Describe the operation of a thermostatic mixing valve in a domestic hot water system and its role in scald prevention.

A thermostatic mixing valve (TMV) in a domestic hot water system automatically blends hot and cold water to deliver water at a constant, safe temperature. Its primary role is scald prevention, protecting users from burns caused by excessively hot water. TMVs operate using a thermostatic element, typically a wax-filled or liquid-filled cartridge, that expands or contracts in response to temperature changes. This element controls the flow of hot and cold water into the mixing chamber. The TMV has three inlets: one for hot water, one for cold water, and one outlet for the mixed water. The hot water inlet is connected to the hot water supply, which is typically maintained at a temperature above 140°F (60°C) to prevent Legionella growth. The cold water inlet is connected to the cold water supply. The thermostatic element senses the temperature of the mixed water at the outlet. If the mixed water temperature is below the setpoint, the element allows more hot water to flow into the mixing chamber. If the mixed water temperature is above the setpoint, the element allows more cold water to flow into the mixing chamber. This automatic adjustment ensures that the mixed water temperature remains constant, even if the hot or cold water supply temperatures or pressures fluctuate. TMVs are available in different sizes and types, including point-of-use TMVs, which are installed at individual fixtures, and whole-house TMVs, which are installed at the water heater outlet. TMVs are essential components in domestic hot water systems, providing safe and comfortable water temperatures while also preventing scalding.