In the market for a new furnace? Noticing AFUE, listed everywhere? You may be scratching your head, and wondering, “what in the heck is AFUE?” In this month’s, Comfort Talk with KCC, let’s discuss just that.

With a new gas furnace comes attached to it, an Annual Fuel Utilization Efficiency rating (AFUE). An AFUE rating basically boils down to the amount (by percentage) of fuel being used by your system, to keep your space warm.

Take for example, a unit with an 80% AFUE rating. It uses 80% of the fuel that is put into it, while 20% of that fuel is wasted. While, say, a unit with a 90% AFUE rating, uses 90% of the fuel while 10% of that fuel is wasted (and so on).

But wait… there’s more. These furnace types work differently from one another, as well. (You can see the percentage cutoffs here.) To generate energy from natural gas, the furnace with an 80% AFUE rating uses what’s called, open combustion, while the unit with a 90% rating, uses sealed combustion. Think of the combustion chamber as the place where all the “magic” happens within the furnace. It is where the air mingles with the burning gas, which is what creates the combustion, which creates the heat, which then travels to the heat exchanger. The difference between the two combustions, boils down to where the air is being pulled from. The 80% pulls in air from the surrounding indoor area, versus the 90% pulling in air from outside of the home.

The unit with an 80% AFUE rating ends up producing 20% of exhaust from the gas, while the 90% AFUE rated furnace, only 10%. The 80% rated unit disposes of the exhausted gas via a chimney, and the 90% rated unit, via a plastic pipe that both brings in, and disposes of the air from the outside.

Now that you know what an AFUE rating is, and their influence of the furnaces’ functionality, come back next time when we discuss the pros and cons of each.