What is the major sub-assembly of the water heater?

The major sub-assembly of a water heater typically includes the tank, which stores and heats the water. Other key components within this sub-assembly are the heating elements (in electric water heaters) or the burner assembly (in gas water heaters), as well as insulation to maintain temperature. Additionally, the thermostat and safety features like pressure relief valves are integral to the operation and safety of the water heater.