Navien Tankless Water Heater Gas Smell From Exhaust

Navien Tankless Water Heater Gas Smell From Exhaust [5 Easy Solutions]

The gas smell will come from the exhaust of your Navien Tankless Water Heater due to the following reasons: 

  • Repeated short cycles 
  • Poor combustion 
  • Exhaust gas gets trapped and fail to escape to the open air
  • Gas leak 
  • Gas regulator may too close to the unit

Before breaking down those problems, I will first disclose whether the gas smell on the Navien Water Heater is normal or not. Then, I will describe what causes the unit to spit unburned gas out of the exhaust and how you can solve it. 

So, stay in tune with me till the end. 

Table of Contents

Is It Normal To Smell A Gas From Navien Tankless Water Heater Exhaust?

Yes, it’s normal for a tankless water heater like the Navien one to get some unburned natural gas smell out of the exhaust during ignition. During the start-up, the unit just cranks that gas valve open and starts sparking. 

Such a bit of unburned gas is normal before it gets a good burn going. Keep in mind; this only happens during the ignition phase. The gas smell should dissipate or go away once you achieve the steady state combustion.  

However, if you smell the gas out of the exhaust after getting a steady or complete combustion, I bet there is a problem. In this case, you must call a professional. In the next chapter, I will describe the reasons responsible for the gas smell that comes out of the exhaust. So, keep scrolling. 

This chapter will reveal the 5 possible culprits behind the gas smell coming out of the exhaust. 

01. Repeated Short Cycles

Short cycle is what I suspect first if you smell gas coming from the water heater. The short cycling on a water heater refers to the period where the system runtime is very short. 

In other words, your tankless water heater gets stuck in the start-up cycle and the unit won’t stay long enough to complete a full cycle. When short cycling, your tankless water heater will turn on and off frequently. 

Now the question is- Is a short-cycling water heater problematic? 


It puts an immense workload on your water heater and causes the unit to wear & tear quickly. Also, short cycling of the unit requires more gas and electricity. Smell unburned gas from the exhaust is another side-effect of a short-cycling water heater. 

Several reasons are responsible for the short-cycling issue, including: 

  • Air inlet filter gets clogged 
  • The heat pump or the recirculation pump gets defective 
  • Improperly size the water heater or installed incorrectly 

How To Fix: 

First off, check the air intake filter of your Navien Tankless unit for a clog. To access the air filter, remove the front panel cover. Then, unscrew the nut that holds the air filter in its place. 

Once you remove the nut, remove the air filter assembly and pull out the filter from the plastic assembly. If it gets dirty by lint or dirt, you must clean it. If needed, scrub the filter with a toothbrush and rinse it with fresh water. 

Afterward, let the filter dry and reinsert it into the plastic assembly after drying it. Finally, reinstall the plastic assembly into its place and secure it with the nut. 

Secondly, check the heat pump and ensure it’s operating finely. If the heat pump doesn’t work properly, it will restrict heat absorptions. I recommend you call a professional if the heat pump is malfunctioning. 

And finally, make sure you size the water heater according to your home size. If you install a too big or small unit for your home, the unit will not function efficiently. 

So, the best bait is- contact a professional to inspect whether the system fits your home or not. Also, ensure you install the unit correctly. 

02. Poor Combustion

Poor combustion is a common issue for the exhaust odor that persists throughout the entire cycle. 

The experience I gained throughout the years of using a tankless unit brings me to this point: Tankless water heaters don’t offer the greatest combustion performance. 

If the burn is dirty, you will smell the gas and it will become worse if the burn becomes dirtier. 

Turns out, tankless water heaters come with a higher baseline compared to other types of equipment. When it comes to Navien Tankless Water Heater, it’s not always possible to set the unit according to the factory specs. As a result, you will experience poor combustion. 

How To Fix: 

I highly recommend you contact a certified plumber to look at this problem. It requires a combustion analyzer and a dual port manometer to test and make an adjustment according to factory reset. 

It will be better to chat with the plumber before they come out. When the professional comes to your house, ensure he brings the combustion analyzer and the manometer. Otherwise, it will be just a waste of money if the plumber doesn’t come with these tools as he can’t do any good without them. 

03. Exhaust Gas May Get Trapped

Where do you set up the exhaust vent for the water heat to free up the exhaust gas? If you set the exhaust vent in a confined place, the gas may get trapped and fail to escape to the open air. It will create a zone for the exhaust gas and it seems  the gas smell comes out of the exhaust . 

How To Fix: 

Ensure you install the exhaust vent for the water heater in an open area so that gas from the exhaust goes in the open air. Never place the exhaust in such an area where it’s surrounded by a fence or big tree. I personally recommend you install the exhaust vent on the top of the roof. 

04. Gas May Leak Out Of The Unit

You can also smell a gas if there is a leak in the gas meter, gas valve, or gas regulator. In such cases, I highly recommend you turn the unit off and call the gas supplier company. 

They will check the gas supply line as well as your entire unit. If they identify a leak on the gas supply line, they will fix it for you. On the other hand, you should call a professional if the leak is on the water heater. 

05. Gas Regulator May Too Close To The Unit

What’s the distance between the gas regulator and the Navien Tankless Water Heater? If the gas regulator is within 6’, there may build up a gas pressure in the combustion chamber. Consequently, it may release gas from the combustion chamber when the unit purges before firing up. 

How To Fix: 

The gas regulator should be at least 6’ away from your tankless water heater. So, ensure the gas regulator isn’t within 6’. Turns out, it’s highly recommended to check the gas pressure to see if it’s within spec or not. Regarding this, you should call a professional to measure the gas pressure. 

How To Check Gas Pressure On Navien Tankless Water Heater?

Go along with the following steps to check the gas pressure on your Navien Water Heater:

  • Turn the gas supply off by turning the gas manual valve to the off position.
  • Turn on a hot water faucet. Now, start the unit to purge the gas in the gas line. You should leave the faucet on until the unit shuts down due to lack of gas supply. Then, shut off the hot water faucet. 
  • Unscrew the nut on the pressure port, which you can find on the gas inlet of your water heater. 
  • Get a manometer and attach it to the pressure port and reset it to 0. 
  • Open the gas valve again and ensure there is no gas leak.
  • Turn on multiple fixtures that have high flow rates. 
  • When the unit reaches its maximum burn, check the gas pressure reading on the manometer. It should be between 5” to 10.5” WC for natural gas or 8” to 13.5” WC for propane. 
  • The gas pressure shouldn’t go down or above the value specified by the manufacturer. 

Note: I highly suggest you perform the gas pressure test by a licensed plumber.


As I mentioned earlier, a bit of gas smell out of your tankless water heater exhaust is normal during ignition.  But the concerning fact is if the gas smell persists throughout the entire cycle. 

Luckily, I mentioned all the possible reasons responsible for tankless water heater exhaust gas smell. Just follow the troubleshooting steps to find the source of the gas smell and fix the issue. 

The best bait is to call a certified plumber if you fail to discover the reason behind the gas smell issue. 

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  1. We constently have the E003 error. I’ve noticed a few other things that may or may not be required. If the propane pressure is low…how can the pressure be increased at the unit? Secondly, does the unit require a backflow damper. We live in Santa Fe, NM and experience high winds 80% of the time. When our unit was installed with pvc for the exhaust and a 90 degree elbow on the roof.
    Thanks for any expertise you can provide. We are looking for a Navien service person now.
    Regards, Frank

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