You are legally required to install a safety group. It serves two purposes as the water starts expanding when heated:
1. A safety valve ensures that the hot water tank does not explode when the water is being heated and the pressure inside the tank subsequently increases. The safety valve is dripping during the heating process.
2. A non-return valve ensures that the hot water cannot return into the system and mix with the cold water in the dwelling.
In addition, the safety group is equipped with a stopcock for the cold water. This is an advantage for instance if you need to replace the anode in the hot water tank.