Quick answer
A sulphur or rotten-egg smell from hot water is hydrogen sulphide, produced when bacteria in the cylinder react with a magnesium sacrificial anode. It is unpleasant rather than dangerous, and it is fixable: replacing the magnesium anode with an aluminium-zinc type resolves it permanently. If only the hot tap smells and the cold is clean, the cylinder is confirmed as the source.
Common causes
- Bacterial reaction with a magnesium sacrificial anode rod
- Geyser running below 55°C, which allows bacterial growth in the cylinder
- Long periods of low usage, such as a holiday home or an empty house, letting water stand
- Sediment in the cylinder base giving bacteria somewhere to colonise
What you can do now
- 1Confirm the smell is only on the hot tap — if the cold smells too, the issue is the supply, not the geyser
- 2Raise the thermostat to 65°C and run it through one full heating cycle
- 3Run every hot tap for several minutes to flush the treated water through the pipework
- 4Return the thermostat to 60–65°C afterwards and see whether the smell comes back over the next week
When to call a professional
If the smell persists after a full hot cycle and a flush, book an anode replacement. The plumber will drain the cylinder, swap the magnesium anode for aluminium-zinc, and flush the sediment out at the same time. It is worth pairing with a routine service since the geyser has to be drained either way.
Typical cost in Johannesburg
Anode replacement R650–R1,200
Indicative ranges only. We provide written, itemised quotes before any work.
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