Older homes can be spooky, especially those with dark, dusty basements that are the fodder of many horror movies. Children who venture into these foreboding depths may emerge screaming in terror as they encounter that monster in old basements known as the octopus furnace. A less scary name for these basement monstrosities is the “gravity furnace”. And despite their size and cephalopod appearance, they are not evil except in one regard. Gravity furnaces emerged in the 1800s. Normally coal-fired, they can be adapted to burn gas and other combustibles. They are virtually indestructible and have no moving parts, so they are durable and extremely low maintenance. They work simply, as fire from the combustion chamber sends heated air throughout the house. Gravity forces heavier cool air into the basement where it is reheated to complete the cycle. There are three main drawbacks to gravity furnaces. The first is that they are woefully inefficient at about 50% meaning that every BTU produced to heat the home is matched by another BTU that simply escapes through the chimney. The next drawback is the challenge they pose when homeowners want to replace them with a modern forced air furnace as part of a central HVAC system. Unless that gravity furnace and ductwork is completely removed, any AC equipment will be in the form of window units or perhaps a ductless system. A ductless system will result in a heating and cooling system that is simply too cumbersome when paired with the gravity furnace. The third drawback is the evil that we hinted at earlier. Most gravity furnaces used asbestos as an insulating material. Asbestos is not dangerous when left alone but any disturbance could send asbestos fibers into the air putting the building occupants at risk of inhaling the known carcinogen.