Standard Forced Air Furnaces draw air from
inside the house into the furnace through cold air
ducts, then warms it and delivers it back through
hot air vents. When the cold air is drawn back,
They have pilot lights that warm a heat exchange
unit and a flue for exhausting the gases to the

High Efficiency Furnaces have an additional heat
exchanger that takes the hot exhaust gases and
extracts any available heat before it is sent
outdoors. A high efficiency condensing furnace
takes this extracted heat and condenses it into
water and drains it. The remaining cool gases are
exhausted to the outdoors. By doing this, there is
no hot air escaping from your home. In addition,
the venting can be done with a PVC pipe rather
than up a heat resistant flue in the roof. It also
reduces the amount of pollutants released into
the air.  Finally, there is no risk of carbon
monoxide building up in your home and hence no
need for energy wasteful high/low vents.

Before the era of tightly constructed homes, it
was not uncommon to install furnaces and air
conditioners that had two to four times the
necessary capacity.   Not only is this wasteful in
terms of space and initial cost, but also in terms
of operating cost.   A correct sizing calculation  
service is often offered at little or no cost to
homeowners by gas and electric utilities

proper venting of mechanical room.