The cost of replacing an older furnace involves many aspects. For instance, the size of the furnace and the efficiency rating of the furnace will increase the cost of the furnace. Your HVAC company can recommend the right size furnace for your homes square footage and help you to decide the efficiency you want the furnace to be. The heat load of your house ( the amount of energy required to maintain a steady 65-degree temperature indoors) is the deciding factor when it comes to the size of the furnace you need. A furnace that has 90% efficiency rating means that 90% of the energy in the fuel becomes heat for the home and the other 10% escapes either through the chimney or elsewhere.