Dual Fuel (also known as Hybrid) is a gas furnace matched with a heat pump outdoor unit and indoor coil.  In summer the heat pump provides air condioning.  In spring and fall the heat pump provides heat at a much lower energy cost than a gas furnace.  In the coldest part of winter, an outdoor thermostat signals the heat pump to shut down and the gas furnace to provide heat. 

Many dual fuel systems also meet the new $1,500 Federal Tax Credit standards.