I got a new hot water heater unit, and the guys in the store said we have to install a tempering valve for our new water heater. What is the purpose of the tempering valve and how does this work?

A tempering valve allows some cold water to mix with the hot water to prevent the hot water from potentially scalding. Is this for a residence or a business? My understanding is that residences don’t typically use them since you can simply turn the water temp down to a safe level.

Might want to check with local code. They are required in some places.