Quick answer
A small, occasional drip from the overflow is normal — water expands as it heats and the valve releases the extra pressure. A constant drip, especially overnight when municipal mains pressure peaks, means the pressure-control valve or the T&P valve has worn through. More than one drip per second is not thermal expansion, it is a failed valve wasting heated water onto your driveway and onto your bill.
Common causes
- Worn pressure-control valve — a four to five year wear item, not a defect
- Failed T&P safety valve after repeated over-temperature events from a drifting thermostat
- Mains pressure above the geyser's rated kPa, common in Johannesburg's high-pressure zones
- Scale build-up holding the valve seat open so it can never fully reseat
- No pressure-reducing valve fitted where the municipal supply runs above 600 kPa
What you can do now
- 1Time the drip — more than one drip per second means the valve has failed
- 2Check whether it drips constantly or only during and just after a heating cycle; constant means the valve, cyclical means normal expansion
- 3If you have a pressure gauge, check mains pressure — anything over 600 kPa will keep destroying valves until a PRV is fitted
- 4Put a container under the overflow for an hour to measure how much hot water you are actually losing
When to call a professional
Book the valve replacement before the leak worsens. A weeping PCV becomes a continuous flow, and the wasted hot water shows up on your electricity account long before the plumber does. If valves keep failing every year or two, ask the plumber to test static mains pressure and fit a pressure-reducing valve — otherwise you are replacing the same part indefinitely.
Typical cost in Johannesburg
R950–R1,500 supplied and fitted
Indicative ranges only. We provide written, itemised quotes before any work.
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