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Flickr: Danny O 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. A tighter built home means less outside air infiltrating in the home, which means less oxygen to use for combustion within the furnace. As the furnace burns fuel and oxygen for combustion, it "eats up" surrounding air, which has to be "made up" with fresh air. If air cannot type in the home as easily anymore, this can cause serious issues. As the surrounding air is utilized for combustion, rather than substituted with oxygen, a negative pressure inside the space can happen. This could create combustion problems for the furnace, plus burn up fresh air for anybody inside the home.