Navien Tankless Water Heater Not Turning On: A Pro'S Step-By-Step

There’s nothing quite like the sinking feeling of turning on the hot water tap and being met with an endless stream of cold. You wait, and you wait, but the promised warmth never arrives. It’s a frustrating moment, especially when you rely on your high-efficiency Navien unit for that on-demand comfort.

We get it. As HVACR technicians, we’ve seen this scenario play out in countless homes. The good news is that when your navien tankless water heater not turning on is the problem, the solution is often simpler than you think.

We promise this guide will walk you through the exact same steps a professional would take, from the easiest DIY checks to understanding what your unit is trying to tell you. We’ll preview the simple power and fuel checks, dive into common error codes, inspect for blockages, and clarify exactly when it’s time to call for backup.

Let’s get that hot water flowing again, together.

First Things First: The Simple Checks Before You Panic

Before you start worrying about complex repairs, let’s cover the basics. More often than not, the issue is something simple that can be resolved in minutes. Don’t worry—we’ll walk through these safe checks together.

Is It Plugged In? Checking the Power Supply

It might sound obvious, but you’d be surprised how often this is the culprit! Modern tankless water heaters, including your Navien, need electricity to operate their control panels, fans, and ignition systems.

Check the Outlet: Make sure the unit is firmly plugged into its power outlet. Look for a GFCI (Ground Fault Circuit Interrupter) outlet—the kind with “TEST” and “RESET” buttons. If the “RESET” button has popped out, press it back in until it clicks.
Check the Breaker: Head to your home’s main electrical panel. Look for the breaker labeled “Water Heater” or similar. If it’s tripped (usually in the middle or “OFF” position), flip it firmly to “OFF” and then back to “ON”.

Is the Gas On? Verifying Fuel Flow

No fuel, no fire. Your Navien needs a steady supply of natural gas or propane to heat the water. This is another one of the most common problems with navien tankless water heater not turning on.

Locate the gas shut-off valve on the pipe leading to your water heater. The valve handle should be parallel to the pipe, indicating that gas is flowing. If the handle is perpendicular to the pipe, it’s closed. Gently turn it to the parallel position.

Safety First: If you smell gas (a rotten egg odor), do not touch any electronics or light switches. Leave your home immediately and call your gas utility company from a safe distance.

Check Your Thermostat and Remote Control Settings

Sometimes the issue is just a simple setting. If your Navien has a remote controller, ensure it has fresh batteries and is set to the desired temperature. Make sure the unit itself hasn’t been accidentally turned off or set to a very low temperature via its main control panel.

Understanding Navien Error Codes: Your Heater’s Cry for Help

One of the best features of Navien units is their self-diagnostic system. When something is wrong, your unit will display an error code on its digital screen. This code is your single most valuable clue. This navien tankless water heater not turning on guide would be incomplete without covering them.

Where to Find the Error Code

The error code will be displayed directly on the front panel of your Navien water heater. It’s typically a two or three-digit number, sometimes preceded by the letter ‘E’. Write this number down before you do anything else.

Common Navien Error Codes (and What They Mean)

While your owner’s manual has a full list, here are a few of the most frequent codes we encounter when a Navien tankless water heater is not turning on:

Error Code 003: This means an “ignition failure.” The unit tried to light the burner but failed. This could be due to no gas, a dirty flame sensor, or an internal issue.
Error Code 012: This indicates “flame loss.” The burner lit successfully but then went out. This often points to gas pressure problems or venting blockages.
Error Code E787: This points to a problem with the air pressure sensor, which usually means your intake or exhaust vents are blocked.

The error code is your starting point. It helps you or your technician narrow down the problem from dozens of possibilities to just a few.

Your Guide to Common Problems with a Navien Tankless Water Heater Not Turning On

If the simple checks didn’t work and you have an error code (or even if you don’t), the next step is to investigate the most common physical culprits. These are the issues that prevent the unit from “breathing” or sensing water flow correctly.

Blocked Air Vents or Exhaust

Your Navien needs to pull in fresh air for combustion and push out exhaust fumes safely. If either the intake or exhaust vent is blocked, the unit’s safety sensors will shut it down to prevent a dangerous situation.

Go outside and find where your PVC vent pipes terminate. Look for blockages like leaves, bird nests, snow, or debris. Carefully clear away any obstructions. This simple step often resolves air-pressure-related error codes.

Clogged Water Filters

Your Navien has a small filter on the cold water inlet to prevent sediment from entering the unit. If this filter gets clogged, the water flow rate can drop too low for the unit’s flow sensor to detect, and it simply won’t turn on.

To check this, you’ll need to turn off the water supply to the unit. The filter is typically located right where the cold water line connects. Unscrew it, rinse it thoroughly, and reinstall it. This is a key part of any navien tankless water heater not turning on care guide.

A Frozen Condensate Line (A Common Winter Woe)

High-efficiency condensing units like Navien produce condensation as a byproduct of heating. This water drains out through a small tube, called the condensate line. In freezing weather, this line can freeze solid, causing water to back up into the unit and trigger a shutdown.

If it’s cold outside and your heater has stopped working, trace this small drain tube. If you find a frozen section, you can often thaw it safely using a hairdryer on a low setting or by pouring warm (not boiling!) water over it.

Proactive Maintenance: The Best Practices to Prevent Future Issues

The most effective way to deal with a navien tankless water heater not turning on is to prevent it from happening in the first place. Following a few best practices ensures your unit remains reliable and efficient for years.

The Importance of Annual Flushing and Descaling

If you live in an area with hard water, mineral scale (calcium and magnesium) will build up inside your water heater over time. This buildup insulates the heat exchanger, forcing the unit to work harder and eventually restricting water flow, leading to a shutdown.

Flushing your unit annually with a descaling solution is the single most important maintenance task you can perform. This process dissolves the mineral buildup and restores your heater’s performance. Adopting this is one of the most sustainable navien tankless water heater not turning on strategies, as it maintains peak eco-friendly efficiency.

Keeping Vents and Filters Clean: A Simple Care Guide

Make it a habit to visually inspect your exterior vents every few months, especially during the fall and winter. At the same time, clean the small inlet water filter. These two simple tasks take less than 15 minutes and prevent the most common nuisance shutdowns.

When to Put Down the Tools and Call a Certified Navien Technician

While many issues can be solved with a little DIY spirit, some problems absolutely require a licensed professional. Your safety is always the top priority.

Signs You’re Out of Your Depth

It’s time to call a pro if:

You smell gas.
You’ve tried all the basic steps and the unit still won’t turn on.
The error code points to an internal component failure (e.g., “gas valve fault” or “main board error”).
You see signs of water leaking from inside the unit’s casing.
You are uncomfortable performing any of the steps listed above.

The Dangers of DIY Gas and Electrical Repairs

Working on gas lines or the internal electrical components of your water heater is extremely dangerous if you are not trained and certified. An incorrect repair can lead to gas leaks, carbon monoxide poisoning, or electrical fires. It’s simply not worth the risk. A professional has the tools, training, and diagnostic equipment to fix the problem safely and correctly.

Frequently Asked Questions About a Navien Tankless Water Heater Not Turning On

How do I reset my Navien tankless water heater?

Most Navien models can be reset by simply unplugging the unit from the electrical outlet for about 30 seconds and then plugging it back in. This will often clear temporary error codes. If the error returns immediately, it indicates a persistent problem that needs to be addressed.

Why is my Navien remote control not working?

If the remote is unresponsive, the first step is always to replace the batteries. If new batteries don’t solve the problem, check for a solid connection wire between the remote and the main unit (if it’s a wired remote). For wireless models, you may need to re-pair the remote with the water heater according to the instructions in your owner’s manual.

Can a dirty flame sensor stop my Navien from turning on?

Absolutely. The flame sensor is a critical safety device that confirms a flame is present when the gas valve is open. Over time, it can get coated with carbon buildup. If the sensor can’t detect the flame, it will shut the unit down. Cleaning a flame sensor is a task best left to a technician, as the sensor is delicate and can be easily damaged.

You’ve now walked through the essential troubleshooting steps for when your navien tankless water heater not turning on. By starting with the basics—power, gas, and simple blockages—you can solve many common issues yourself, safely and confidently.

Remember to use your unit’s error code as your guide and never hesitate to call a certified professional when you’re dealing with gas, internal components, or a problem you can’t solve. Regular maintenance is your best defense against unexpected cold showers.

Stay safe, and enjoy that endless hot water!

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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.