A gravity furnace lacks which essential component?

A gravity furnace operates primarily through the natural movement of heated air, relying on density differences to circulate warm air throughout a space. The essential feature of a gravity furnace is that it does not incorporate a blower motor, which is typically used to force air through ductwork in more modern heating systems.

Instead, gravity furnaces often have large ducts that allow hot air to rise and cold air to settle back down, promoting a cycle of heating without the need for mechanical assistance. This design means that gravity furnaces function efficiently with just the heat produced by the combustion of fuel and the natural flow of air.

The other components listed, such as ductwork, pilot light, and heat exchanger, are indeed integral to the operation of a gravity furnace. Ductwork is necessary for air distribution, a pilot light serves as the ignition source for the flame, and the heat exchanger is vital for transferring the heat from the combustion process to the air that is being circulated. Therefore, the absence of a blower motor is what distinguishes a gravity furnace from forced-air systems, making this characteristic essential to its design.