Quick answer
Popping, knocking or kettling comes from scale on the element. Water trapped under the limescale flashes to steam and the bubble collapses with a pop. The noise itself is harmless, but it is a direct warning that the element is running hot behind an insulating layer and will fail within months if the cylinder is not drained and descaled. Noise only during the heating cycle is the giveaway.
Common causes
- Heavy limescale on the element — Johannesburg water is moderately hard and scales steadily
- Sediment built up in the bottom of the cylinder trapping water underneath it
- Air trapped at the top of the cylinder after a supply interruption or repair
- Thermostat set high enough to accelerate scale precipitation, typically above 70°C
- A geyser never serviced past year five, where scale has had time to bond to the element
What you can do now
- 1Note when the noise happens — only during heating cycles confirms scale rather than plumbing
- 2Check the thermostat setting; above 70°C accelerates scaling with no useful benefit
- 3Listen for whether it is getting louder month on month, which tracks the scale thickness
- 4Do not attempt to drain the cylinder yourself — a partly drained geyser that gets switched on destroys the element instantly
When to call a professional
Book a geyser service: drain, flush, descale, and replace the element and sacrificial anode. Done every three to four years this roughly doubles geyser lifespan in Johannesburg, and it costs a fraction of the replacement it prevents. If the noise is accompanied by slower heating, treat it as urgent — the element is already losing output.
Typical cost in Johannesburg
Service + descale R950–R1,500 · With element R1,800–R2,400
Indicative ranges only. We provide written, itemised quotes before any work.
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