Wood-burning stoves are old devices getting a green upgrade. A new generation of stoves can heat a room or even a small home by efficiently burning biomass such as compacted sawdust or agricultural waste. Specially formulated wood pellets burn with minimal waste or pollution, and they can even be used in boilers or furnaces to update an existing heating system. The pellets must be delivered and fed into the stove or furnace, so these systems require more effort than most heating systems. Malfunctioning wood stoves can emit dangerous air pollutants.