Why does my hot water smell like rotten eggs?

What's actually causing the smell

That rotten egg or sulphur smell is hydrogen sulphide gas dissolved in the water. It forms inside the hot water tank through a reaction between two things: the sacrificial anode rod, and naturally occurring sulphate-reducing bacteria. Those bacteria are harmless to drink and live happily in warm water. The anode rod is a metal rod, usually magnesium, that sits inside the tank and corrodes on purpose so the tank walls don't. When the bacteria feed off the hydrogen the anode gives off, they produce that sulphur gas. The smell is often worse first thing in the morning, or after the system has sat unused, like when you get back from holidays.

Is it dangerous?

The good news: no, it isn't dangerous. The concentration of hydrogen sulphide in a home hot water system is far too low to harm you, and the bacteria behind it aren't the kind that make you sick. It's a nuisance smell, not a health hazard. One quick thing worth checking is whether the smell is only in the hot water, not the cold. If both taps are smelly, or you're on tank or bore water, the source might be the water supply itself rather than the hot water unit, and that's a different job.

How Terry fixes it

Getting rid of the smell usually takes a two-part fix, and it's plumbing work rather than something to tackle yourself:

• Flush and treat the tank: draining and flushing clears out sediment and disturbs the bacteria colony. Sometimes the tank is treated to knock the bacteria back further.
• Replace the anode rod: if the standard magnesium anode is feeding the reaction, swapping it for an aluminium or zinc alloy anode often stops the smell for good, because it changes the chemistry that produces the gas. Terry checks the anode's condition anyway, since a spent anode leaves the tank unprotected.

It's also a good reminder that the anode rod is a wear part. Terry recommends having it checked roughly every 5 years. A healthy anode extends the life of the tank considerably, and a tank that's gone years without one being looked at has often eaten right through the anode, which is when rust and leaks start.

Why the smell comes back

A common mistake is to flush the tank, enjoy a few clear weeks, then have the smell return. That's usually because only half the job got done. Flushing knocks the bacteria back, but if the same magnesium anode keeps feeding the reaction, the colony rebuilds and the sulphur smell comes with it. That's why Terry pairs the flush with an anode swap where it makes sense, so the chemistry that drives the smell is changed, not just reset. Homes on soft or low-mineral water tend to see the problem more often, so if yours has a history of it, the anode change is usually the smarter move.

What not to do

Don't just turn the temperature down to mask it, because the tank needs to stay hot enough, around 60°C, to keep bacteria in check. If anything, running the system a little hotter for a while can help knock the bacteria back, but the lasting fix is the flush and anode swap. And leave the tank work to a licensed plumber. Draining a hot tank, handling the anode and refilling it correctly is straightforward for a professional but easy to get wrong.

If your hot water has got a whiff of rotten eggs, it's an easy one for Terry to sort. Call 0415 274 542 for a fixed-price flush and anode check, servicing Berala, Lidcombe, Auburn and Western Sydney.