Leaks are the first part of the visible signs that your hot water cylinder needs attention. If you have a leak and it is from the bottom of the cylinder, or there are signs of rust, then this normally means that the internal workings of your hot water cylinder are not great and it’s time to think about a replacement.
Before a cylinder can leak externally, the water must have already escaped the inner tank. If a leak is coming from the body or base of the cylinder, it is likely that the internal source leak started approx 7-10 years ago.
A failed element or thermostat in a cylinder of 15+ years age indicates that the cylinder has an internal tank leak and should be replaced. As water is heated inside the cylinder, the water expands and causes the cylinder to undergo a pressure increase, so the internal tank becomes more pressurised. This pressure increase results in water escaping from a pin hole leak in the inner tank (which is made of metal and is in contact with water from the moment it was filled up 15+ years ago), then dripping into the element/thermostat housing. When a new element or thermostat is installed in this circumstance, the source leak is not repaired and it is only a matter of short time before the new element or thermostat also fails. Better to replace in full and “shut the door” on it for another 15-20 years.