Not long ago I showed a buyer a house with a gravity furnace and got the response I often get… what is that???
Often nicknamed an 'octopus' furnace because of its multiple 'arms' they date back to the late 1800's and early 1900's. They were originally designed to burn coal, but later switched to burn oil or natural gas. There aren't too many left but you still sometimes find them in older homes, especially ones where the owners have lived there for many decades.
Although they are only about 50% energy efficient and take up huge amounts of space older homeowners often don't replace them because, like the Energizer Bunny, they just keep going and going and going… with few moving parts, there isn't much that can break down.
As its name implies, air circulation in a gravity furnace relies on gravity… the principle that warm air rises and cool air falls. There typically is no fan on a gravity furnace… that is why there are so many 'arms' and why they are so large. Since no fan is pushing the air, the air pressure is lower as the warm air gently enters the room and there tend to be fewer hot spots. However, there is no way to provide central air conditioning or warm air to the basement with this type of furnace… and there is no filter since that obstructs air movement.
When a home has a gravity furnace, the new homeowners will usually choose to replace it with a new energy-efficient forced air furnace and air conditioning system… which requires all new ductwork. Additionally, almost all gravity furnace systems are insulated with asbestos… ok as long as it remains sealed, but needs asbestos abatement if removed because of the health hazard.