All furnaces, regardless of what type of fuel they burn, require venting to remove the byproducts of combustion from a home.

It's for this important reason the Trane Home Comfort Institute recommends that if you're replacing your furnace, you ask your heating contractor whether your current chimney or flue pipe is adequate for the new equipment. If not, new venting will be required.

Here are some other facts about venting that the experts say you should know:

If a new gas furnace is installed as replacement, it is usually placed in the same location as the old furnace. Most building codes require that a furnace be installed as close as possible to the chimney, and that the length of the vent pipe between the furnace and the chimney be as short as possible. This ensures that the flue gases will remain hot enough to maintain a gaseous state throughout their journey to the outside.

Furnaces with efficiency or AFUE ratings of 78% and 80% generate gases at temperatures up to 500°F, which is hot enough for them to be buoyant and to permit natural draft venting. If combustion products should cool enough to condense before leaving the chimney, water vapor could corrode the venting system.

Condensing gas furnaces, which have efficiency ratings of 90% and higher, reduce the temperature of the flue gases to as low as 100°F, permitting venting through plastic pipe in a side wall similar to a gas dryer or through the roof. This feature provides much more flexibility in the placement of the furnace.

Finally, experts at the Trane Home Comfort Institute say that if you're replacing an oil furnace with a new gas furnace that will use an existing chimney for venting, you should have the vent pipe and chimney cleaned and checked for proper sizing before you begin operation of the new unit. This is recommended because many of the new higher efficiency gas furnaces use smaller vents.