A rotten-egg smell from hot water is usually a reaction between naturally occurring bacteria and your water heater's anode rod, not a gas leak.
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Quick answer
A sulphur or rotten-egg smell from hot water, but not cold, is typically caused by naturally occurring sulfate-reducing bacteria in the tank reacting with hydrogen released from the magnesium anode rod, producing hydrogen sulfide gas. This is a water chemistry issue inside the tank, not a gas leak, and it's usually resolved by switching to a different anode rod type, such as aluminum-zinc or a powered anode, or by having the tank flushed and disinfected, rather than by anything involving the home's actual gas supply.
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Every tank-style water heater includes a sacrificial anode rod, usually magnesium or aluminum, whose job is to corrode preferentially in place of the steel tank, protecting the tank from rusting through over its service life. As that anode corrodes, it releases hydrogen. In most tanks that hydrogen simply dissipates without issue, but if certain naturally occurring sulfate-reducing bacteria are present in the water supply or have colonized the tank, they can use that hydrogen in a chemical reaction that produces hydrogen sulfide gas, the compound responsible for the distinct rotten-egg smell.
A useful diagnostic clue: if the smell appears only in hot water and not in cold water from the same tap, that points strongly toward this anode-and-bacteria reaction happening inside the tank, rather than an issue with the incoming municipal supply itself.
It's worth being clear that this hydrogen sulfide smell in water is a completely different situation from a natural gas leak, even though both can smell sulphurous, since utilities intentionally add a similar-smelling compound to natural gas for detection. If the smell is specifically tied to running hot water at the tap, and there's no odour near the gas appliance itself or its connections, it's very likely the water chemistry issue described here. If you're ever unsure, or if you smell something near the water heater's gas connection or venting rather than at the tap, treat it as a possible gas concern and act accordingly.
| Anode type | Effect on sulphur smell |
|---|---|
| Magnesium (most common factory default) | Most reactive, often the biggest contributor to the bacterial reaction |
| Aluminum-zinc | Often reduces or eliminates the smell while still protecting the tank |
| Powered (impressed current) anode | Doesn't corrode the same way, commonly resolves the smell entirely |
Switching from a magnesium anode to an aluminum-zinc anode is often the first and simplest step a technician tries, since it still provides sacrificial protection for the tank but is less likely to fuel the bacterial reaction. A powered anode, which uses a small electrical current instead of sacrificial metal corrosion, avoids the hydrogen production mechanism altogether and is a common solution for households where the smell keeps returning even after an anode swap.
Alongside changing the anode type, a technician may recommend flushing the tank thoroughly and, in some cases, disinfecting it to reduce the bacterial population inside. This is particularly relevant if the tank has gone a long time without maintenance, since sediment at the tank bottom can also harbour bacteria and worsen the smell independent of the anode material.
Raising the tank's thermostat temporarily to a higher setting for a period can also help reduce bacteria levels, since sulfate-reducing bacteria are less tolerant of higher temperatures, though this should be balanced against scald risk and done under a technician's guidance rather than left permanently at a higher setting without consideration for that risk.
If a sulphur smell is present in both hot and cold water, or if you're on a private well rather than municipal supply, the source is more likely the water itself rather than an anode reaction specific to the water heater, and that calls for a different approach involving water testing and possibly whole-home treatment rather than simply changing the anode rod.
Safety note
If you ever smell gas near the water heater's gas connections, venting, or elsewhere in the home, leave the area and call FortisBC at 1-800-663-9911. Anode rod replacement and gas water heater servicing should be performed by a Technical Safety BC licensed gas fitter.
Hydrogen sulfide from this bacterial reaction inside the tank is generally a nuisance issue affecting odour rather than an immediate safety hazard, but it's still worth addressing, and any suspected natural gas smell should always be treated seriously.
Switching to an aluminum-zinc or powered anode resolves the smell in many cases, but not universally, since bacteria levels and water chemistry vary, so a technician may combine it with flushing or disinfection.
The reaction producing hydrogen sulfide happens inside the water heater tank itself between the anode and bacteria, so cold water bypassing the tank typically doesn't carry the same smell.
Removing the anode rod entirely isn't recommended, since it leaves the steel tank unprotected against corrosion and will shorten the tank's service life considerably.
Higher sustained temperatures can reduce bacterial activity, but this needs to be balanced against scald risk at the tap, so it's best discussed with a technician rather than adjusted on your own without guidance.
It can occur with both municipal and well water, but private wells are somewhat more prone to it depending on local groundwater chemistry, which is worth mentioning if you're troubleshooting a well-supplied home.
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