Furnaces come in 3 types of efficiencies. Low, mid and high. The lowest efficient furnaces are typically 40 years old or more. You?d be surprised how many of these are still in use in homes. I live in Cincinnati, former home of Williamson furnace company. You can identify a Williamson furnace, because they are typically green in color, and are roughly twice the size of a newer furnace. Williamson?s and other furnaces of this ilk, are considered very inefficient by today?s standards. They have standing pilot lights, no draft induction motors or dampers inside the flue pipe, and are often times over-sized for the square footage of the home. The efficiency of this type of furnace is probably less than 50%, meaning for every dollar you pay in natural gas, you get 50 cents or less in heat supplied to your home. The rest of the heat escapes up the chimney flue, lost forever.
Mid efficiency furnaces started cropping up in the 1970?s. Generally a mid efficiency furnace is 80% or more efficient. So for every dollar spent, 8o cents or more is actually being used to heat the home, while the rest escapes up the chimney. Mid efficiency furnaces are still used and installed in homes today. Often times they are used in milder climates and places where the winter months are shorter.