In conclusion, the most environmentally friendly way to heat your home would be through the use of renewable energy sources such as solar, wind or geothermal energy. Biomass boilers and passive solar home designs are also environmentally friendly and sustainable heating options.

High-efficiency gas furnaces can provide an annual fuel utilization efficiency (AFUE) of around 90% to 98.5%. That means they lose from 1.5% to 10% of the heat energy through flue gasses. Older gas furnaces may have an AFUE as low as 56%.

There are dozens of variations of home heating systems available, but the vast majority fall into one or more of these four distinct categories: forced air, electric, hydronic, and geothermal.

Air-Source Heat Pumps: Heat pumps are incredibly efficient since they simply transfer heat to and from the outside air. If yours can be powered using renewable electricity (like solar), then you get even more sustainability bonus points.