Tankless Water Heater Exhaust Smells Like Gas

The Short Answer: If you smell gas (a rotten-egg or sulfur odor) indoors, or a carbon monoxide alarm sounds, leave the building now and call the gas utility or 911 from outside. A smell only at the outdoor vent while the heater runs can be exhaust or unburned fuel from poor ignition. Either way, stop using the heater and have a qualified technician check the combustion and venting.

What to do right now

If the odor is inside the house, in the room with the heater, or strong enough that you can smell it from a doorway, treat it as a gas leak:

• Get everyone, including pets, outside.
• Do not operate light switches, appliances, the garage door opener, or the heater’s controls on the way out. Do not use a phone inside.
• Call the gas utility’s emergency number or 911 once you are outside and away from the building.
• Do not go back in, and do not look for the leak yourself. Leak testing with soap solution is for after the area has been made safe, and is the technician’s job.

Navien’s NPE-2 owner manual (opens in a new tab) gives the same basic instructions: do not try to light any appliance, do not touch electrical switches or use a phone in the building, and call your gas supplier from a neighbor’s phone, or the fire department if you cannot reach the supplier.

If a CO alarm goes off, follow the same steps. Carbon monoxide has no smell, so the absence of an odor does not mean the air is safe (CPSC carbon monoxide guidance (opens in a new tab)). Rinnai’s SENSEI manual (opens in a new tab) recommends a CO alarm in the hallway near the bedrooms in each sleeping area.

Raw gas, exhaust, or something else?

Utilities add a sulfur-type odorant to natural gas and propane so leaks can be noticed. Exhaust from a properly burning heater should not smell like that. Use where and when you notice the smell to decide what you are dealing with:

If you cannot tell which row fits, treat it as gas and leave.

Why exhaust ends up smelling like gas

Several faults can push fuel or exhaust where it should not be. All of them are inside the gas, combustion, or vent system, so the diagnosis and the fix belong to a qualified technician.

• Failed or delayed ignition. When the burner does not light promptly, gas released during the attempt can be blown out the vent by the fan. Heaters log this as ignition or flame faults: Navien E003 (ignition failure) and E012 (flame loss) on NPE-2 models, and Rinnai code 11 (no ignition) and code 12 (no flame). Repeated resets mean repeated releases.
• Blocked intake or exhaust. Navien lists E110 (abnormal air pressure) with the instruction to make sure the exhaust pipe is free of obstructions; Rinnai code 10 covers air supply or exhaust blockage and a full condensate trap. Poor airflow leads to poor combustion and odor. See Navien exhaust blockage.
• Separated or damaged vent pipe. Rinnai’s diagnostic guide warns that poorly sealed vent joints can cause exhaust gas leakage. Navien has a specific code, E762, for a vent pipe not properly inserted.
• Dry condensate trap. On condensing heaters the trap’s water seal keeps exhaust out of the drain line. Noritz’s NRC98 owner’s guide (opens in a new tab) says the trap must be refilled after the heater has been out of use for a long time, to stop dangerous exhaust gases from entering the building.
• Gas supply or settings problems. Wrong gas pressure, or a heater set up for the wrong fuel, burns poorly. Navien has codes for wrong gas type (E773) and a gas-type setting mismatch (E788).

What you can check safely

Only if there is no gas smell indoors and no CO alarm:

• Turn the heater off at its power button and do not keep resetting a code. Write the code down.
• Look at the outdoor vent and intake from outside for snow, ice, nests, leaves, or a cover left in place. Do not reach into a vent on a heater that is running. Navien’s manual asks owners to inspect the vent termination and air intake once a year.
• Look, without touching, at any visible vent pipe indoors for joints that have pulled apart, cracks, corrosion, or melting. Navien and Noritz both say to stop using the heater if the vent is damaged, separated, holed, blocked, or corroded.
• Check that the CO alarms in the home work.

Do not remove the front cover. Navien’s manual warns that operating the heater with the cover open can cause CO poisoning. Do not tighten gas fittings or adjust anything on the gas line.

What the technician will check

A qualified gas technician will leak-test the gas piping and the heater’s gas connections, check supply and manifold gas pressure against the rating plate, confirm the heater is set up for the fuel it is burning, inspect the burner and ignition system, and test combustion at the vent. They will check the vent and intake for proper assembly and materials, and on condensing models confirm the condensate trap is full. The heater should not go back into service until the cause is found.

Frequently Asked Questions

Q: Is a smell at the vent normal when a tankless heater starts? A: A rotten-egg smell is not. Exhaust from a properly burning heater should not smell like fuel gas. A puff of odor at each start, especially with a whoosh or bang, suggests delayed ignition, which needs service.

Q: The smell is only outside at the vent. Can I keep using hot water? A: Stop using the heater until it has been checked. Unburned fuel or poor combustion at the vent can get worse, and the same fault can send exhaust indoors if the vent or intake is compromised.

Q: Why does my heater keep showing an ignition code along with the smell? A: Ignition codes (Navien E003, Rinnai 11) mean the burner did not light on that attempt. Each reset is another attempt that can release gas. Leave the heater off and have the gas supply, settings, and ignition checked.

More causes of odor around water heaters are covered in water heater smells.