Tankless Water Heater Smells Like Sulfur
On this page (6 sections)
The Short Answer: First decide where the smell is. A rotten-egg smell in the air near a gas tankless heater may be natural gas or propane, so leave and call the utility. A sulfur smell only in the hot water is usually hydrogen sulfide from the water supply or bacteria growing in warm, stagnant water such as a recirculation loop. Most tankless heaters have no anode rod to replace.
Air or water?
| Where you smell it | Likely cause | What to do |
|---|---|---|
| In the room, near the heater or gas line | Possible gas leak | Leave; call from outside |
| Near the vent outside, mainly when the heater fires | Exhaust or a combustion problem | Have the heater inspected; see tankless exhaust smells like gas |
| In hot and cold water | Hydrogen sulfide in the water supply, often from a well | Water testing and treatment |
| Only in hot water, worse after the water sits | Bacteria in a recirculation loop, storage or buffer tank, or seldom-used pipes | Find the stagnant water |
| Only in hot water, home also has a storage tank in the hot line | The tank’s anode reacting with the water | Treat the tank, not the tankless |
| Near a floor drain or the relief valve drain | Dry drain trap | Refill the trap with water |
Why tankless and tank heaters differ
In conventional storage tanks, sulfur-smelling hot water often comes from a reaction between a magnesium anode, sulfate in the water, and bacteria in the warm tank. Most gas and electric tankless heaters have no storage and no anode, so the smell usually originates somewhere else. Heating can make an existing hydrogen sulfide smell more noticeable because warm water releases the gas more readily.
Common water-side sources
- Well water. Hydrogen sulfide is common in some groundwater. If cold water also smells, especially when first run, the source is the well, not the heater.
- Recirculation. Recirculation systems keep warm water moving through a loop. If the loop runs at moderate temperatures and has sections where water stagnates, bacteria can grow.
- Buffer or storage tanks. Some tankless models have a small built-in buffer, and some installations include a separate tank. Either can harbor bacteria or contain an anode.
- Vacation or low use. Water that sits in the heater and pipes for weeks can develop an odor that fades after flushing taps.
What you can check
- Smell the cold water at several taps and at an outdoor spigot before the heater.
- Run hot water for a few minutes. Note whether the smell fades (stagnant water) or persists (supply or ongoing growth).
- Note whether the smell started after a recirculation system or a storage tank was added.
- Check for a second water heater or tank in the hot-water line.
What a plumber or water specialist will check
A water treatment specialist can test for hydrogen sulfide, sulfate, iron, and sulfur-reducing bacteria and recommend treatment such as oxidation, filtration, or chlorination at the well. A plumber can review the recirculation loop’s temperature and layout, and disinfect or flush the heater and loop following the manufacturer’s procedure. If a separate storage tank is the source, its anode can be changed to a type suited to your water.
For tank heaters, see water heater smells like dead animal and the water heater smells hub. For RV heaters, see RV water heater smells like rotten eggs.
Frequently Asked Questions
Q: Should I replace the anode rod in my tankless heater? A: Most tankless heaters do not have one. Check your manual. If your system includes a storage tank, its anode may be involved.
Q: Is sulfur-smelling hot water harmful? A: At the low levels that cause odor, hydrogen sulfide is mainly a nuisance, but it can indicate bacteria or other water problems. Testing tells you what you are dealing with.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
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