Greenwood Technologiesoffers smoke-free wood-fired furnaces that burn 85 percent efficient. Leaves no smoke and virtually no ash. Environmentally-friendly furnaces are as safe as a water heaters, and as easy to use as a fireplace. They can permanently reduce consumers’ heating bills by 70%. In the coldest parts of the country, 5-6 cords of wood are all that’s required to provide ample heat throughout the entire winter season.