Navien Water Heater Error Code 351 00 - Your Step-By-Step

That sinking feeling when you turn on the shower and get nothing but icy water is bad enough. But seeing a cryptic navien water heater error code 351 00 flashing on the display can be downright confusing and frustrating.

Don’t worry—you’ve come to the right place. We’re here to demystify this error for you. As HVACR professionals, we see this issue regularly, and the good news is that it’s often something a homeowner can safely investigate and sometimes even resolve without a service call.

We promise to help you understand the problem. This complete guide will walk you through exactly what this code means, the most common culprits behind it, and a safe, step-by-step checklist you can follow.

In this article, we’ll preview the simple checks for closed valves, how to clean a critical filter that often gets clogged, and how to know for sure when it’s time to call in a pro. Let’s work together to get that hot water flowing again.

What Exactly is Navien Error Code 351 00? Unpacking the Message

First things first, let’s translate what your Navien tankless water heater is trying to tell you. In the simplest terms, Navien error code 351 00 means “Low Water Pressure Detected” or “No Water in the Unit.”

Think of it as a smart safety feature. Your high-efficiency Navien unit needs a certain amount of water flowing through its heat exchanger to operate safely. If it doesn’t sense adequate water pressure or flow, it will shut down and display this code to prevent the unit from firing up dry, which could cause serious damage to its internal components.

This error isn’t a sign of a catastrophic failure. More often than not, it’s a symptom of a simple restriction in the water supply leading to the heater. This navien water heater error code 351 00 guide is designed to help you find that restriction.

The Most Common Causes Behind Error Code 351 00

Before you can fix the problem, you need to understand where it’s coming from. We’ve found that this specific error code usually points to one of a handful of common problems. Let’s look at the most likely suspects.

Closed or Partially Closed Water Valves

It sounds almost too simple to be true, but this is the number one cause. Your water heater has several valves. If the main cold water inlet valve was accidentally bumped or not fully opened after a plumbing repair, it will restrict the flow enough to trigger the error.

Clogged Inlet Water Filter Screen

Your Navien unit has a small screen filter on the cold water inlet connection. Its job is to catch sediment, rust, and debris from the water supply before it enters the sensitive internal parts of the heater. Over time, this screen can become completely clogged, severely restricting water flow and causing error 351 00.

Frozen Water Lines (A Winter Warning)

If the error appears suddenly during a cold snap, a frozen pipe is a strong possibility, especially if your water heater is located in a garage, crawlspace, or near an exterior wall. Even a small section of frozen pipe can stop water flow entirely.

Low Municipal or Well Water Pressure

Sometimes the problem isn’t with your heater at all. If your entire home is experiencing low water pressure due to a municipal supply issue, a failing well pump, or a faulty pressure-reducing valve, your Navien will be the first to let you know by shutting down with this error code.

A Faulty Internal Sensor

While less common, the error can also be triggered by a malfunctioning internal component, such as the water pressure sensor or flow sensor. These parts tell the unit’s main control board how much water is flowing. If one of them fails, it might send a false signal, causing the heater to shut down even when water pressure is fine. This is a job for a qualified technician.

Your Safe DIY Troubleshooting Guide for Navien Water Heater Error Code 351 00

Ready to do some detective work? We’ll walk you through a few safe checks together. Following these steps in order will help you systematically rule out the most common issues.

IMPORTANT SAFETY FIRST:

Before you begin any inspection, it’s wise to turn off the electrical power to your water heater at the circuit breaker. While most of these checks involve plumbing, it’s always the safest practice to de-energize the appliance you’re working near.

Step 1: Check All Your Water Valves
This is your first and easiest check. Locate the cold water pipe that enters the bottom of your Navien unit. You will see a valve handle on this line. For the valve to be fully open, the handle should be parallel to the direction of the pipe. If it’s perpendicular, it’s closed. Make sure it’s turned all the way to the open position.
Also, check the main water shut-off valve for your entire house to ensure it is fully open.
Step 2: Inspect and Clean the Inlet Water Filter
This is one of the most effective how to navien water heater error code 351 00 fixes. The inlet filter is located right where the cold water line connects to the heater. Here’s how to clean it:
- Turn off the cold water shut-off valve to the heater.
- Open a hot water faucet somewhere in your house (like a bathtub) to relieve any pressure in the lines.
- Place a small bucket or towel under the cold water connection on the heater to catch any drips.
- Carefully unscrew the cold water line connection from the heater. Just inside the port, you will see a small, thimble-shaped screen.
- Gently pull this screen out using your fingers or a pair of needle-nose pliers.
- Clean the screen thoroughly under a faucet, using an old toothbrush to gently scrub away any sediment or scale buildup. Hold it up to the light to ensure it’s completely clear.
- Carefully re-insert the clean filter, re-attach the water line, and slowly turn the water valve back on. Check for any leaks.
Step 3: Check for Whole-House Low Pressure
Is the water pressure low everywhere? Go to a sink farthest from your water heater and turn on the cold water full blast. Then do the same with the hot water. If both streams are weak, the issue is likely with your home’s overall water supply, not just the Navien unit.
Step 4: Reset Your Navien Unit
After you’ve performed the checks above and restored water flow, it’s time to reset the unit. Simply press and hold the power button for a few seconds to turn it off, wait a minute, and then press it again to turn it back on. The error code should clear. If it returns immediately, there’s another issue at play.

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

Your safety is the top priority. While the steps above are safe for most handy homeowners, there are clear signs that it’s time to call a licensed and insured professional who is certified to work on Navien products.

If you’ve performed all the DIY checks and the navien water heater error code 351 00 persists.
If the error code immediately reappears after a reset.
If you are not comfortable performing any of the steps, especially cleaning the filter.
If you suspect you have frozen pipes and are unsure how to thaw them safely.
If you see any signs of water leaking from the unit’s casing. This could indicate a more serious internal problem.
If you suspect the issue is an internal component like a faulty sensor, as this requires specialized diagnostic tools and parts.

Trying to fix complex internal issues can lead to further damage or unsafe operation. A professional technician can accurately diagnose the problem and get it fixed right the first time.

Best Practices for Preventing Future Navien Error Codes

An ounce of prevention is worth a pound of cure, especially with high-tech appliances. Following a simple navien water heater error code 351 00 care guide can save you from future headaches and no-hot-water emergencies.

Schedule Annual Professional Maintenance

The single best thing you can do for your tankless water heater is to have it professionally serviced once a year. During this service, a technician will descale the heat exchanger, clean all the filters, check sensor operations, and ensure everything is running at peak efficiency. This is one of the most important navien water heater error code 351 00 best practices.

Regularly Clean Your Inlet Filter

Now that you know how to clean the inlet water filter, make it a part of your home maintenance routine. If you have hard water or older pipes, check it every six months. For those with better water quality, checking it annually is usually sufficient.

Consider a More Sustainable Water Solution

If you find that your filter is constantly clogging with sediment, it’s a sign of poor water quality. Installing a whole-home sediment filter can act as a first line of defense, protecting not just your water heater but all of your plumbing fixtures and appliances. This is an eco-friendly navien water heater error code 351 00 solution that improves longevity and efficiency, making it a truly sustainable investment.

Frequently Asked Questions About Navien Error Code 351 00

Can I just keep resetting my Navien water heater when I see error 351 00?

No, this is not recommended. The error code is a warning that the unit is not getting enough water to operate safely. Continually resetting it without addressing the root cause can put stress on the components and potentially lead to more significant damage. You must solve the water flow problem first.

How much does it cost to fix a Navien error code 351 00?

The cost can vary dramatically. If the fix is simply opening a valve or cleaning the filter yourself, the cost is $0. If you need a professional to come out and clean the filter, you can expect to pay a standard service call fee. If an internal sensor has failed, the cost will include the part and the labor, which could range from a few hundred dollars upwards.

Is error code 351 00 a dangerous problem?

The error code itself is not dangerous; it is a safety mechanism preventing a dangerous situation. It indicates that the system has safely shut down. The danger would come from trying to bypass these safety features or improperly servicing the unit.

Does hard water make error 351 00 more likely?

Yes, indirectly. While hard water scale (limescale) is more famous for causing overheating errors by insulating the heat exchanger, the mineral content can also contribute to clogging the small inlet filter screen more quickly, which directly leads to error 351 00.

Tackling a water heater error code can feel intimidating, but now you have a clear action plan. The navien water heater error code 351 00 is most often a simple flow problem that you can safely investigate.

By following the steps to check your valves and clean the inlet filter, you can often resolve the issue yourself and restore your hot water in under an hour. And just as importantly, you now know the clear signals for when it’s time to call for professional backup.

Stay safe, be confident, and enjoy that reliable 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.