Navien Tankless Water Heater Exhaust Smells Like Gas - Your Pro Guide

That faint, acrid smell near your Navien tankless water heater’s exhaust vent… it’s enough to make any homeowner’s heart skip a beat. Your mind immediately jumps to the worst-case scenario. Is my house going to explode? Is there a massive gas leak? It’s a genuinely unsettling experience, and you’re right to be concerned.

As an HVACR professional with years of field experience, I’m here to guide you through this. I promise to help you understand what’s happening, what you can safely check yourself, and when you absolutely must call for professional help. We’ll cut through the panic and get to the practical steps.

In this comprehensive guide, we’ll cover the immediate safety actions you must take if you suspect a gas leak. We will then explore the common reasons why your navien tankless water heater exhaust smells like gas, helping you differentiate a minor issue from a true emergency. By the end, you’ll have a clear action plan and the confidence to handle this situation safely and effectively.

First Things First: Your Immediate Gas Smell Safety Checklist

Before we diagnose anything, your safety is the absolute priority. If you smell gas, especially if it’s strong or inside your home, you must act immediately. Natural gas is flammable and can be dangerous.

Follow these steps without exception:

Stop What You’re Doing: Do not operate anything electrical. This means no light switches, no phones (not even a cell phone inside the house), no appliances, and no thermostats. A tiny spark can ignite gas fumes.
No Open Flames: Extinguish any candles, cigarettes, or other open flames immediately.
Evacuate If Necessary: If the gas smell is strong, persistent, or inside your home, leave the building immediately. Do not pause to grab belongings. Leave the doors open behind you to help with ventilation.
Call for Help from a Safe Distance: Once you are safely outside and away from the building, use your cell phone or a neighbor’s phone to call your local gas utility’s emergency number. Their number is usually on your gas bill. If you cannot find it, call 911. They will dispatch a technician to shut off the gas and locate the source of the leak.

Only after the gas company has declared the area safe should you or an HVAC technician begin troubleshooting the appliance itself. Never take chances with a gas smell.

Why Your Navien Tankless Water Heater Exhaust Smells Like Gas: Common Causes

Once you’ve ensured there isn’t an active, dangerous leak, we can explore why you’re noticing this odor specifically at the exhaust vent. This is a common issue, and understanding the potential causes is the first step in our navien tankless water heater exhaust smells like gas guide.

Normal Startup Puff vs. A Persistent Problem

Sometimes, a very faint, brief odor of gas (or more accurately, the odorant called mercaptan that’s added to gas) can be present for a few seconds right as the unit ignites. This is because a tiny amount of unburnt gas may pass through before the electronic ignition sparks.

The key here is that it should be extremely brief—lasting no more than two or three seconds—and very faint. If the smell lingers, gets stronger, or happens every time the unit cycles, it is not normal and points to an underlying issue.

Incomplete Combustion: The Most Likely Culprit

The most common reason for a persistent fuel smell from the exhaust is incomplete combustion. In simple terms, your Navien unit isn’t burning the natural gas or propane completely and efficiently. The unburnt fuel is then expelled through the exhaust vent, creating that tell-tale odor.

Several things can cause this:

Blocked Vents: Your high-efficiency Navien has two pipes: an intake for fresh combustion air and an exhaust for flue gases. If either of these is blocked by leaves, snow, ice, or even a bird’s nest, the unit can’t “breathe” properly. This starves the flame of oxygen, leading to a rich fuel mixture and that gas smell.
Dirty Burner Assembly: Over time, dust and debris can accumulate on the burner, disrupting the proper mixing of air and fuel.
Improper Gas Pressure: If the gas pressure delivered to the unit is too high or too low, it will throw off the precise air-to-fuel ratio required for clean combustion.

A Cracked Heat Exchanger: The Serious Scenario

This is one of the more serious common problems with navien tankless water heater exhaust smells like gas. The heat exchanger is where the heat from the flame is transferred to your water. If it develops a crack, it can allow unburnt gas and dangerous flue gases—including deadly, odorless carbon monoxide—to mix and escape. A gas smell from the exhaust can be an early warning sign of this critical failure. This is not a DIY repair and requires immediate professional service.

Gas Leaks Near the Unit

It’s also possible there’s a very small gas leak at a fitting or connection right at the unit itself. The draft created by the combustion fan could be pulling these fumes into the burner chamber and pushing them out the exhaust. While the leak isn’t in the exhaust itself, the smell presents there.

Your Safe DIY Inspection Guide

If the gas smell is very faint, only occurs outdoors, and you feel comfortable, there are a few safe checks you can perform. This is where our how to navien tankless water heater exhaust smells like gas tips come in handy. Remember, if you are ever in doubt, stop and call a professional.

Step 1: Visually Inspect the Vents

Go outside to where your PVC intake and exhaust pipes terminate. Look closely at the openings.

Are they covered in snow or ice?
Is there a bird’s nest, hornet’s nest, or leaves packed inside the screen?
Has landscaping or a snowdrift blocked the pipes?

Carefully clear away any obstructions. Sometimes, this simple step is all it takes to restore proper airflow and solve the problem.

Step 2: Listen and Look for Error Codes

Stand near your unit while someone inside turns on a hot water tap. Listen to the unit as it fires up. Do you hear any unusual popping, sputtering, or banging sounds? These can indicate an ignition problem.

Next, look at the digital display on the front of the Navien unit. These smart water heaters are excellent at self-diagnosing. If there’s an internal fault, it will often display an error code. Write the code down and consult your owner’s manual or a quick online search for “Navien error code [your code]”. This gives you—and your technician—a huge head start.

When to Stop and Call a Licensed HVAC Professional

Knowing your limits is the most important part of any DIY endeavor, especially with gas appliances. Your safety is not worth the risk of a small repair bill. Call a qualified technician immediately if you encounter any of the following:

The gas smell is strong, persistent, or inside your home.
You’ve cleared the vents, but the smell continues.
The unit is displaying an error code you can’t resolve.
You see black soot or signs of scorching around the exhaust vent.
You hear loud or unusual noises coming from the unit during operation.
You are unsure or uncomfortable at any point in your inspection.

A professional has the specialized tools, like a combustion analyzer and electronic gas leak detector, to pinpoint the exact cause of the problem safely and accurately.

Best Practices for Prevention: Your Navien Care Guide

The best way to deal with problems is to prevent them. Following a few navien tankless water heater exhaust smells like gas best practices will keep your unit running safely and efficiently for years, contributing to a more sustainable and eco-friendly home.

The Importance of Annual Maintenance

Just like your car, your tankless water heater needs a yearly tune-up. During a professional maintenance visit, a technician will:

Clean the burner and ignition assembly.
Inspect the heat exchanger for any signs of wear or cracking.
Check and adjust gas pressure.
Clean the intake and inline water filters.
Flush and descale the unit to remove mineral buildup, which is vital for efficiency and longevity.

Keeping Vents Clear Year-Round

Make it a habit to check your exterior vents regularly. In the fall, clear away fallen leaves. In the winter, ensure snow and ice don’t build up and block them after a storm. In the spring, check for insect or bird nests.

Embrace Eco-Friendly Operation

One of the key benefits of navien tankless water heater exhaust smells like gas awareness is that it’s tied directly to efficiency. A smell indicates incomplete combustion, which means you’re wasting fuel. A properly maintained, cleanly burning unit uses less gas to heat your water, lowering your utility bills and reducing your carbon footprint. This is the heart of a sustainable navien tankless water heater exhaust smells like gas strategy.

Frequently Asked Questions About Navien Exhaust Smells

Is it normal to smell a little gas when my Navien first turns on?

A very faint, momentary whiff of the gas odorant right at ignition can sometimes occur and is not usually a cause for alarm, as long as it disappears within 1-3 seconds. A smell that is strong, lingers, or happens every time is not normal and needs to be investigated.

What does incomplete combustion smell like?

It often smells just like unburnt natural gas or propane because that’s essentially what it is. The real danger of incomplete combustion is that it also produces high levels of carbon monoxide (CO), a colorless, odorless, and highly toxic gas. The gas smell is your warning sign that a dangerous situation could be developing.

Can a dirty filter cause a gas smell on a Navien tankless water heater?

Navien units don’t have a traditional furnace-style air filter. However, the screen on the air intake vent acts as a filter. If that screen gets clogged with debris, it restricts airflow and can cause the exact same incomplete combustion issues, leading to a gas smell.

Don’t let a strange smell from your water heater disrupt your peace of mind. By following these safety steps, performing simple checks, and knowing when to call in the experts, you can address the issue with confidence.

Your home’s safety is paramount. An investment in professional maintenance is an investment in both the longevity of your equipment and the well-being of your family. 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.