Never run your pw with no water running through it or you will damage your pump as you experienced. The water coming in from the garden hose also cools the pump. If you let your pw run without pulling the trigger, the water gets hot. Once it reaches a certain temp, the thermal relief valve opens to let cooler water in. Same concept as the cooling system in your car except the thermal relief valve is the thermastat. If you run your car with no coolant or if the thermastat fails to open, it will overheat rather quickly.