The most efficient furnace for a large home is a direct-vent furnace using propane or natural gas, according to Consumer Reports. Modern gas furnace designs can achieve average fuel use efficiencies of 95 percent, while oil furnaces achieve a maximum of around 90 percent.
Many homeowners install too large a furnace for their home, especially when heating a large residence. Doing so partially defeats the benefits of a high-efficiency gas furnace, as the initial cost of the furnace eliminates some of the savings from lower fuel bills. A furnace that is too large for a home also cycles on and off more frequently than a furnace that is correctly sized, leading to more repairs and lower overall efficiency.