What is this plumbing "temperature regulator" called, and how do I adjust it?
Thermostatic Mixing Valve
According to the installation instructions (PDF)



Let the water flow for at least two minutes to allow supply temperature to stabilize.
Calibrate the mixed water outlet temperature by placing a thermometer in the mixed water stream.
To adjust the setting of the valve, loosen locking cap screw with hex wrench, see Figure 4. Cap must be lifted 1/4&quot; to adjust temperature. To increase the temperature, turn counter clockwise. To decrease temperature turn clockwise.
Lower handle and tighten screw.
Check outlet temperature.