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Difficulty concentrating or mind going blank 9. Their emotional appeal The practical options for energy range from the standard fuels, natural gas, petroleum (or oil), and coal. Fossil fuels are referred to as nonrenewable energy sources because, once used, they are gone. Scientists are exploring the practicality of other sources called renewable energizes. These include sun, wind, geothermal, water, and biomass. The renewable power resources are very important in long range energy planning simply because they will not be depleted. Today's building codes state spaces using furnaces have to have opportinity for creating combustion air because it used. Newer homes have ducting installed, during construction, which brings outdoors externally with an area nearby the furnace. This ducting, usually called make-up air duct, or combustion make-up air, is sized in accordance with the capacity from the furnace.