Navien Tankless Water Heater Keeps Running - Your Step-By-Step

Hear that low, persistent hum? It’s the sound of your Navien tankless water heater… running. And running. Even when every faucet in the house is tightly shut. It’s a frustrating sound, not just for the noise, but for what it represents: wasted energy, unnecessary wear, and a problem you can’t quite put your finger on.

I get it. As an HVACR technician, I’ve been in countless homes where a homeowner points to their unit and says, “It just won’t shut off.” The good news is, you’ve come to the right place. While a navien tankless water heater keeps running for several reasons, many of them are surprisingly simple to diagnose.

Don’t worry—we’re going to walk through this together, step by step. I promise to give you the expert insights to understand what’s happening inside that box on your wall.

In this comprehensive guide, we’ll uncover the most common culprits, from tiny hidden leaks to misunderstood settings. You’ll get a clear, safe checklist to follow and learn to spot the exact moment when it’s time to call in a certified pro. Let’s get that heater running only when you want it to.

First, Let’s Understand: Why Does a Tankless Heater “Run”?

Before we dive into troubleshooting, let’s quickly cover how your Navien is supposed to work. Unlike a traditional tank that constantly heats a large volume of water, your tankless unit is a marvel of on-demand efficiency. It’s designed to be completely dormant until one thing happens: you open a hot water tap.

When you do, a flow sensor detects the moving water and signals the unit to fire up the burner and heat the water as it passes through. When you close the tap, the sensor signals the unit to shut down. Simple, right?

So, when your unit keeps running, it believes there is a demand for hot water, even if you’re not creating one. Our job is to figure out what’s sending that false signal. The “running” sound you hear could be one of a few things:

The Blower Fan: This fan runs to purge exhaust gases after a heating cycle and can sometimes run for a short period. Continuous running, however, is a problem.
The Burner Firing: If you hear the “whoosh” of the burner igniting and feel heat from the vent, the unit is actively heating water.
The Recirculation Pump: Many Navien models have an internal pump to provide faster hot water. This pump can be programmed to run at certain times, which can be mistaken for a problem.

The Top 5 Reasons Your Navien Tankless Water Heater Keeps Running

Let’s get into the detective work. Most of the time, the issue falls into one of these five categories. We’ll start with the most common and easiest to check. This is your essential navien tankless water heater keeps running guide to solving the mystery.

Culprit #1: The Hidden Hot Water Leak

This is, by far, the most frequent cause. The flow sensor in your Navien is incredibly sensitive. It only takes a tiny, almost imperceptible flow of water—as little as 0.25 to 0.5 gallons per minute—to trick it into thinking a faucet is open.

A dripping faucet, a leaky showerhead, or a pinhole leak in a pipe somewhere in your walls is enough to keep the unit firing up. This is one of the most common problems with navien tankless water heater keeps running issues.

How to Check for Hidden Leaks:

Visual Inspection: Carefully check every single faucet, showerhead, and toilet in your home. Look under sinks and behind the washing machine. Don’t forget outdoor spigots if they are on the same system.
The Pro Tip – Your Water Meter: Find your home’s main water meter. Make sure no water is being used anywhere in the house. Look at the meter; most have a small triangle or dial that detects very low flow. If that dial is spinning, even slowly, you have a leak somewhere.

Culprit #2: A Faulty or Debris-Clogged Flow Sensor

The flow sensor is the gatekeeper. It’s a small part inside the unit with a paddle or turbine that moves when water flows past it. If hard water scale, sediment, or a piece of debris gets lodged in this sensor, it can get stuck in the “on” position.

In this case, the sensor is constantly telling the control board, “We have water flow!” even when there is none. The unit obediently stays ready or fires up, trying to heat water that isn’t moving.

Fixing this usually involves cleaning or replacing the sensor, a task that is often best left to a technician as it requires opening the unit and handling sensitive components.

Culprit #3: The Internal Recirculation Pump & Settings

Does your Navien have “ComfortFlow,” “HotButton,” or a built-in recirculation system? This feature is fantastic for getting instant hot water, but it’s also a common source of confusion. The system works by slowly circulating hot water through your pipes so it’s ready when you need it.

Your unit likely has different recirculation modes:

Intelligent Mode: The unit learns your hot water usage patterns and circulates water during those times.
Always-On/Manual Mode: The pump runs much more frequently or continuously to keep the loop hot.

It’s very possible your unit is simply operating as programmed. Check your control panel or user manual to see which mode you’re in. Switching to the intelligent mode or a more conservative schedule can solve the “problem” and is a great example of sustainable navien tankless water heater keeps running best practices.

Culprit #4: Crossover in a Plumbing Fixture

This one is a bit trickier but a classic plumbing puzzle. “Crossover” happens when cold water is able to leak into the hot water pipes, creating a small but persistent flow that triggers your tankless heater.

Where does this happen? The most common culprit is a faulty single-handle faucet (kitchen, bath, or shower) where the internal cartridge or valve has failed. A poorly installed or failing mixing valve can also be the cause.

How to Test for Crossover:

Go to your Navien unit and turn off the hot water supply using the valve on the hot water outlet pipe.
Go to any faucet in your home and turn on only the hot side.
After a few seconds, the trapped hot water should stop flowing. If it continues to dribble or run cold, you have a crossover issue somewhere in your plumbing.

Culprit #5: A Malfunctioning Main Control Board

Think of the control board as the brain of your water heater. While rare, it is possible for the board to malfunction and send continuous signals to the fan or burner, even with no input from the flow sensor.

This is a diagnosis of last resort after all other possibilities have been exhausted. A faulty board is a strictly professional repair and requires a certified technician to diagnose and replace correctly.

Your Safe DIY Troubleshooting Checklist

Feeling a bit overwhelmed? Let’s simplify. Here is a step-by-step checklist you can safely follow. This is how to navien tankless water heater keeps running diagnostics should begin.

Listen and Observe: Get close to the unit. Is it just the fan, or can you hear the burner ignite? Is an error code displayed on the screen? Note your observations.
Begin the Leak Hunt: Systematically check every faucet, fixture, and appliance connected to the hot water line. Use the water meter trick for confirmation.
Check Recirculation Settings: Grab your user manual. Identify your recirculation mode and try changing it to see if the behavior stops. This is one of the most important navien tankless water heater keeps running tips.
Perform the Crossover Test: Shut off the hot water outlet at the heater and check your faucets as described in the section above.
Inspect the Condensate Line: Ensure the small plastic tube draining from the bottom of your high-efficiency unit is not clogged or frozen. A blockage can sometimes cause pressure switches to act up, leading to erratic behavior.

The Benefits of Fixing a Constantly Running Tankless Heater

Getting your Navien to operate correctly isn’t just about stopping an annoying noise. The benefits are significant and directly impact your wallet and the environment. This is where a proper navien tankless water heater keeps running care guide pays off.

Energy Savings: A tankless heater firing unnecessarily is burning gas and using electricity for no reason. Fixing the issue leads to immediate savings on your utility bills, making for a more eco-friendly navien tankless water heater keeps running solution.
Water Conservation: If the cause is a hidden leak, you’re not just wasting energy—you’re wasting hundreds or even thousands of gallons of water over time.
Longer Unit Lifespan: Constant, unnecessary operation puts extra wear and tear on critical components like the igniter, fan, and heat exchanger. Proper operation extends the life of your investment.
Peace and Quiet: A properly functioning on-demand heater should be silent most of the time. Restoring that silence brings peace of mind.

When to Call a Navien-Certified Technician: Know Your Limits

Your safety is the number one priority. While the checks above are safe for most homeowners, there is a clear line you should not cross. It’s time to put down the tools and call a professional if:

You have completed the DIY checklist and still can’t find the cause.
You suspect the issue is internal, like the flow sensor or control board.
You see any error code displayed on the unit’s screen.
You are not 100% comfortable performing any of the steps.
You smell gas. If you ever smell gas, leave the area immediately, call your gas company from a safe distance, and then call a qualified technician.

When you call for service, ask if the technician is certified or has experience with Navien products. These are sophisticated units, and specialized knowledge goes a long way.

Frequently Asked Questions About a Navien Tankless Water Heater That Keeps Running

Why is my Navien making a humming sound even when not in use?

This is often related to either the post-purge cycle, where the fan runs for a minute or two after heating to clear the exhaust, or the internal recirculation pump. If the hum is constant and lasts for many minutes or hours, it’s likely one of the issues we discussed above, such as a leak or a stuck flow sensor.

Can a hard water problem cause my tankless heater to run constantly?

Yes, indirectly. Hard water is rich in minerals like calcium and magnesium, which can build up as “scale” inside your unit. This scale can cause the flow sensor to become clogged or stick, tricking the unit into running. This is why annual descaling and flushing is one of the most crucial navien tankless water heater keeps running best practices.

How often should I have my Navien tankless water heater serviced?

Navien, like most manufacturers, recommends annual service by a qualified technician. This service typically includes inspecting the unit, cleaning filters, and descaling the heat exchanger. Regular maintenance is the best way to prevent the common problems with navien tankless water heater keeps running issues and ensure years of efficient, trouble-free operation.

Tackling an issue like a constantly running water heater shows you’re a proactive homeowner, and that’s something to be proud of. By following this guide, you have a clear path to diagnosing the problem and understanding how your system works.

You’re now equipped to find the simple fixes and, just as importantly, to know when to make the safe call to a professional. Enjoy that endless hot water—only when you actually need it!

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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.