If your have an old monster-sized furnace in the basement with huge ducts that snake off in various directions, your home may be heated by a gravity furnace. Gravity furnaces were installed in homes at the turn of the 20th century and well into the 1940s. Depending upon locally available fuels, they may burn coal, wood, oil, or natural gas.

See How a Forced-Air HVAC System Works for an explanation of a typical heating system and note that an old gravity system is different in that it doesn’t “force” air. Instead of utilizing a blower to push heated air into rooms, a gravity furnace allows the heated air to rise by natural convection through large ducts into rooms.

Because it doesn’t have a blower, a gravity furnace is quieter than a forced-air furnace and doesn’t stir up dust and allergens by blowing air—but that’s where the benefits end. Though these big monstrosities do a pretty good job of heating, they waste a tremendous amount of energy compared to today’s high-efficiency furnaces. Not only do they send about 50% of the heat straight up their chimney, but the heated air also takes longer to reach distant rooms.

Because asbestos was commonly used for fireproofing and insulating these furnaces and their ducts, the chances are good that an old gravity system presents an asbestos hazard. This white, fibrous wrap is a known carcinogen that can cause asbestosis when airborne. It should not be disturbed other than a by a certified asbestos abatement company. (For more information, see Case Study: Asbestos Ductwork Removal.)

If you intend to be in your home for more than a couple of years and your budget will allow, it would likely pay to replace your outdated gravity furnace with a new, high-efficiency furnace. Doing so will probably reduce your heating bills by half. But be advised that this is a big job that requires complete removal and replacement of the old system, usually including the ductwork, patching floors, installing new registers, and more.