Ducts that carry heated air from the furnace to heated spaces in your home  often travel through unheated spaces (like a basement or crawl space) on the way. If the ducts are not sealed and insulated, they can waste a lot of the heat.
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Sealing and insulating heating ducts in accessible areas is inexpensive and not difficult to do yourself if you are handy and don't mind a bit of a mess.
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This action will give you 800 pounds of CO2 savings annually.