Your boiler is responsible for heating water, even though it doesn’t actually boil it. A pump circulates hot water through the entire house. The most common issue with boilers is a leak in the pump, the one part of the boiler that is constantly working. It is quite common for the pump valve or seal to break, allowing water to leak. The expansion tank allows the boiler to compensate for the water expansion at really high temperatures. But leaks in your boiler can happen beyond the pump as potential leaks can happen in the pipes. Additionally other components can get damaged too, like the boiler tank itself, which can get rusty with time, or the pressure relief valve.