If we are the one installing the new water heater for you, we will help you verify if there is a need for a thermal expansion tank. Moreover, you can also check with your water utility if there is a need for an expansion tank.
Heated water increases water pressure. This is because the water expands in volume when heated. Thermal expansion is the term pertaining to this effect. For an "open water system", an increase in water pressure that exceeds the limit of the water heater can flow back into the city main. Hence, dispersing the excess pressure. There is no need for a thermal expansion tank.
However, a "closed water system", prevents the expanding water from moving back into the main supply line. As a result, thermal expansion can build an unexpected dangerous pressure increase in the water heater system and piping. During each heating cycle, this rapid pressure increase can trigger the pressure relief valve repeatedly causing untimely breakdown.
The recommended method of managing this is to connect a thermal expansion tank. In the cold water line between the water heater and check valve, this is where the expansion tank should be installed.  It is designed with an air cushion that decreases as the system pressure increases. Therefore, reducing the pressure build-up and repeated operation of the pressure relief valve.
For more information, you can contact us.