Navien Tankless Water Heater Leaking Inside - Your Pro Diagnostic &

That little puddle of water under your Navien unit. It’s a sight that can make any homeowner’s heart sink. You invested in a high-efficiency tankless water heater for its reliability, and now there’s a leak. Don’t worry—you’ve come to the right place.

I’m going to walk you through this just like I would with a client in their home. We’ll figure out what’s going on together. Many times, a navien tankless water heater leaking inside isn’t a catastrophe. It could be something simple you can identify yourself.

In this comprehensive guide, we promise to help you safely diagnose the source of the leak. We’ll preview the most common culprits, provide a step-by-step inspection you can perform, and clearly explain when it’s time to put down the tools and call a licensed professional for your safety.

First Steps: What to Do Immediately When You Find a Leak

Before we even think about finding the source, your first priority is safety. Water, electricity, and natural gas (or propane) are a dangerous combination. Follow these steps in order before doing anything else.

Step 1: Shut Off the Power

Locate the circuit breaker that controls your Navien unit and switch it to the OFF position. If you’re unsure which breaker it is, it’s safest to turn off the main breaker to your entire home. Do not just unplug the unit if it’s sitting in water.

Step 2: Turn Off the Water Supply

Your Navien unit has an isolation valve kit with a cold water inlet (usually marked with a blue handle). Turn this handle 90 degrees so it is perpendicular to the pipe. This will stop more water from entering the heater.

Step 3: Close the Gas Valve

If you have a natural gas or propane model, find the gas shutoff valve on the line leading to the unit. It typically has a yellow or red handle. Turn it 90 degrees to shut off the gas supply. You should not smell gas, but this is a critical safety measure.

Diagnosing Your Navien Tankless Water Heater Leaking Inside: Common Culprits

With the unit safely shut down, it’s time to play detective. The source of the water tells a story. Here are the most common problems with a navien tankless water heater leaking inside, from the simple to the serious.

Is It a Leak or Just Condensation?

Navien units, especially the high-efficiency condensing models (NPE and NPN series), produce condensation as a normal part of their operation. This water is supposed to exit through a dedicated condensate drain line.

Check if the dripping is coming from the plastic PVC vent pipes or the condensate trap. A clogged condensate line can cause water to back up and drip, mimicking a leak. This is often a simple fix—clearing the blockage can solve the problem.

Loose Pipe Fittings and Connections

This is the most common and often easiest issue to fix. Over time, vibrations can cause the threaded connections on the hot and cold water lines to loosen slightly. Take a flashlight and a dry paper towel and check these points:

The cold water inlet connection.
The hot water outlet connection.
The pressure relief valve connection.

If you find moisture, a gentle tightening with a wrench (just a quarter-turn past hand-tight) might be all you need. Be careful not to over-tighten, as this can damage the fittings.

The Pressure Relief Valve (PRV)

The Temperature and Pressure Relief (TPR or PRV) valve is a safety device designed to release water if pressure or temperature inside the unit gets too high. If you see water dripping from the PRV’s discharge pipe, it could mean one of two things:

High Water Pressure: Your home’s water pressure might be too high (above 80 PSI). A plumber can test this and install a pressure-reducing valve.
Faulty Valve: The valve itself could be failing or have debris stuck in its seat, preventing a proper seal. Replacing a PRV is a job best left to a professional.

Cracked Heat Exchanger

This is the most serious potential problem. The heat exchanger is the core component that heats your water. If it cracks due to freezing, excessive scale buildup, or a manufacturing defect, it will cause a significant internal leak. A leak from the heat exchanger is often a fatal flaw for the unit and almost always requires a full replacement by a certified technician. This is not a DIY repair.

Internal O-Rings, Gaskets, and Seals

Inside your Navien are numerous O-rings and gaskets that seal various components like the flow sensor, water adjustment valve, and manifold. Over years of heating and cooling cycles, these rubber components can become brittle and fail, leading to small, persistent drips inside the casing.

A Step-by-Step Navien Tankless Water Heater Leaking Inside Guide for DIYers

Feeling confident and have the right tools? Let’s take a closer, safe look inside. This inspection is about observation, not disassembly. Remember our navien tankless water heater leaking inside best practices: safety first, and when in doubt, call for help.

Step 1: Gather Your Tools

You won’t need much. Grab a good flashlight (a headlamp is even better), some paper towels, and your smartphone to take pictures.

Step 2: Carefully Remove the Front Cover

Most Navien models have a few screws on the bottom or sides of the front panel. Remove them and gently lift the cover off. Set it aside somewhere safe. This exposes the “guts” of your water heater.

Step 3: The “Paper Towel Test” to Pinpoint the Drip

With the power, water, and gas OFF, use dry paper towels to wipe down every visible pipe, connection, and component inside the unit. Then, watch closely. Sometimes the leak is slow. You can also lay paper towels on the bottom of the unit to see where the first drip lands.

Step 4: Inspecting Connections and Valves

Use your flashlight to get a clear view of everything. Look for signs of corrosion (white or greenish buildup), mineral deposits, or active dripping around:

The points where the copper pipes connect to the main manifold.
The base of the pressure relief valve.
The flow sensor and water inlet/outlet assemblies.

Step 5: Document Everything

Take clear photos of where you see water. If you end up calling a technician, showing them these pictures can save valuable diagnostic time. Note the location and the frequency of the drip (e.g., “one drip every 10 seconds from the right-side connection”).

When to Stop and Call a Professional: The DIYer’s Red Flags

Your safety is more important than saving a few dollars. Here are the absolute red flags that mean you should stop immediately and call a certified Navien technician.

You see water dripping onto or near any electrical components, circuit boards, or wiring harnesses.
The leak appears to be coming from the center of the unit, which could indicate a failed heat exchanger.
The unit is displaying an error code along with the leak.
You smell gas at any point. Leave the house immediately and call your gas company from a safe distance.
You simply don’t feel comfortable. There is no shame in calling a pro. That’s what we’re here for!

Your Proactive Navien Tankless Water Heater Care Guide to Prevent Leaks

The best way to handle a leak is to prevent it. Following a simple maintenance schedule is one of the most sustainable and eco-friendly navien tankless water heater leaking inside prevention strategies. You save water, save energy, and extend the life of your unit.

The Importance of Annual Flushing

Mineral scale (limescale) is the number one enemy of any tankless water heater. It insulates the heat exchanger, forcing it to work harder and creating hot spots that can lead to stress cracks. Flushing your unit annually with a descaling solution is the single most important maintenance task you can perform.

Inspect for Corrosion and Wear

Every six months, take a quick look at the unit’s exterior plumbing. Check for any signs of green or white buildup on the pipe fittings. Catching a slow drip early can prevent water damage and a much bigger repair bill down the road.

Check Your Water Pressure

You can buy a simple water pressure gauge that screws onto a hose bib for under $20. If your home’s pressure is consistently over 75-80 PSI, you are putting unnecessary stress on every pipe, fixture, and appliance in your home, including your Navien.

Frequently Asked Questions About a Leaking Navien Tankless Heater

How much does it cost to fix a leaking Navien?

The cost varies dramatically based on the cause. Tightening a loose fitting might be part of a standard service call fee ($100-$250). Replacing a faulty valve or sensor could range from $300 to $600. A heat exchanger replacement is the most expensive repair and can often cost $1,500 or more, at which point replacing the unit may be more economical.

Is a small drip from my Navien tankless heater a big deal?

Yes. Any unmanaged water leak is a big deal. What starts as a small drip can lead to catastrophic water damage, mold growth, and component failure inside the unit. Address it immediately.

Why is my Navien leaking from the bottom?

A leak at the bottom of the casing can come from several places. It could be a clogged condensate drain overflowing, a leak from an internal component dripping down, or a faulty connection at the very bottom of the unit. The key is to remove the cover to trace the water back to its true source.

Dealing with a navien tankless water heater leaking inside can be stressful, but by following these steps, you can safely assess the situation and make an informed decision. Remember to prioritize safety, document what you find, and never hesitate to call a qualified professional when you’re out of your depth.

You’ve got this. Stay safe, stay dry, and stay comfortable!

Author
Recent Posts

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.