What type of control system is found in a gravity heating system?

In a gravity heating system, the open expansion tank is essential for allowing the system to operate effectively. This type of system relies on the natural circulation of water due to differences in water density as it heats up. An open expansion tank provides a means for water to expand and contract without creating excessive pressure in the system.

As water heats, it expands and some of its volume is allowed to escape into the atmosphere through the open tank, preventing pressure buildup and allowing for a stable water level. This setup is particularly important in gravity systems where it is crucial to manage the system's water volume and pressure levels, ensuring proper circulation and function.

The other options, while important in various contexts, either do not pertain to the specific operation of a gravity heating system or serve different purposes within other types of heating or cooling systems. Therefore, the open expansion tank is the appropriate choice for this context.