Navien Tankless Water Heater Error Code Es15 - A Step-By-Step DIY Fix

That dreaded moment: you step into the shower expecting a hot, relaxing stream, but instead, you’re met with lukewarm water and a blinking error code on your Navien tankless water heater. Seeing that little “ES15” pop up can be frustrating, but don’t worry—we’re here to help.

I’m an HVACR technician, and I’ve seen this exact issue countless times. The good news is that the navien tankless water heater error code es15 is one of the most common and often one of the easiest for a homeowner to fix safely.

We promise this guide will give you the confidence and the clear, step-by-step instructions to troubleshoot and resolve this error yourself. You’ll learn what causes it, how to fix the primary culprit, and when it’s time to call for professional backup.

Let’s get that hot water flowing again. We’ll walk through this together.

What Does the Navien Tankless Water Heater Error Code ES15 Actually Mean?

Think of error codes as your water heater’s way of talking to you. It’s telling you exactly what’s wrong so you can fix it. In this case, the ES15 code is straightforward and points to a water flow issue.

Essentially, your Navien unit isn’t sensing enough water moving through it to safely ignite the burner and start heating. It’s a built-in safety feature to prevent the unit from overheating and damaging itself.

There are two primary culprits behind this code:

A Clogged Cold Water Inlet Filter: This is the cause more than 90% of the time. Over months and years, tiny bits of sediment, minerals (from hard water), and debris from the water lines build up on a small screen filter where water enters the unit. Eventually, this buildup restricts the flow, triggering the ES15 code.
A Malfunctioning Water Flow Sensor: Less common, but possible. This sensor is the component that measures the rate of water flow. If it fails or gets stuck, it might incorrectly tell the control board that there’s no flow, even when there is.

This article will focus on the most common problem—the clogged filter—which is a perfect task for a capable DIYer. This is a crucial part of any good navien tankless water heater error code es15 care guide.

Before You Begin: Essential Safety Precautions

Safety always comes first. Before you touch a single pipe or valve, we need to properly and safely shut down the unit. Don’t skip these steps! It only takes a minute and ensures you won’t have any surprises.

First, gather your tools. You won’t need much:

A small bucket or bowl
A few old towels (spills happen!)
A pair of channel-lock pliers or an adjustable wrench
A soft-bristle brush (an old toothbrush is perfect)

Now, let’s secure the unit. Follow these steps in order:

Turn Off the Electricity: Go to your home’s main electrical panel and flip the breaker that powers your tankless water heater to the OFF position.
Shut Off the Gas Supply: Find the gas valve on the line leading to your Navien unit. The handle should be parallel with the pipe when it’s open. Turn it 90 degrees so it’s perpendicular to the pipe. This shuts off the gas.
Close the Water Valves: Your unit should have three valves on the pipes below it. Close the cold water inlet (usually blue) and the hot water outlet (usually red) by turning their handles perpendicular to the pipes.

With the power, gas, and water off, we’re ready to get to work safely.

Your Complete Navien Tankless Water Heater Error Code ES15 Guide

Alright, with the unit safely powered down, let’s tackle this error head-on. This section will provide the how to navien tankless water heater error code es15 solution that works for most homeowners.

Step 1: Locate the Cold Water Inlet Filter

Look at the bottom of your Navien unit. You’ll see the cold water pipe connecting to it, typically on the right side. The filter is located right at this connection point.

You’re looking for a small plug or cap, often with a hexagonal or slotted head for a wrench or screwdriver. It’s usually situated just behind the cold water service valve you just closed.

Step 2: Safely Drain and Remove the Filter

Even with the water off, there’s still some water and pressure in the lines. First, go to a nearby sink and open the hot water tap to relieve any remaining pressure.

Place your bucket and towels directly under the filter plug. Using your pliers or wrench, slowly turn the plug counter-clockwise to loosen it. A small amount of water will drain out—this is normal. Once it stops dripping, you can fully unscrew and remove the plug.

Step 3: Inspect and Clean the Filter Screen

Attached to the plug or just inside the port, you’ll find a small, thimble-shaped screen. This is your culprit! It will likely be covered in a gritty paste of sediment, rust flakes, or hard water scale.

To clean it, follow these simple navien tankless water heater error code es15 tips:

Rinse It: Hold the screen under a faucet and gently rinse it. Most of the loose debris should wash away.
Brush It: Use your soft-bristle brush to gently scrub away any stubborn buildup. Be careful not to damage the delicate mesh screen.
For Heavy Scale: If it’s caked in white, chalky mineral scale, you can soak it in a cup of white vinegar for about 15-20 minutes. The acid will dissolve the minerals, and you can then brush it clean.

Your goal is to have the screen looking clean and clear, so light can easily pass through the mesh.

Step 4: Reinstall the Filter and Restore Power

Once the filter is clean, carefully slide it back into its housing. Screw the plug back in by hand to ensure it isn’t cross-threaded, then give it a gentle final snug-up with your wrench—about a quarter-turn past hand-tight is plenty. Do not over-tighten it, as this can crack the plastic housing.

Now, we’ll reverse the shutdown procedure:

Turn On the Water: Slowly turn the handle on the cold water inlet valve. Listen for the sound of water filling the unit. Check carefully for any drips or leaks around the filter plug you just tightened.
Turn On the Gas: Turn the gas valve handle back so it’s parallel with the pipe.
Turn On the Power: Go back to your breaker panel and flip the switch for the water heater back ON.

The unit will power up, and the ES15 error code should be gone! Go test a hot water tap to confirm you have hot water again.

What If Cleaning the Filter Doesn’t Clear the ES15 Code?

If you’ve thoroughly cleaned the filter and the error code persists, we need to consider other, less common problems. This is often the point where calling a licensed technician is the best course of action, as these checks can be more complex.

Investigating a Potential Water Flow Sensor Issue

The water flow sensor is a small internal part with a paddle wheel or turbine that spins as water passes through. If this sensor is stuck with debris or has failed electronically, it can’t send the “go” signal to the burner. Replacing this part requires opening the unit’s casing and is not recommended as a DIY job for safety and warranty reasons.

Proactive Care: Best Practices to Prevent Error Code ES15

The best way to deal with error codes is to prevent them from ever happening. Adopting a few navien tankless water heater error code es15 best practices will extend the life of your unit and improve its efficiency.

The Importance of Regular Flushing (Descaling)

The sediment you cleaned from the filter is also building up inside the unit’s delicate heat exchanger. Descaling, or flushing the unit with a vinegar solution, removes this mineral buildup. This is the single most effective maintenance task you can perform. It promotes a more sustainable navien tankless water heater error code es15 solution by keeping the unit running at peak efficiency, saving energy.

Consider a Whole-House Water Treatment System

If you live in an area with hard water, the ultimate eco-friendly navien tankless water heater error code es15 prevention method is to treat the water before it ever reaches the heater. A water softener or a sediment pre-filter will capture these minerals and debris, protecting your investment and ensuring you rarely, if ever, see an ES15 code again.

Frequently Asked Questions About Navien Error Code ES15

Is it safe for me to fix the ES15 error myself?

Yes, absolutely. Cleaning the cold water inlet filter is considered routine maintenance and is a perfectly safe and simple task for a homeowner, as long as you follow the safety shutdown procedure to turn off the power, gas, and water before you begin.

How often should I clean my Navien’s inlet filter?

As a general rule, checking and cleaning it once a year is a great practice. However, if you have very hard water or are on a well system, you may benefit from checking it every six months.

Can I just ignore the ES15 error code?

No, you should not ignore it. The error code is a safety mechanism. Ignoring it means you won’t have any hot water, and if the unit tries to operate intermittently under low-flow conditions, it could put unnecessary strain on its components.

Does a water softener help prevent the ES15 error?

It helps tremendously! A water softener removes the calcium and magnesium minerals that cause scale buildup. This scale is the primary “glue” that makes sediment stick to the filter screen. A softener is one of the best long-term solutions for preventing flow-related errors.

Congratulations on taking the initiative to understand and fix your Navien water heater! By cleaning that small filter, you’ve just performed a key maintenance task that keeps your system healthy and efficient.

Remember, routine care is the key to longevity. A little bit of prevention goes a long way in ensuring you have reliable, on-demand hot water whenever you need it.

Stay warm, stay comfortable, and enjoy that perfectly hot shower!

Author
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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.

Navien Tankless Water Heater Error Code Es15 – A Step-By-Step DIY Fix