A rumbling noise from your hot water cylinder is almost always caused by sediment or mineral scale build-up on the heating element or at the tank bottom. As the element heats water through the sediment layer, it makes popping, rumbling, or banging sounds. This reduces efficiency and can shorten cylinder life.
Christchurch's water is relatively soft and low in minerals compared to many NZ cities, meaning sediment build-up is less severe here. However, over 10+ years, even soft water leaves some scale deposits, particularly if the thermostat has exceeded 70°C at any point.
Sometimes. A plumber can flush the cylinder to remove loose sediment, or replace the element if scale-encrusted. However, if the cylinder is over 10 years old, the cost of flushing and element replacement often doesn't make economic sense versus replacing the unit.
If rumbling is accompanied by: longer reheating time, higher power bills, discoloured water, or age over 12 years — the cylinder is likely near end of life. Replacement is the better investment, especially with a heat pump upgrade to reduce running costs.