What device revolutionized hot water heat design by facilitating closed systems?

The circulator is the device that revolutionized hot water heating design by facilitating closed systems. In a hot water heating system, circulators are used to pump water through the boiler and into the heating units, often in a closed-loop system. This setup allows for efficient thermal transfer and maintains consistent water temperature throughout the system.

By enabling water to circulate continuously, the circulator helps in efficiently distributing heat to various areas of a building without the need for open systems, which can lose heat and water due to evaporation and exposure to the atmosphere. The use of circulators therefore contributes to increased energy efficiency and improved comfort by ensuring that heated water is directed where it is needed most.

While other devices listed might play important roles in the heating system—such as flow control valves that regulate the water flow, radiators that act as heat exchangers, and expansion tanks that accommodate changes in water volume due to thermal expansion—they do not have the same direct impact on the design of the system as circulators do. Circulators are fundamental in creating the effective closed systems that are now standard in modern hot water heating applications.