Depending on the size of your house, a wood burning stove can produce a lot of heat. But since you already have forced-air from your
air conditioning unit, I would look at a wood pellet or wood burning furnace. The other option that could be the cheapest but also requires a little more research is heating with waste oil. You can find sites out there where people have modified their wood burning stoves and furnaces to burn used vegetable oil and other waste oils. The biggest problem or potential problem is finding a source for waste oil.