The furnace receives all the air from the home, via the duct system, and then treats that air, either by cooling it or heating it.  The furnace is usually a box approximately 3 feet by 2 feet by 2 feet. Most modern furnaces fall into one of three application types; up flow, horizontal flow, or down flow.

The biggest difference in gas furnaces is the efficency rating. There are two main catagories of efficency rating, the 80% efficient furnaces and the 90% efficient furnaces. One major difference between the two, is the 90% efficient furnace has a secondary heat exchanger. The standard 80% effieient furnace is vented in metal piping. When this piping is touched it is very hot. This heat is going up the chimney. The 90% efficient furnace has a secondary heat exchanger. This secondary heat exchanger, takes the heat that would have been vented through the metal piping, and gone out the chimney, and passes air across it again. This process removes the heat, that would have gone out the chimney, and makes it available to heat the home. This is the process that increases the efficiency from 80% to 90%. The secondary heat exchanger removes so much heat, that the gases actually condense. When these gases condense, water is formed. And that is why a 90% efficient furnace needs to be connected to a drain.

  If you are trying to decide the long term cost savings of upgrading your next furnace purchase to a new 90% efficient furnace, computing the cost difference is really simple.  Look at your gas bill and determine the months that you are using the most gas. This will usually be the winter months. Take 10% of the bill. That would be roughly what you would save, (keep in mind other gas appliance usage and judge accordingly.) Next you have to ask yourself, how long am I going to be in this home, and how many years of gas savings will it take to to off set the increased cost (the price difference between the 80% and 90% efficient models).

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