Quick answer
Scalding water means the thermostat has failed closed or has been set too high. This is a genuine safety risk, not a comfort issue — at 65°C a child can be badly scalded in under five seconds, and above that the margin disappears entirely. If turning the thermostat down changes nothing within a heating cycle, the thermostat is stuck and the next thing to operate will be the T&P safety valve.
Common causes
- Thermostat stuck closed so the element never switches off
- Thermostat dial set above 65°C, sometimes after a service
- Failed tempering or mixing valve at the geyser no longer blending the outlet
- Sensor bulb not seated properly against the cylinder after a repair
- Solar system dumping excess heat into the cylinder with no tempering valve fitted
What you can do now
- 1Turn the thermostat down to 60°C and wait one heating cycle
- 2Switch the geyser breaker off at the DB board if the water is dangerously hot in the meantime
- 3Warn everyone in the house, particularly where there are children or elderly family members
- 4Check whether the overflow has started discharging, which means the T&P valve is now operating
- 5Do not rely on mixing at the tap as a fix — the risk is the next person who opens it fully
When to call a professional
Call immediately if the thermostat setting makes no difference. A stuck-on thermostat keeps heating until the T&P valve lifts, and that valve is the last safety device between the cylinder and a failure. A plumber will replace the thermostat, verify the cut-out operates, and can fit a tempering valve so the outlet is limited regardless of what the cylinder is doing.
Typical cost in Johannesburg
Thermostat R650–R1,100
Indicative ranges only. We provide written, itemised quotes before any work.
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