Three-way valves in hydronic heating systems are used for what purpose?

Three-way valves in hydronic heating systems serve the primary purpose of mixing water from two different sources, which is why mixing is the correct answer. These valves typically blend the hot water from a boiler with cooler return water from the heating system to achieve a desired temperature for the circulating water.

By controlling the ratio of hot and cold water, three-way valves help maintain a consistent and efficient temperature throughout the heating system. This mixing process not only enhances comfort levels in a space but also optimizes energy efficiency, as it allows the system to operate closer to the desired temperature set point.

In contrast, options like heating and cooling do not accurately describe the valve's primary function, as these terms refer to the overall purpose of a hydronic system rather than the specific operation of the valve. Circulation involves the movement of water through the system but does not specifically address the function of mixing temperatures, which is the key role of a three-way valve.