What is a closed-loop heating system?

A closed-loop heating system is characterized by its ability to circulate heated water within a contained system without allowing any of that water to escape into the environment. This design ensures that the same water continuously recirculates through the boiler, distribution system, and radiators or heating elements, maintaining a consistent temperature and efficiency throughout the system.

In contrast, other systems would not align with this definition. For instance, a system that allows water to escape during operation would be classified as an open-loop system, where water is used once and then discharged, requiring additional water to be drawn in for subsequent heating cycles. Systems that utilize only direct contact heating methods do not fit the definition of a closed loop, as they generally involve water being directly heated and could lose water in the process. Lastly, while a boiler operating under low pressure is a relevant characteristic regarding its operational safety and efficiency, it does not define the closed-loop nature of the system. Thus, the essence of a closed-loop heating system lies in its sealed circulation of heated water, making choice B the correct description.