If your hot water smells like rotten eggs but the cold tap runs clean, the cause is almost always hydrogen sulphide gas forming inside your water heater. Do a quick sniff test on both taps, check whether the smell is isolated to hot water, and if you also smell gas anywhere near the unit, treat it as an emergency and get everyone clear of the area. Otherwise, this is usually a DIY-fixable heater issue, not a supply crisis.
TL;DR:
- Most cases involve hydrogen sulphide gas produced by bacteria reacting with magnesium anode rods, especially in hot water only scenarios.
- Running a quick diagnostic, like testing multiple fixtures and checking the anode rod, helps identify whether the heater or supply is at fault.
- Flushing the tank, replacing the anode with an aluminium-zinc version, and conducting a high-temperature sterilisation are key steps to eliminate smells.
- Persistent odours after these steps may indicate a failing tank lining or broader equipment issues, often requiring professional replacement.
- Regular maintenance, such as annual flushing and anode inspection, prevents bacteria buildup and keeps the smell from recurring.
Table of Contents
- What that rotten-egg or musty smell in your hot water actually means
- How to work out whether the heater, your fixtures, or the supply is to blame
- Step-by-step fixes: flushing, disinfection, anode swaps and sediment removal
- Keeping the smell from coming back
- When it’s time to call in a professional
- Field perspective: what years of smelly hot water calls actually teach you
- Sorting out smelly hot water for good with Priority Plus Plumbing
- Where to check the science and get further guidance
- Sources
- FAQ
What that rotten-egg or musty smell in your hot water actually means
Different smells point to different culprits, and getting this right saves you from chasing the wrong fix. A rotten-egg or sulfur smell in hot water almost always comes down to hydrogen sulphide gas, produced when sulfate-reducing bacteria inside the tank react with the magnesium anode rod. It’s the anode’s job to corrode instead of your tank, but that same chemistry feeds the bacteria that make the gas.
A sewage-like smell is a different animal entirely. That usually traces back to a drain, trap, or vent issue rather than the water itself, particularly if it’s strongest around a specific sink or shower drain.
Musty or earthy odours are commonly linked to bacteria or biofilm sitting in a faucet aerator or a rarely-used branch of pipe, and cleaning the fixture often clears it up. A metallic tang points to corroding pipes or a failing anode rod releasing dissolved metal into the water.
Here’s a quick way to map smell to cause:
- Rotten eggs, hot tap only: hydrogen sulphide from bacteria and anode reaction inside the heater
- Rotten eggs, hot and cold: likely a supply-side or well-water issue, not your heater
- Sewage odour near a drain: dry trap, vent blockage, or drain-line problem
- Musty or earthy smell: biofilm in an aerator or a stagnant pipe branch
- Metallic taste or smell: corrosion in pipework or a spent anode rod
How to work out whether the heater, your fixtures, or the supply is to blame
Before you touch a spanner, run through a short diagnostic sequence. It takes ten minutes and tells you exactly where to focus.
- Smell both taps. Run hot water for 30 seconds and sniff, then do the same with cold. Hot-only odour means the heater; both taps smell means supply or well water.
- Test more than one fixture. If only one tap smells, the problem is local, likely a dirty aerator or a dry trap nearby.
- Try the glass test. Fill a glass from the hot tap and let it sit for a few minutes away from the drain. If the smell develops or strengthens in the glass, it’s in the water. If it fades once you’re away from the sink, the drain is the likely source.
- Check the aerator. Unscrew it and give it a rinse under running water. A gunky aerator is a common, easily fixed cause of musty smells.
- Note whether it’s citywide or just your place. If neighbours on the same supply report the same smell, contact your water utility rather than reaching for tools.
Pro Tip: When our technicians arrive on a hydrogen sulphide call, the first thing checked is anode condition. A heavily corroded magnesium rod covered in a dark, gritty film is usually the smoking gun, and it tells us in seconds whether we’re dealing with bacteria in the tank or something further down the line.
Step-by-step fixes: flushing, disinfection, anode swaps and sediment removal
Once you’ve confirmed the smell is coming from the heater, there’s a logical order to work through fixes. Each step builds on the last, and skipping ahead often means the smell just comes back.
- Flush the tank. Turn off the power or gas to the heater, connect a hose to the drain valve, and run water out until it’s clear. For most residential tanks this takes 15 to 20 minutes and clears out loose sediment feeding bacterial growth.
- Consider thermal sterilisation. Raising the thermostat to a sufficiently high temperature for a few hours can kill off sulfate-reducing bacteria in the tank. This carries a real scalding risk, so reset the thermostat to a safe household setting (around 50°C) immediately afterwards, and warn everyone in the house before you start.
- Look at chlorination for stubborn cases. Shock chlorination of the tank is one of the standard responses to heater-generated hydrogen sulphide, but it involves handling concentrated chlorine solution safely and flushing it out completely before use. Most homeowners are better off having a licensed plumber do this step.
- Replace the anode rod. Magnesium anodes are the more reactive metal and the more common trigger for smell. Swapping to an aluminium-zinc anode often resolves the issue because it’s less reactive with the bacteria involved, though it also means slightly less corrosion protection for the tank itself.
- Check for sediment buildup. If your hot water runs cloudy, is noisier than usual, or the flush water came out gritty, sediment is likely accumulating and insulating the bottom of the tank, which can accelerate both bacterial growth and heating inefficiency.
The most durable outcome usually combines more than one of these steps. A combined approach of flushing, sterilisation and an anode swap tends to hold up far better than any single fix on its own, and relying on just one often means dealing with the smell again within months.
Stop and call a licensed plumber if: your unit is gas-powered and needs internal inspection, you’re not confident accessing an electric element safely, or you’ve tried flushing and an anode swap and the smell persists. If you ever smell gas near the unit itself, that’s a genuine emergency, not a smelly-water problem.
For a broader look at hot water faults beyond odour, our guide to hot water not working covers related warning signs worth knowing.
Keeping the smell from coming back
Once you’ve fixed it, a light maintenance routine keeps it that way. Sulfate-reducing bacteria thrive in stagnant, sediment-heavy conditions, so the goal is simple: keep water moving and keep the tank clean.
- Flush your tank annually, or every six months if you’re on well water or have hard water.
- Inspect the anode rod every one to two years and replace it before it’s fully corroded.
- Run taps you rarely use at least once a fortnight to stop stagnant water sitting in dead-end pipe branches.
- Clean aerators every few months, especially in bathrooms that see less daily use.
- Avoid leaving a heater switched off and empty for extended periods, since stagnant water is exactly what bacteria need to establish.
On thermostat settings, don’t run your heater permanently hot as a “prevention” measure. Occasional short sterilisation cycles are fine, but sustained high temperatures raise scalding risk for no lasting benefit. For more on how heater settings affect both smell and running costs, see our piece on hot water and high bills.
When it’s time to call in a professional
Some situations are worth escalating straight away rather than working through the DIY steps first.
- Any gas smell near a gas hot water unit means evacuate and call your gas provider’s emergency line immediately, not a plumber first.
- Persistent odour after flushing, sterilising, and replacing the anode suggests something beyond a simple fix.
- Black staining or flaking inside the tank or in drained water points to lining damage, not just bacteria.
- Cold-tap odour across multiple outlets in your home, or reports from neighbours on the same supply, means it’s time to contact your water utility rather than your plumber.
A professional visit typically includes a full inspection of the anode and tank condition, a proper disinfection if needed, sediment removal, and water testing if supply contamination is suspected. Before any work starts, ask for their licence number, what specific tests they’ll run, and a clear, upfront price. Our hot water repair guide covers the kind of issues a proper diagnostic visit tends to uncover.
Field perspective: what years of smelly hot water calls actually teach you
Most callouts for smelly hot water aren’t mysteries. Technicians triage by checking the anode first, because it explains the majority of cases in a single glance. The mistake we see most is homeowners over-sterilising a tank repeatedly instead of swapping the anode, which wastes effort and still leaves the underlying cause untouched. Occasionally the smell keeps returning no matter what’s replaced, and that’s usually a sign the tank lining itself has failed. At that point, repeated fixes are a waste of money, and replacement is the honest answer. Priority Plus Plumbing has spent over 25 years making exactly that call for Sutherland Shire homes, backed by 24/7 availability and CCTV inspection capability when drains are part of the puzzle.
— Peter Honey
Sorting out smelly hot water for good with Priority Plus Plumbing
Priority Plus Plumbing is the straightforward alternative to guessing your way through anode swaps and chlorine dosing yourself, our technicians diagnose the exact cause on the first visit and fix it properly the same day in most cases. Our hot water technicians handle the full range covered above, tank disinfection, anode inspection and replacement, sediment flushing, and full system replacement when a tank’s lining has failed beyond repair.
Every service call starts with a proper inspection, not a guess, and includes upfront pricing before any work begins, so you know the cost before we start. Safety comes first on every job: gas units get a full leak check, and any electrical work near the heater is isolated properly before we touch it. If you’re dealing with a smell that won’t shift, or you’d rather have a licensed plumber handle the sterilisation and anode swap from the start, book a hot water system repair with Priority Plus Plumbing today, and we’ll have someone out to sort it properly.
Where to check the science and get further guidance
- NHMRC’s drinking water guidelines cover taste and odour standards for Australian tap water.
- Health WA’s page on hydrogen sulphide explains the gas and its health considerations in plain terms.
- Portland’s drinking water troubleshooting guide is a useful practical reference for sorting plumbing issues from supply issues.
Sources
- Australian drinking-water guidelines — taste and odour (NHMRC)
- Hydrogen sulphide and public health (Health WA)
- Hydrogen sulfide and sulfur bacteria in well water (MN Dept. of Health)
- Rotten egg smell in tap water? Start at the hot tap (TapWaterData)
FAQ
How do I get rid of a bad smell in my hot water?
Flush the tank, replace the magnesium anode with an aluminium-zinc version, and consider a brief thermal sterilisation cycle. Combining all three gives the most lasting result rather than relying on just one fix.
Why does my hot water stink but not cold?
A hot-only smell means the odour is being created inside the heater itself, typically hydrogen sulphide from bacteria reacting with the anode rod, rather than coming in through your water supply.
Is it normal for hot water to have a smell?
It’s a common issue, especially in tanks with a magnesium anode, but it’s not something to ignore. The smell itself is usually more of a diagnostic clue than a health emergency, though persistent odour still needs addressing.
Why does my hot water heater smell like sulfur?
Sulfate-reducing bacteria living in the tank react with the magnesium anode rod to produce hydrogen sulphide gas, which is what creates that distinctive rotten-egg smell. It’s a chemical process inside the unit, not a fault with your water supply.