Which of the following devices adds cool water to domestic hot water to reduce temperature and the risk of scalding?

The device that adds cool water to domestic hot water to reduce temperature and mitigate the risk of scalding is the thermostatic mixing valve. This valve functions by blending hot water from the water heater with cold water from the supply line to achieve a safe and consistent outlet temperature for taps and showers. This is particularly important in preventing injuries, especially for vulnerable populations such as children and the elderly, who are more susceptible to scalding from hot water.

In contrast, a pressure relief valve is designed primarily to protect against excessive pressure in a plumbing system by releasing water when pressure exceeds a certain threshold. A backflow preventer is used to prevent contaminated water from flowing back into the clean water supply, thereby ensuring the safety of the drinking water. Flow restrictors limit the volume of water passing through a faucet or fixture but do not control water temperature. Therefore, the thermostatic mixing valve is specifically the device designed to manage temperature effectively and safely in a domestic hot water system.