In order for a flame to burn it must have a fuel source, oxygen, and a spark or something to ignite it. A gas furnace burns fuel to create heat for heating various things such as your home. This means it needs all three criteria in order to heat. The furnace fuel can be natural gas, propane, or even oil. The spark can be a pilot light, spark ignitor, or hot surface ignitor. This leaves oxygen as the final element, which comes from the air surrounding the furnace. This air used by the furnace is referred to as combustion air.
Older homes were not built as “tight” as newer homes. In other words, newer homes are built with better insulation, better windows, and less gaps and holes. This makes the home more energy efficient by not allowing heat to leave the home as easily in the winter, or enter in the summer.
A tighter built home means less outside air infiltrating into the home, which in turn means less oxygen to be used for combustion inside the furnace. As the furnace burns fuel and oxygen for combustion, it “eats up” the surrounding air, which must be “made up” with fresh air. If air can not enter the home as easily anymore, this could cause serious issues. As the surrounding air is used for combustion, and not replaced with fresh air, a negative pressure in the space can occur. This could create combustion problems for the furnace, plus use up fresh air for anyone in the home.
Today’s building codes state spaces using furnaces must have means for making up combustion air as it used. Newer homes have ducting installed, during construction, which brings fresh air from the outside to an area near the furnace. This ducting, usually called make up air duct, or combustion make up air, is sized according to the capacity of the furnace.
One major problem is most homeowners do not realize the importance of this ducting near their furnace. They usually notice cold air coming in through it during the winter, and in an effort to keep the house warmer and save energy, it usually ends up getting a towel or something of the sort stuck in the end to stop the cold.
If you have furnace, and a newer home with combustion air duct near the furnace, realize its importance, and please do not cover or block it. It serves an important purpose, and needs to remain open to the outside. In the same respect, if you are finishing out an unfinished space which houses the furnace, make sure this ducting ends inside the same room as the furnace.