We can heat our well insulated 900sqft of living space to 65-70°f using around 2 cords of wood each year with our Rocket Mass Heater. We typically wake up to 65°f and burn a fire in the morning and another fire in the evening. A "J" tube burn chamber covered by a 55gal barrel creates radiant heat as well as a thermal pump which forces exhaust gasses horizontally though ducting in a large box filled with pea gravel. The gravel extracts most of the heat from the exhaust radiating it out slowly after the fire burns out. By the time the exhaust reaches the chimney it's 120-200°f down from 300-500°f at the barrel. The combustion is nearly total so very little smoke is exhausted and maximum BTUs are used to heat the space. Built with materials found at most big box hardware stores Rocket Mass Heaters are cheap to build, easy to source, and incredibly efficient. It is the favorite contraption I've built on the homestead.