There is no such thing as an "overflow" on a water heater. It is a temperature and pressure release valve, and it is there to prevent the water heater from over-pressurizing when something is wrong. No water should ever come out of it unless something is wrong.
When water is heated, it expands. The expansion tank is meant to give somewhere for this expanding water to go. Something in your system is not working properly and the drainage is a symptom of that problem.