A rotten egg odour in hot water from a Bosch tankless unit is typically a bacterial or sediment issue in the water itself, not a gas leak, but it still deserves a proper check.
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Quick answer
A rotten egg smell in hot water from a Bosch tankless water heater is almost always caused by sulfur-reducing bacteria reacting with sediment or minerals inside the plumbing or heat exchanger, not by natural gas. It is worth confirming this distinction quickly: a true gas odour smells different and calls for leaving the home immediately. For a water-related sulfur smell, a licensed technician can flush the unit, inspect the heat exchanger, and check the water supply to resolve it.
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The single most important thing to sort out first is whether the odour is coming from the water itself or from the gas supply. Utilities add a distinct sulfur-like scent to natural gas specifically so leaks are noticeable, and that smell is present in the air around the appliance and gas line, not only when a hot tap is running. A water-borne rotten egg smell, by contrast, is noticed specifically when hot water runs from a tap and tends to fade once the water has run for a moment.
If you smell gas in the air near the appliance, in a closet, or anywhere in the home independent of running a tap, treat it as a possible gas leak: leave the home immediately and call FortisBC at 1-800-663-9911 from outside. Do not operate switches, light anything, or investigate further yourself. The remainder of this article addresses the far more common scenario, where the smell is tied specifically to hot water use.
A sulfur or rotten egg smell in hot water is most often caused by naturally occurring sulfate-reducing bacteria that live in water systems and react with sediment, organic matter, or certain metal components to produce hydrogen sulfide gas. Because these bacteria thrive in warm, low-flow conditions, they show up in hot water far more often than cold, and they are common in any water heater, tank or tankless, gas or electric.
On a Bosch tankless unit, the odour typically comes from sediment or biofilm accumulating in the heat exchanger, the inlet filter, or in a length of pipe near the unit rather than a built-in tank, since there is no standing reservoir of water inside a tankless heater. Homes on private wells in the Fraser Valley are more prone to this issue because groundwater naturally carries more sulfate and mineral content than Metro Vancouver's treated reservoir supply.
A technician investigating a sulfur smell on a Bosch tankless system starts by confirming the odour pattern: whether it affects only hot water, only certain fixtures, or the whole home, and whether it is constant or intermittent. This narrows down whether the source is the water heater, a specific branch of piping, or the incoming water supply.
The service typically includes flushing the unit's heat exchanger and inlet filter to clear sediment and biofilm, inspecting for any anode-style component or metal fitting that could be reacting with the water, and checking water pressure and flow settings, since stagnant or low-flow conditions encourage bacterial growth. On well-fed properties, the assessment may also point toward water treatment upstream of the heater.
Throughout, the technician confirms there is no coincidental gas-related issue at the appliance, since a proper diagnostic always rules out combustion and venting concerns before focusing on water quality.
A sulfur smell in hot water is a water-quality and maintenance issue in the overwhelming majority of cases, not a sign the tankless heater has failed. A professional flush and filter cleaning resolves it in most homes, and the appliance itself typically continues to perform normally afterward.
Replacement of the unit is rarely warranted for odour alone. It becomes a consideration only if a technician finds that internal components have degraded from long-term untreated sediment or scale buildup, which is uncommon on a properly maintained system that receives periodic descaling.
| Where the smell shows up | Likely cause | Typical next step |
|---|---|---|
| Air near appliance, not tied to water use | Possible gas leak | Leave home, call FortisBC 1-800-663-9911 |
| Hot water only, all fixtures | Sediment or bacteria in the heater/heat exchanger | Professional flush and inlet filter cleaning |
| Hot water at one fixture only | Localized pipe or fixture issue | Inspect that branch line or fixture |
| Consistent smell, well-fed property | Naturally sulfate-rich groundwater | Water treatment assessment ahead of heater |
Because natural gas is scented to smell similar to sulfur, always treat an unexplained odour near a gas appliance seriously until it has been properly identified. Leave the home first and call FortisBC at 1-800-663-9911 if you have any doubt. Any work on the gas supply, venting, or combustion side of a tankless unit must be carried out by a Technical Safety BC licensed gas fitter; CANRO holds gas licence LGA0043870 and boiler licence LBP0046450.
Safety note
If an odour resembling rotten eggs or sulfur is present in the air around the appliance and not only in running water, treat it as a possible natural gas leak: leave the home immediately and call FortisBC at 1-800-663-9911. Gas fitting, venting, and combustion work must be performed only by a Technical Safety BC licensed gas fitter.
A water-borne sulfur smell caused by bacteria or sediment is generally a nuisance and maintenance issue rather than a health emergency, though persistent odour is worth having checked, especially on well water.
Smell the air directly around the appliance without running any tap. A true gas odour is present independent of water use, while a water-related sulfur smell only appears when hot water is running.
Sulfate-reducing bacteria thrive in warmer, slower-moving water, which is why the odour typically develops in the hot water side of a plumbing system rather than the cold side.
In most cases yes. A professional flush of the heat exchanger and inlet filter clears the sediment and biofilm that bacteria feed on, resolving the odour for typical households.
Rarely. A sulfur odour is almost always resolved through cleaning and maintenance rather than requiring replacement of the unit itself.
Yes. Private well water often carries more natural sulfate content than Metro Vancouver's treated municipal supply, making the odour more frequent and sometimes recurring in well-fed homes.
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