Quick answer
Hot water that smells like rotten eggs almost always means hydrogen sulphide gas is forming inside the tank: harmless sulphate-reducing bacteria in the water feed on the magnesium anode rod and release the gas. It is not a gas leak. Flushing and sanitizing the tank, then swapping the magnesium anode for an aluminum-zinc or powered anode, fixes it in most Burlington homes.
If your hot water smells like rotten eggs but the cold water from the same tap smells fine, the problem lives inside your water heater, not in the municipal supply. The odour is hydrogen sulphide, and in a Burlington, Oakville or Hamilton home on treated Lake Ontario water it is being produced right in the tank. Here is what is happening, how to confirm it, and what actually stops it.
What causes the rotten egg smell in hot water
The smell comes from sulphate-reducing bacteria that live in almost every water supply in small numbers. They are not a health threat, but inside a warm tank they find two things they love: sulphate in the water and a steady supply of free electrons from the magnesium anode rod that protects the steel tank from rust. As the anode corrodes it feeds the bacteria, and the by-product is hydrogen sulphide gas dissolved in your hot water.
Three conditions make it worse. A tank set below about 49°C sits in the temperature range where these bacteria multiply fastest. Water that sits unused for days — a cottage, a rental between tenants, or a Millcroft home that was empty for a two-week vacation — gives them time to bloom. And a water softener speeds up anode corrosion, so softened homes tend to get the smell sooner.
How to tell if it's the water heater or the water supply
- Run the cold tap alone for a minute and smell it. If the cold water smells of sulphur too, the source is upstream of the heater — typical for homes on private wells in rural north Burlington (Kilbride, Lowville, Mount Nemo) and parts of Flamborough, where sulphur in the groundwater is common.
- If only the hot side smells, the tank is the source. Municipal water in the urban parts of Halton Region and Hamilton is chlorinated lake water and rarely carries a sulphur odour on its own.
- If the smell is strongest at one fixture only, check that fixture's drain and P-trap first — sewer gas from a dry trap is often mistaken for a water heater problem.
- A smell that appears after the house sat empty for a week or two, then fades with daily use, points squarely at bacteria in the tank.
How to get rid of the smell for good
A one-time fix is a flush and sanitize: the tank is drained, filled with a hydrogen peroxide solution (a litre or two of 3% peroxide is the usual dose for a 40- or 50-gallon tank), left to sit for an hour or two, then flushed through every hot tap in the house. Peroxide kills the bacteria and leaves no chlorine taste. This buys relief, but with the same magnesium anode still in the tank the odour usually returns within months.
The permanent fix is to change the anode. An aluminum-zinc anode feeds the bacteria far less than magnesium and stops the reaction in most homes. A powered anode — a titanium rod driven by a small plug-in transformer — removes the reaction entirely and never wears out, which is the right call for a softened home. Both still protect the tank from corrosion, so you are not trading smell for a shorter tank life.
Getting the old anode out is the hard part. It sits under a hex head on top of the tank, is usually seized after years of heating cycles, and in a low-ceiling basement — common in 1950s and 60s bungalows in Aldershot and Roseland — there is not enough headroom to pull a straight rod. A segmented (flexible) anode solves that. This is a routine water heater repair; most run $150–$800 depending on the anode type and how stubborn the old one is.
Does turning up the temperature fix it?
Raising the tank to 60°C slows the bacteria and reduces the smell, and it is the approach Ontario's plumbing code takes in new homes — a 60°C tank paired with a thermostatic mixing valve so taps never deliver more than 49°C. If you turn the tank up without a mixing valve, scald risk rises sharply, especially for children and older adults. Treat temperature as a helper, not the cure; the anode swap is what solves it. Our answer on what temperature to set a water heater to covers the trade-offs.
When the tank should be replaced instead
If the tank is past 8–12 years, the anode has likely been consumed down to a bare wire and the smell is a sign the steel itself is now corroding. Spending on a new anode at that point buys little. A like-for-like tank replacement lands at $1,200–$2,500 installed, and a new tank with the right anode from day one is the cleanest fix. Our guide on how long a water heater lasts in Ontario helps you judge where yours sits.
ZK Mechanical's TSSA-licensed technicians handle anode changes, sanitizing flushes and tank replacements across Burlington, Oakville and Hamilton. See our water heater services in Burlington or book a visit and tell us whether the smell is on the hot side only — it shortens the diagnosis.