Navien Tankless Water Heater No Hot Water

Navien Tankless Water Heater No Hot Water [5 Easy Solutions]

Your Navien Tankless Water Heater will deliver no hot water due to the following reasons: 

  • No power (both electric and gas) goes to the unit
  • Dirty cold inlet water filter
  • Bad flow sensor
  • Heat exchanger gets clogged by scale buildups
  • Defective 3-way valve

In this Navien troubleshooting guide, I will walk you through every step to troubleshoot those issues. Plus, I will show you how to fix those water heater problems. 

So, read on between the lines to discover the solution and let the water heater produce hot water again. 

Table of Contents

This chapter will describe the reasons that cause you water heat to deliver no hot water. 

01. No Power To The Unit

The Navien Tankless Water Heater is generally operated by gas. But you need to connect it with electricity. If you don’t plug it into a properly grounded electric outlet, you can’t turn the unit on. And certainly, the water heater won’t heat water until you start it up. 

Similarly, the water heater won’t ignite if no gas flows into its combustion chamber. In short, no gas means no ignition. Turns out, feeding the unit with an inappropriate fuel type can cause the hot water heater to not heat the water. 

How To Fix: 

Start with ensuring that you plug the tankless water heater into an electric outlet. Most importantly, make sure you insert the power cord into a properly grounded electric outlet that supplies the electric power the unit needs. 

Indeed, ensure plug the water heater into a 120VAC electric outlet. Don’t forget to inspect the condition of that outlet you use to power up the heater. If there is any electric issue, call a certified electrician for help. 

Apart from proper electric power, the Navien tankless water heater also requires the right amount of gas for combustion. Inspect the main gas control valve and ensure you turn it to the On position. 

Next, ensure the unit gets proper gas supply pressure.

The following chart will show you the proper gas supply pressure you need to ignite the water heater, depending on the gas type: 

Gas TypeMinimumMaximum
Natural3.5 in WC10.5 in WC
Propane8 in WC13 in WC

Finally, use the right type of gas your water heater requires. Check the unit and ensure what type of gas it needs for ignition. 

02. Dirty Cold Inlet Water Filter

A water heater requires a minimum amount of water flow per minute to heat the water. What amount of water flow is proper mostly depends on the size of a water heater. If the water flow is inappropriate, your tankless water heater will produce no hot water. 

And a cold inlet water filter is first to blame if there is an issue with the water flow. If this filter gets clogged by dirt or minerals, you will never get the proper flow. You must clean the cold inlet water filter periodically to maintain proper water flow. 

I will show you how to clean that filter in the ‘How To Fix’ section. So, continue to read. 

How To Fix:

To clean the cold inlet water filter, follow the steps described below: 

1. Turn off the unit by pressing the power button on the front panel. 

2. Place a plastic bucket underneath the unit to catch water that will come out of the unit. 

3. Shut off the gas valve by turning it clockwise. 

Closing the gas valve
*Closing the gas valve*

4. Either close the water supply valve or shut off the water supply to your entire plumbing system. 

5. Open all the hot water faucets. 

6. Pull out the recirculation inlet filter from the bottom of the unit. 

Location of recirculation filter, cold inlet filter, and drain plug

7. Open the pump drain plug. 

8. Remove the cold water inlet filter from the bottom of your water heater. 

9. Let the water drain from your water heater. 

10. As it takes time to drain the water, I recommend you check the cold water inlet filter for a clog. Rinse the filter with cold water. If needed, scrub it with a brush. 

11. Once you clean the cold water inlet filter, reinsert it to its place and tighten it up. 

12. Refill the filter by following these steps in reverse. 

03. Defective Flow Sensor

A bad or broken flow sensor can also be the culprit behind this issue. The flow sensor actually measures how much water comes into the unit and sends this information to PCB. If the sensor senses a burst of cold water entering into the system, the controller board adjusts the flame. 

But the controller fails to adjust the flame correctly if it receives intermittent signals because of a broken sensor. Consequently, you will get cold water from the unit. 

How To Fix: 

You must replace the flow sensor if it’s at fault. If you are mechanically inclined, watch out this tutorial to learn how to change the flow sensor on Navien: 

However, it will be best to call a qualified technician to do this task for you if you are uncomfortable with the replacement. 

04. Dirty Heat Exchanger

One common water heater issue that can throw a wrench into work is a dirty heat exchanger. Over time, the heat exchanger gets clogged by scale and mineral deposits. 

If you live in a hard water area or operate the unit at higher temperature settings, it will form the scale or lime deposits more quickly. 

And a dirty heat exchanger fails to heat the water efficiently. As a result, you will get no hot water out of your Navien tankless water heater. 

How To Fix: 

Descaling the water heater is an easy solution to this problem. Before flushing the unit, ensure you have the following pieces of equipment at your fingertips: 

  • A 5-gallon capacity plastic bucket
  • Cleaning solution diluted with water
  • 3 hoses
  • A water circulation pump

Then, follow the flushing directions shown in this tutorial: 

05. 3-way Valve Gets Defective

A defective 3-way Valve is another caveat that prevents your Navien tankless water heater from producing hot water. 

How To Fix: 

It will take a lot of elbow grease to replace the 3-way valve. So, I strongly recommend you hire a professional to change the defective 3-way valve with a new one. However, if you are mechanically inclined, you can check this tutorial:


How do you reset a Navien tankless water heater?

Resetting a Navien Tankless Water Heater is easier than turning on a water heater. Just press the Back button on the front panel and it will reset this tankless unit successfully.

What would cause a tankless water heater to go cold?

Low water pressure is first to blame if your tankless water heater goes cold. Every tankless water heater requires a minimum water flow to activate the unit to heat the water. If the water pressure is getting lower than the recommended activation flow rate, it will shut off the burner completely. Consequently, your tankless unit will go cold.

What are the most common problems with tankless water heaters?

Flame failure or false flame detection, ignition failure, scale buildup, clogged air inlet filter, and system overload are some common issues with a tankless water heater.

Is there a reset button on a tankless water heater?

Yes, every tankless water heater has a reset button. If you talk about Navien, you can find this button on the front panel known as Back button. Indeed, the Back button meets the purpose of a reset button.


Getting no hot water out of the Navien tankless water heater is a common problem. I broke down everything responsible for this issue. Just follow each troubleshooting step I mentioned and it will help you bring the unit back to its normal condition. Best of Luck!

Read Also:

Similar Posts

Leave a Reply

Your email address will not be published. Required fields are marked *