What type of air/water control is employed in gravity heating systems?

The correct choice is the open-loop system. In gravity heating systems, an open-loop system is used to circulate water or air. This type of system relies on the natural circulation of water due to differences in temperature and density; warmer water rises while cooler water sinks.

Gravity heating systems do not use pumps to circulate water, unlike closed-loop systems, which are more common in modern heating applications and use a pressurized setup. In an open-loop system, once the water is heated, it flows through the system and returns back to the source to be reheated, making it distinct from a closed-loop system.

This understanding of how gravity heating utilizes an open-loop design highlights its reliance on natural physical principles rather than mechanical means, which defines its operation. The role of valves like the two-way and expansion valves pertains to more complex or controlled systems, which are not typical in gravity heating systems fundamentally designed around open-loop principles.