The check valve on a water heater is a safety device that protects the water heater from excessive water pressure and water temperature. Either one or both of these situations together could result in a powerful explosion.
No water is discharged by the check valve when the water heater is functioning normally. As of 2015, check valves on residential water heaters are set to discharge when the water pressure exceeds 150 pounds per square inch per gallon. If water temperature rises above 210 degrees Fahrenheit, the check valve discharges water. These standards have been established by trade groups, such as the American Society of Mechanical Engineers (ASME).