What component controls the outlet temperature of a circulating water heater?

The storage tank thermostat is the component that directly controls the outlet temperature of a circulating water heater. It achieves this by sensing the water temperature within the storage tank and regulating the heating elements or burner accordingly. When the thermostat detects that the water temperature has fallen below a preset level, it activates the heating mechanism to bring the temperature back up to the desired setting. This ensures that the water being supplied is at an optimal temperature for usage.

In contrast, other components have different functions: the pressure regulator is responsible for maintaining a consistent pressure within the system, the temperature relief valve provides safety by releasing water if the temperature exceeds safe limits, and the expansion tank accommodates fluctuations in water volume due to temperature changes without causing overpressure. None of these functions directly control the output water temperature, which is why the storage tank thermostat is the appropriate choice for this question.