What type of heating system is characterized by gravity flow warm air?

The correct answer is associated with a heating system that relies on the principle of gravity to move warm air from a furnace. In a gravity flow warm air furnace, the warm air naturally rises and circulates through the living spaces, utilizing the natural tendency of warm air to ascend. This system does not require any additional mechanical assistance to distribute heat, making it distinct from forced air systems, which utilize fans to push heated air through ducts.

Gravity flow systems are typically older and built into structures designed for this method of heating. They can be efficient in their simplicity, though modern heating systems typically employ forced air, which provides more controlled and uniform heating throughout a space.

Other heating options mentioned do not focus on the gravity flow of warm air. Forced air furnaces actively move heated air, while pump circulating hot water boilers do not involve air circulation at all, as they rely on water to transfer heat instead of air. Gravity flow steam boilers deal with steam rather than air, making them another different type of heating system. Thus, the unique characteristic of gravity flow warm air circulation confirms that the identified answer is accurate.