot water issue in a tankless water heater, the air intake filter seems to be the root reason. On the top left of the heater inside, you will see an air intake screen that is sued to draw fresh air from outside. Plus, it keeps dirt and bugs out of the system. 

So, higher chances are that the air intake filter is plugged up by dirt and lint that is preventing enough combustible air to enter. That’s the fact, the heater won’t ignite due to lack of air and consequently, you won’t get the hot water, you’re desiring. You can fix this issue only by cleaning it. 

How To Clean Air Intake Filter In Tankless Water Heater

Follow the below steps to inspect and clean the air intake filter:

Step 1: Before starting, switch off the power connection to your water heater.

Step 2: Take a flathead screwdriver and unscrew all the nuts and bolts, holding the face plate.

Step 3: Once you have removed the cover plate, you will see a filter screen on the top left side. If you’re confused, look at the below picture, indicating the element:

 air intake filter screen

Step 4: The filter will be secured in place with a screw. Unscrew that and pull out the filter towards you.

Step 5: Clean out all the lint and dirt from the screen under the running water. You can scrub the screen with a toothbrush.

clean out all the lint and dirt from the air filter

Step 6: Once cleaning is done, let the air filter dry and then reassemble it as like before. Finally, restore the power supply to the heater. 

4. Interruption In Power Supply

If still, you are not getting hot water from your Navien water hater, the issue is most likely related to degradation in the power supply. There are a few factors that you need to consider to track down the power supply problems:

interruption in power supply
  • Tripped circuit breaker.
  • The unit isn’t plugged into a grounded electric outlet.
  • Insufficient voltage supply.
  • Worn out wiring.
  • Blown out fuse. 

How To Fix

Starting by checking the electrical main panel. Verify whether the circuit breaker is tripped. If so, push the circuit breaker into the “ON” position to get back it into normal operation.

Check for blown fuses and replace the bad fuses. Plus ensure that your heater is properly grounded and there is no electric separation in the outlet. 

Inspect all the wiring leading to the water heater. Replace the wires since any of them are damaged chewed or burnt. 

Now, all is okay and still, the same issue persists, understand the culprit is incorrect voltage going to the outlet. Measure the voltage reading with a voltmeter. 

Take note that, a tankless water heater requires a 240-volt dedicated circuit with a 120-amp breaker. If your voltmeter doesn’t give the reading in this range, contact a skilled technician to handle the task.

5. Ignition/Flame Failure

Ignition or flame failure is one of the leading reasons in case you’re encountering no hot water in your Navien tankless water heater. If the flame of the burner fails to hold or ignite, your heater will never produce hot water. 

Go through the below responsive factors behind this issue with their effective solutions: 

i). Gas Supply Issue

The gas supply issue is the leading one of the flame failure in the heater. Either gas valves are not fully opened or you may be using an undersized gas line for your heater. Another possibility could be a defective 3-way Valve inside the heater.

gas supply issue

How To Fix:

  • First off, open the gas valve all the way. Also, ensure you are using the right size gas line. 
  • Check the gas supply pressure water heater. It should be (3.5-10.5) in WC for natural gas and (8-13) in WC for propane. 
  • Inspect and replace the defective 3-way Valve. Contact a professional to handle the task.

ii). Bad Igniter Or Flame Rod

Non-operation igniter or flame rod is another bigger reason for heating issues in the tankless system. The igniter is an essential part of the tankless water heater that allows the ignition to start by generating the spark. 

tankless water heater flame rod assembly

In case of defection in the igniter or flame rod assembly, it fails to do so. So, we recommend inspecting the assembly and replacing it if needed. 

iii). Dirty Burners

To the fact of no hot water from the Navien water hater, the dirty burner would be a major reason to blame. Chances are there is condensation or debris on the burner that is failing it ignite. So, inspect the burner and clean it using fine sandpaper of 100 grit.  

6. Mineral Buildup In The Heat Exchanger

The tankless water heater is enough susceptible to becoming partially blocked by mineral or sediment buildup. Especially the heat exchanger may clog by hard water minerals like calcium and magnesium that disrupt its efficiency.

The heat exchanger is mainly designed to raise the unit temperature by transferring heat from one source to another. So normally, it won’t let to heat the water due to plugged up. 

Most of the time, sediments also spread to the entire plumbing system like pipes, water filters and other parts

If you are experiencing such an issue with your heater, only flushing the system is the only solution to get rid of this problem. 

How To Flush A Tankless Water Heater

Follow the below few phases to flush descale your tankless water heater:

Phase 1: Turn Off The Power Supply & Isolation Valves

In the first step, unplug your water heater from the power connection and turn off the gas valve. Then, turn off the isolate valves (blue inlet valve & red outlet valve), connected to your water heater. The valve’ lever should be perpendicular to the pipe.

Phase 2: Connect The Hoses

Next, remove the service port cap from the valve. Attach the hoses to the inlet and outlet valves. It’ll be better to watertight the valves using pliers.

connect the hoses

Phase 3: Connect The Hoses To The Circulation Pump

Now, attach the other end of the hoses to the circulation pump. Drop the pump discharge side in a 5-gallon bucket.

Phase 4: Add Cleaning Agent & Open The Valves

In this step, add a cleaning agent to the bucket. Alternatively, you can use vinegar. Open the inlet and outlet valves. Let the chemical water circulate in the heater for about 45 minutes to one hour.

Phase 5: Flush Out The Cleaning Agent

Once done, turn off the pump and both valves. Disconnect the cold inlet valve hose and keep the other hose connected.

flush out the cleaning agent

Turn on the cold water valve and flush out the remaining cleaning agent through the hot water side.

Phase 6: Re-assemble The Heater

Turn off the cold water inlet valve, disconnect the hose and put everything back in place. 

7. Low Water Flow

The tankless water heater unit requires a minimum water flow per minute to produce hot water. If somehow there is not sufficient water flow as the recommendation, the heater will fail to heat up the water.

Most of the time, it happens due to using multiple hot-water fixtures at a time, overloading the system. That’s the point, your heater fails to fulfil the demand of all, leaving cold water at last. Plus, the partially closed water shut-off valve is also responsible for this issue.

How To Fix

  • First off, head to the water shut-off valve and open it all the way.
  • Besides, check for leaks in the shut-off valves and fix them.
  • Ensure that you are using the right size tankless water heater according to your family size.
  • If needed, inspect the plumbing pipes for any blockages.  Also if there is a pressure regulator installed in your plumbing system, make sure it is set in between 40-60 psi.  

5 Major Navien Tankless Water Heater Error Code For No Hot Water

Most often users encounter several error codes on the digital display and stop heating water. So, unlike the above major reasons, all these error codes are also concerning factors in the case of the heater heating problem.

navien tankless water heater error code

So, it’s better to get knowledge about all related error codes for this issue. Have a look through the below table, highlighting the codes, their means, and the quick solutions:

Error CodesError IndicationQuick Solutions
E002Low water pressure> Increase the water pressure & the unit will reset itself.
E003Ignition failure> Ensure the main gas supply is on & the igniter is sparking.
E004False flame detected> Ensure the electrical cord is ground and verify that the igniter delivers a spark.
> Replace the bad flame rod assembly.
E012Flame loss> The gas valve should be fully open.
> Check the burner & flame rod assembly.
E515Abnormal operation of the PCB> Try a new outlet or replace the PCB if necessary. 


How to reset the Navien Tankless water heater?

First, find the reset button on the front control panel, located opposite the power button. Just press the button to reset your tankless water heater. And the heater system will reset. 

How to prevent corrosion in tankless water heater heat exchangers?

Flushing the water heater tank once a year is the most effective way to prevent hard water mineral accumulation in the heater element. Also, you can install a spin-down sediment filter to preserve the heater’s efficiency. 

What cleaning agent should I use for flushing the tankless water heater?

There are a bunch of flush kits available in the market. Choose biodegradable modules added a descaler for harmless cleaning of the heater. Avoid using chemical solutions of any kind in this case.  

Wrapping Up!

Well! That’s all from us here about Navien tankless water heater no hot water issue. Hope that all the queries and confusion about the problem should go away now. 

Throughout this guide, we have input all responsive factors and solving methods that will prove enough effective to tackle the issue with ease. 

Follow each of our suggested processes right in order. Do let us know if you face any more troubles while doing so. We’ll give our level best to help you out!   

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 .

Leave a Comment