Why Does My Hot Water Smell of Rotten Eggs?

Updated July 2026 · Emergency Plumber Chelsea

The smell comes from hydrogen sulphide, the same gas that gives rotten eggs their odour, dissolved in your water. In a hot water system it forms when naturally occurring sulphates in the water meet bacteria that thrive in warm, low-oxygen conditions. The reaction is often speeded up by the magnesium sacrificial anode fitted inside many hot water cylinders to protect the tank from corrosion. The anode does its job of protecting the steel, but as a by-product it can feed the reaction that makes the gas. This is why the smell so often appears on the hot side only.

Before anything else, work out whether it is a gas leak rather than a water problem, because the two smell similar to some people and one is an emergency. Natural gas is given a deliberate sulphur-like odour so you can detect leaks. If you smell it near the boiler, the hob or the meter rather than at a running tap, if it is stronger when appliances are on, or if anyone feels dizzy or unwell, treat it as a gas escape. Leave the property, do not touch switches, and call the National Gas Emergency Service on 0800 111 999 from outside. A water smell, by contrast, only appears when you run a tap and is tied to the water itself.

Once gas is ruled out, the single most useful test is hot versus cold. Run the cold tap and smell it, then run the hot and smell that. If only the hot water smells of eggs, the problem is almost certainly inside your hot water cylinder or hot pipework, most often the anode reaction or bacteria in a rarely used tank. If both hot and cold smell, the issue is more likely in the incoming supply, a filter, or occasionally the wider water main, and it is worth checking with a neighbour to see whether they have the same thing. Cold-only smells are unusual and point to the supply rather than your heating.

The second test is one tap versus all taps. If every hot tap in the home smells, the source is central: the cylinder or the main hot run. If only one tap smells, especially a bathroom basin used infrequently in a Chelsea second bathroom or guest suite, the cause is often stagnant water sitting in a dead leg of pipe or in a tap you rarely open. Running that outlet hard for a few minutes will usually flush it and tell you whether the smell is local or systemic. Note which taps are affected before you call, as it saves diagnostic time.

If the smell is on the hot side and comes from a cylinder, the classic first fix is to flush the tank through with hot water to clear the bacteria and any settled debris. Run the hot taps for several minutes to move a full tank of water through the system, ideally after turning the cylinder thermostat up temporarily. Sulphate-reducing bacteria are killed by heat, so raising the store to around 60 degrees for a period can clear a mild case. Take care afterwards, because water at that temperature scalds, and turn the thermostat back to a safe setting once done. In hard-water Chelsea this also helps limit the scale and sediment that give bacteria somewhere to sit.

Where a flush does not hold, the anode is the usual culprit and there are two routine options. The first is to replace the standard magnesium sacrificial anode with an aluminium or zinc-alloy anode, which produces far less hydrogen sulphide while still protecting the tank. The second, on cylinders designed for it, is to fit a powered impressed-current anode that protects the steel electronically without feeding the reaction at all. Removing the anode entirely is not advisable, as it shortens the life of the cylinder. A competent plumber can identify which anode your cylinder takes and change it as a straightforward job.

Disinfecting the system is the other proven route, particularly for tanks that have sat unused. A chlorine-based sanitising dose run through the cylinder and hot pipework kills the bacteria colony, after which a thorough flush removes both the chlorine and the smell. This is a common fix in Chelsea for holiday homes, pieds-a-terre and flats that are empty for stretches, where warm water stands still for weeks and gives bacteria ideal conditions. Pair it with a habit of running the taps when you return, and the smell often stays away.

Do not ignore a persistent egg smell even though it is rarely harmful, because it usually means bacterial growth or a corroding component that will get worse. Avoid pouring random cleaning chemicals or bleach down taps in the hope of masking it, as that can damage seals and does nothing to the source inside the cylinder. Do not simply crank the thermostat to maximum and leave it, both for scald safety and because a badly corroded anode or an ageing cylinder needs proper attention rather than heat alone. If the tank itself is old and rusting, replacement may be more sensible than repeated anode changes.

Some older Chelsea homes have a cold water storage tank in the loft feeding the hot cylinder, and a neglected tank there can be the real source. If the storage tank lacks a proper lid, has debris or stagnant sediment in it, or is fed intermittently, bacteria multiply and taint everything downstream. In that case the fix is to clean and disinfect the storage tank and fit a close-fitting cover, not just to treat the cylinder. This is worth checking in period mansion flats and townhouses where a traditional gravity-fed system is still in use.

If you have a combi boiler rather than a stored cylinder, a rotten-egg smell is far less common, because there is no tank of standing warm water and no sacrificial anode for bacteria to feed on. When it does happen on a combi, the smell usually traces back to the incoming supply or to a rarely used outlet, so the hot-versus-cold and one-tap-versus-all tests matter even more. On stored systems heated by an electric immersion, the same anode and bacteria rules apply, and a cylinder whose immersion has failed can also leave water sitting cool and stagnant long enough to turn, so it is worth confirming the immersion or boiler is actually heating the tank as it should.

It also helps to know what does not cause the smell, so you do not chase the wrong fix. A water softener, common in hard-water Chelsea, does not create hydrogen sulphide by itself, though a poorly maintained or rarely regenerated softener can occasionally harbour bacteria that add to it. Limescale and sediment are not the direct source either, but they give bacteria a rough surface to colonise, which is one more reason to keep a cylinder flushed and serviced in this part of London. A brown or discoloured tinge to the water is a separate issue again, usually iron or disturbed sediment rather than sulphide, and is worth mentioning to the engineer if you see it alongside the smell.

Call Emergency Plumber Chelsea on 020 3002 6667 if flushing does not clear the smell, if it returns quickly, or if you would rather have the cylinder, anode and storage tank checked properly. We diagnose which part of the system is producing the hydrogen sulphide, flush and disinfect where needed, change or upgrade the anode, and advise honestly if an old cylinder is better replaced. We cover SW3, SW10 and nearby postcodes, and every job is quoted before major work. If at any point you suspect a gas escape rather than a water smell, call National Gas Emergency on 0800 111 999 first.

Quick tips

• Smell the cold tap and the hot tap separately first: hot-only points to the cylinder or anode, both sides points to the supply.
• If only one rarely used tap smells, run it hard for a few minutes to flush stagnant water from a dead leg.
• A magnesium sacrificial anode is the common cause: swapping to an aluminium/zinc or powered anode usually cures it for good.
• Never confuse it with a gas leak. If the smell is by the boiler or meter, not a tap, leave and call National Gas on 0800 111 999.
• For flats left empty for weeks, run the hot taps on return to clear standing water before bacteria build up.

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