Navien Tankless Water Heater Won'T Stay On - A Pro'S Step-By-Step

There’s nothing more frustrating than stepping into the shower, expecting a wave of comforting hot water, only to be hit with an icy blast seconds later. You hear your Navien tankless water heater click on, then mysteriously click off. It’s a common problem, but the good news is that you’ve come to the right place.

We know how jarring it is when your high-efficiency unit isn’t performing as it should. The thought of complex repairs and cold showers can be stressful. But don’t worry—we’re here to help you get that steady stream of hot water back.

We promise this guide will walk you through the most common reasons a Navien tankless water heater won’t stay on. We’ll cover simple, safe checks you can perform yourself, explain what might be happening inside your unit, and clarify exactly when it’s time to call in a certified technician.

In this article, you’ll learn how to read error codes, inspect critical airflow paths, check for simple blockages, and understand the basic principles that keep your Navien running smoothly. Let’s get that water heated up!

First Things First: Let Your Navien Tell You What’s Wrong

Before we start taking things apart, let’s do the smartest thing first: listen to the machine. Modern Navien units are equipped with a sophisticated self-diagnostic system. When something is wrong, it will usually display an error code on its front panel.

This code is your single best clue. Think of it as your water heater’s way of saying, “Hey, the problem is over here!”

How to Find and Interpret Common Navien Error Codes

Simply look at the digital display on the front of your unit. If the system has detected a fault, it will flash a number, often preceded by an “E.”

While your user manual has a full list, here are a few common culprits when your Navien tankless water heater won’t stay on:

Error Code E003 (or 03E): This means Ignition Failure. The unit tried to light the burner but failed. This could be a gas supply issue or a problem with the igniter itself.
Error Code E012 (or 12E): This indicates Flame Loss. The burner successfully ignited but the flame went out unexpectedly. This is a classic symptom of the “on-then-off” problem and often points to gas pressure, venting, or a dirty flame sensor.
Error Code E110 (or 110E): This signals a Blocked Exhaust or High Wind condition. The system detects that exhaust gases aren’t venting properly, a critical safety issue that will shut the unit down immediately.
Error Code E227 (or 227E): This points to a False Flame Detection, which can be caused by a faulty sensor or mainboard issue.

If you see a code, write it down. This simple step can save you (or your technician) a ton of diagnostic time. Resetting the unit by unplugging it for a few minutes and plugging it back in can sometimes clear a temporary fault, but if the code returns, it’s time to dig deeper.

The “Big Three” of Tankless Troubleshooting: Air, Fuel, and Water

At its core, your Navien needs three things to work correctly: a steady flow of air for combustion, a consistent supply of fuel (natural gas or propane), and proper water flow to tell it when to turn on. When your unit short-cycles, one of these three is almost always the cause.

This section provides a complete navien tankless water heater won’t stay on guide to checking these core components safely.

Is Your Navien Getting Enough Air? Inspecting Intake & Exhaust Vents

Your Navien unit “breathes” through two pipes that lead outside—one for fresh intake air and one for exhaust. If either of these is blocked, the unit can’t operate safely and will shut down.

Here’s how to perform a quick visual inspection:

Locate Your Vents: Find where the PVC pipes exit your home. This is usually on a side wall or through the roof.
Check for Obstructions: Look for anything that could be blocking the openings. Common culprits include leaves, dirt, bird nests, snow, or ice.
Clear Debris Carefully: If you see a blockage, carefully clear it away. Make sure the termination caps or screens are clean and free of debris.

Pro Tip: Navien systems are very sensitive to proper venting. If the pipes are too long or have too many bends, it can also cause flame loss. While you can’t change the installation, knowing this can help your technician diagnose a tricky issue.

Checking the Fuel Supply (Gas Pressure & Lines)

This is where we add a big dose of caution. Working with gas lines is not a DIY job and should only be handled by a licensed professional. An incorrect adjustment can be extremely dangerous.

However, there are a couple of safe checks you can do:

Check Other Gas Appliances: Is your gas stove or furnace working correctly? If not, you may have a problem with the gas supply to your entire home. Contact your utility company.
Verify the Gas Valve is Open: Find the small gas shut-off valve on the pipe leading to your Navien. The handle should be parallel (in line with) the pipe, which means it’s open. If it’s perpendicular, it’s closed.

A very common reason for flame loss (Error E012) is insufficient gas pressure. Your Navien needs a specific, stable pressure to run. If other large gas appliances (like a furnace) kick on, they can temporarily drop the pressure enough to cause your water heater to shut off.

Water Flow Issues: The Hidden Culprit

Your Navien doesn’t know you’ve turned on the hot water tap. Instead, it relies on an internal flow sensor. If this sensor doesn’t detect a sufficient flow rate, it won’t fire up or will shut down prematurely. This is one of the most common problems with navien tankless water heater won’t stay on situations.

The number one cause of flow issues is a clogged filter. You have a small screen filter on the cold water inlet to protect the unit from sediment. Here’s how to check it:

Turn Off Power, Water, and Gas: Safety first! Unplug the unit or turn off the breaker. Close the main water valve leading to the heater (usually a blue handle on the isolation valve kit) and the gas valve.
Relieve Water Pressure: Open a nearby hot water tap to drain residual water and relieve pressure in the lines.
Locate and Clean the Filter: On the bottom of your unit, the cold water inlet pipe has a small, removable filter. You may need pliers to carefully unscrew the plug. Gently remove the screen and rinse it thoroughly under a faucet until it’s clean.
Reassemble and Test: Re-insert the clean filter, tighten the plug, and slowly turn the water back on, checking for leaks. Then, restore power and gas and test a hot water tap.

This simple cleaning solves a surprising number of issues and is a key part of our navien tankless water heater won’t stay on tips.

Advanced Troubleshooting: Internal Components and Maintenance

If the basics didn’t solve your problem, the issue may lie with an internal component or a maintenance oversight. This section will help you understand what a technician will look for next.

Flame Sensor and Igniter Issues

Inside the combustion chamber are two critical parts: the igniter (which creates the initial spark) and the flame sensor (a thin metal rod that confirms a flame is present). Over time, the flame sensor can get coated with residue.

When this happens, the sensor can’t properly detect the flame, even though it’s burning. The system’s brain thinks the flame has gone out and, as a safety measure, shuts off the gas supply. This is a textbook cause of the “on-then-off” cycle. Cleaning or replacing a flame sensor is a job best left to a professional, but understanding its function is key to diagnosing the problem.

Condensate Line Blockages

As a high-efficiency condensing unit, your Navien produces a small amount of acidic water (condensate) as a byproduct of combustion. This water drains out through a small tube.

If this tube gets clogged with debris or freezes in the winter, the water will back up inside the unit. A safety sensor will detect this and shut the system down to prevent a leak. Check the clear or white plastic tubing leading from your unit to a drain. If you see standing water in it, it may be blocked and need to be carefully cleared.

A Proactive Care Guide: Prevention is the Best Medicine

The best way to deal with a Navien that won’t stay on is to prevent the problem in the first place. Following a few navien tankless water heater won’t stay on best practices can save you headaches and extend the life of your unit.

Adopting a routine is a sustainable and eco-friendly navien tankless water heater won’t stay on solution, as a clean unit is an efficient unit.

Annual Flushing: Your Best Defense Against Scale

If you live in an area with hard water, mineral scale (calcium and magnesium) is your unit’s worst enemy. This scale builds up inside the delicate pipes of the heat exchanger, restricting water flow and forcing the unit to work harder.

Annual descaling or “flushing” is the most important maintenance task you can perform. This involves circulating a solution (like diluted white vinegar) through the unit to dissolve these mineral deposits. While DIY kits are available, we recommend having a professional perform this service to ensure it’s done thoroughly and correctly.

When to Call a Certified Navien Technician

We’re all for empowering homeowners, but safety is paramount. You should stop troubleshooting and call a professional immediately if you encounter any of the following:

You Smell Gas: Do not touch anything electrical. Leave the house immediately and call your gas company from a safe distance.
Persistent Error Codes: If you’ve tried a reset and basic checks but the error code keeps returning, it’s time for an expert.
Water Leaking from the Unit’s Casing: A leak from a pipe fitting is one thing, but a leak from the main body of the heater indicates a serious internal problem.
You’re Uncomfortable with Any Step: If you don’t feel 100% confident, it’s always better to call a pro. That’s what we’re here for!

Frequently Asked Questions About Navien Water Heaters

Why does my Navien water heater click on and off repeatedly?

This is called short-cycling. The most common causes are a dirty inlet water filter restricting flow, low gas pressure, or a dirty flame sensor that can’t confirm a stable flame is present, causing the unit to shut down for safety.

How do I reset my Navien tankless water heater?

The simplest way to perform a “soft reset” is to unplug the unit from the electrical outlet (or turn off its dedicated breaker), wait for five minutes, and then restore power. This can clear temporary electronic glitches but won’t fix an underlying mechanical or sensor issue.

What are the benefits of fixing a Navien tankless water heater that won’t stay on?

The primary benefit is restoring reliable, on-demand hot water! Additionally, fixing the underlying issue ensures your unit is running at peak efficiency, which saves you money on energy bills and prevents more costly damage down the road. A properly functioning unit is also a safer unit.

We hope this comprehensive navien tankless water heater won’t stay on care guide has empowered you to tackle this common issue. By starting with the simple checks—error codes, vents, and filters—you can often solve the problem yourself or, at the very least, provide valuable information to your service technician.

Remember that consistent maintenance is the key to years of reliable service from your high-tech water heater. A little proactive care goes a long way in preventing those cold morning surprises.

Here’s to steady, endless hot water. Stay safe and stay comfortable!

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David Clark

Founder & Lead HVAC Specialist at HVACRLY

David Clark is the founder of HVACrly and a certified HVAC specialist with 10+ years working on residential and light-commercial systems. He turns complex heating, cooling, ventilation, and refrigeration topics into clear, actionable guides focused on safety and energy efficiency. When he’s not on a service call, he’s testing tools and writing step-by-step tutorials.