Geyser safety: what the two cut-outs actually do

A storage water heater holds pressurised water at around 60 °C, a few feet above where you stand undressed. It deserves more thought than it usually gets.

Three devices, three different jobs

The thermostat

This is the working control. It senses tank temperature, switches the element off when the water reaches the set point, and switches it back on when it drops. It operates constantly through the day and is the reason leaving a geyser on does not mean it is consuming power continuously.

A thermostat is a mechanical part and it can fail. When it fails, it usually fails closed — the element keeps heating and does not stop. Which is why there is a second device.

The thermal cut-out

This does nothing at all in normal use. It sits there waiting. If the temperature climbs past a safe threshold — because the thermostat has failed — the cut-out breaks the circuit entirely.

It is deliberately independent of the thermostat. One failure should never become a hazard, and that only holds if the backup does not share a failure mode with the thing it is backing up.

The pressure relief valve

Water expands as it heats. In a sealed tank that expansion has nowhere to go, so pressure rises. The relief valve opens and releases a little water before pressure reaches a dangerous level.

A small amount of dripping from this valve while heating is normal and means it is working. Continuous heavy discharge is not normal and should be looked at.

What high supply pressure does

Higher floors and booster pumps both raise inlet pressure. A tank rated to 4 bar in a system running near that continuously is working at the edge of its design, and the relief valve is compensating far more often than it was meant to.

The fix is either a pressure reducing valve on the inlet, or a tank rated for the pressure the building actually produces. This is worth establishing before buying rather than after a failure.

Electrical safety

• Earthing is not optional. An element sits in water. If it develops a fault and there is no earth path, that fault has to find another one.
• The circuit must be rated for the load. A 2000 watt element on undersized wiring is a heat source in your wall.
• An accessible isolating switch so power can be cut without hunting for the main board.
• An RCD or ELCB on the circuit if your board supports it. It disconnects on detecting current leaking to earth, and in a wet room that is genuinely worth having.

Signs to act on immediately

• Any tingling sensation from the water or taps — switch off at the isolator and call an electrician. This is not something to observe for a few days.
• The MCB tripping when the geyser runs.
• Water noticeably hotter than the setting should give — a failing thermostat.
• Continuous heavy discharge from the relief valve.
• Any moisture, staining or rust around the unit.
• Rumbling or banging while heating — sediment, and it is time to descale.

Sensible habits

• Keep the thermostat around 55–60 °C. Higher increases scald risk, deposits scale faster and saves nothing.
• Check the mounting once a year. A full 25 litre tank is over 30 kg on your wall.
• Where children or elderly people use the bathroom, remember that water above 60 °C can scald in seconds.
• Switch it off at the isolator if you are away for an extended period.