A wall furnace or floor furnace can be an efficient way to heat your house. These models from US Stove use either coal or wood for fuel, and can connect with your existing central duct system. Solid fuel furnaces are not just for smaller homes. Sizes are available that are rated for as much as 3600 square feet and feature powerful blowers to spread the warmth throughout your house. These furnaces are not approved for use with mobile or modular homes.