Why Does My Hot Water Smell Like Rotten Eggs? Causes and Fixes

You turn on the hot tap to wash your hands and you are hit by the unmistakable smell of rotten eggs. It is one of the most common — and most misunderstood — water quality complaints that Ontario homeowners bring to plumbers. The good news: the cause is almost always identifiable and fixable. The better news: it is usually not a sign of anything seriously wrong with your water supply.

This guide from Mechora Group's plumbing team explains exactly what causes that smell, why it affects your hot water but not your cold, and walks through every solution from a simple temporary fix to the most permanent remedy available.

What Causes the Rotten Egg Smell in Hot Water?

The smell is caused by hydrogen sulphide gas (H2S). H2S has a powerful, distinctive odour — the characteristic "rotten egg" smell — detectable by the human nose at extremely low concentrations, often below one part per million. It is produced inside your water heater tank through a biological and chemical reaction involving three components:

• Sulphate-reducing bacteria (SRB) — a type of anaerobic bacteria naturally present in many water supplies at low levels. They are harmless in normal quantities.
• Sulphur compounds — specifically sulphates (SO₄²⁻) naturally dissolved in the water supply. Ontario's water supplies vary in sulphate content, with groundwater sources typically having higher concentrations than Great Lakes surface water.
• The magnesium anode rod — a sacrificial metal rod installed in virtually every tank water heater to protect the steel tank walls from corrosion. The standard material is magnesium.

Here is the reaction in plain terms: Sulphate-reducing bacteria in the tank use the sulphates in the water as an electron acceptor in their metabolic process. The magnesium from the anode rod accelerates this reaction, providing electrons that the bacteria use to chemically reduce sulphate into sulphide. The sulphide dissolves in water and outgasses as H2S when the hot water exits the tap.

The magnesium rod is not the original "cause" — you would sometimes get mild H2S without it, but the magnesium dramatically amplifies the reaction. This is why simply removing the anode rod eliminates the smell but is not a recommended solution — the rod's job of protecting your tank is important.

Why It Affects Hot Water But Not Cold

If you run the cold water and it smells perfectly fine, then switch to hot and get the rotten egg odour, this is characteristic and strongly points to the water heater as the source. There are two reasons hot water is affected while cold is not:

Temperature. Sulphate-reducing bacteria are thermophilic — they prefer warm environments. The interior of a water heater tank, typically maintained at 49–55°C, is an ideal environment for their growth. Cold water at ground temperature (around 8–12°C in Ontario) is too cold for them to thrive. The bacteria are present throughout the supply at harmless levels but only proliferate actively inside the warm tank.

Anaerobic conditions. The inside of a water heater tank, especially the bottom where sediment accumulates, is oxygen-depleted — an anaerobic environment. Sulphate-reducing bacteria are anaerobic and require this low-oxygen environment to function. Cold water in the supply lines is under pressure and exposed to oxygen at point of use, which is inhospitable to the bacteria.

The anode rod. The magnesium anode rod is inside the water heater. Cold water never contacts it. The rod's chemistry only accelerates H2S production inside the tank itself.

How to Confirm the Source

Before treating the problem, confirm that the water heater is indeed the source and not a sewer gas issue or a sulphur problem in your supply:

Test 1 — Hot vs. Cold: Run your cold kitchen tap for 30 seconds and smell the water. No smell? Run the hot water for the same time. Strong rotten egg smell? Your water heater is the source.

Test 2 — After Idle Periods: Does the smell worsen after the water heater has sat unused overnight or over a weekend? This is characteristic of the bacterial cause — bacteria have had more time to produce H2S in the standing water.

Test 3 — Does It Fade? Run the hot water for 2–5 minutes. Does the smell decrease after a while? This indicates you are flushing the tank's standing water through and replacing it with fresher water that hasn't had as long to react.

If the smell is present in both hot and cold water, or is present throughout the home without running water at all, you may have a sulphur issue in your water supply (requiring whole-house filtration) or a sewer gas problem — which requires prompt attention. Contact a plumber to investigate sewer gas sources.

Solutions: From Temporary to Permanent

Distinguishing H2S Smell from Sewer Gas

Both hydrogen sulphide from a water heater and sewer gas from a plumbing drainage issue have objectionable odours, but they are distinct and come from different sources:

Hot water heater H2S smell:

• Present when running the hot water tap
• Absent from cold water (in most cases)
• Strongest after the heater has been idle
• Fades after running hot water for a few minutes
• Smells specifically like sulphur/rotten eggs — a clean, sharp chemical smell

Sewer gas smell:

• Present without running water — emanating from drain openings
• Often stronger in specific rooms (near a floor drain, in a bathroom with a dried-out P-trap)
• More complex odour — organic decomposition, not just sulphur
• Often caused by a dried-out P-trap (pour water into infrequently used floor drains or sinks), a cracked drain pipe, a failed wax ring under a toilet, or a missing trap on a fixture

If you smell a sewage-like odour emanating from drains or in specific rooms without running water, this is not a water heater problem. A cracked sewer pipe or improperly vented drain can allow sewer gas — which contains methane in addition to H2S — into the living space. This warrants a prompt call to a licensed plumber for drain and sewer inspection.

Ontario Water Quality Context

Ontario's water quality varies significantly by region and water source. Municipal water supplies across the GTA — primarily drawn from the Great Lakes — have relatively low sulphate content, and the rotten egg smell issue is correspondingly less common in homes supplied by Lake Ontario or Lake Simcoe water than in homes on private wells or groundwater-fed municipal supplies.

That said, even very low sulphate concentrations in the water supply can support sulphate-reducing bacteria in the warm anaerobic environment of a water heater tank, particularly in older units with depleted anode rods. The smell does not require a supply water problem — it requires only the right bacteria, the right temperature, and a magnesium anode rod.

For homeowners on private well water, sulphate content is generally higher and the rotten egg smell problem is correspondingly more prevalent. A complete water test from an accredited lab will quantify sulphate levels and help determine whether in-tank solutions (anode rod replacement) or supply-side treatment is the appropriate approach.

Mechora Group Water Heater Services

Mechora Group Inc. provides full water heater services across the GTA including anode rod inspection and replacement, tank flushing and maintenance, water heater installation (tank and tankless), and advice on water quality issues affecting water heater performance. We serve Toronto, Mississauga, Brampton, Vaughan, Markham, and Oakville.

If you are dealing with hot water smell and want a permanent solution — or if your water heater is approaching the end of its service life and you are considering a new installation — contact us for an honest assessment and a fair quote.