Typically, a hot water tempering valve is installed to give you an additional volume of hot water. To give an example of how it works, you raise the water heater thermostat setting to 140+ degrees and at the same time, you set the tempering valve to 120 degrees. By doing this, you are tempering down the outgoing water. The stored volume of hot water will last longer. One of the downsides to raising the stored water temperature is that it will unavoidably cause the glass lining of a water heater to deteriorate faster than normal. This will result in a shorter life expectancy.
Sometimes a hot water tempering valve is installed to help guarantee that the people using the hot water cannot get scalded. For instance, it’s not unusual to see a tempering valve installed on water heaters in a nursing home or daycare. Elderly and children are more likely to be scalded by 120-degree water. Most of the time the temperature of the heater remains at 120 degrees but the valve is set to say 90 degrees.