House plans . By William Kibbel III Coal furnace chimney now used for gas. Wm Kibbel Owners of old homes are often very proud of their working fireplace. They understand the function of its chimney and can see the smoke discharging at the highest structure above the house. I often see a puzzled expression, however, when I begin to discuss the other chimney, or second flue of a single chimney.
Many homeowners never consider the fact that the central heating system also vents into a chimney.
When I point this out and start talking about health and safety, the expression turns to one of concern. The original central heating system in most older homes was fueled by coal or oil. Many homeowners replace these old furnaces with more efficient, modern systems, or convert to cleaner burning gas heat. Much of the combustion heat from older boilers and furnaces was lost up the chimney. This escaping heat warmed the chimney and created an adequate updraft.