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Furnace Efficiency and AFUE Rating

on January 20, 2017

Furnace Efficiency and AFUE RatingEnergy conservation is an important factor when it comes to saving heat, fuel, and money every month.  If you’re considering upgrading or replacing your existing furnace with a new system, you’ve likely seen electric and gas furnaces labeled with an AFUE rating.  This rating exists to help you identify how efficient the system is.  But you shouldn’t rely on the AFUE rating alone!  Energy efficiency in a new furnace is dependent on many variables so, while the rating is the first thing you should watch for, there are plenty of other items to check before purchasing a new system too.

What Is AFUE?

AFUE stands for Annual Fuel Utilization Efficiency.  It’s a measurement of how well a unit performs by comparing the amount of fuel used per BTU of heat produced.  This estimate is not designed to describe every time you turn the unit on.  Instead, the AFUE rating is designed to reflect the average efficiency of the unit over the course of a full season of use.  It compares the amount of heat produced to the amount of heat possible from its fuel source.  As an example, if you’re using a gas furnace with an AFUE rating of 85%, then it produces 85 BTU worth of heat for every 100 BTU available from the propane used.

This is not the total BTU delivered into your home. There are still losses due to vents, attic ductwork, steam piping, drafts in the home, or poorly-insulated outdoor units.  The AFUE rating is only an efficiency rating for how well your furnace operates compared to directly consuming the fuel.  Because AFUE is designed to reflect how much energy is delivered versus fuel used, electric furnaces typically have a higher AFUE rating.  That does not necessarily mean they are more efficient.  Since fuel used for electric furnaces is burned at a power plant, AFUE ratings for electrical furnaces are rated based on electricity conversion, which is typically closer to 100% of the energy being diverted into heat.

What Else Affects Efficiency?

The efficiency of your furnace is affected by many other elements in your home:

  • Leaky Ductwork
  • Closed Vents
  • Clogged Exhausts
  • Drafts and Seals
  • Faulty Thermostats
  • Lack of Insulation

All of these things are key methods of heat loss in your home.  If you find that your home is difficult to heat, then check for drafts and make sure that your home is still insulated well against the cold.  Draw curtains at night to keep the warmth in and open them when in direct sunlight to allow heat to radiate into the home.

You may also find that you’ve simply set the thermostat too high for your furnace to keep up.  Try dropping the thermostat to 70 degrees or less and wearing a sweater.  That way you can help your furnace last longer, reduce fuel consumption, and still stay warm.

Should I Replace my Furnace?

The really depends on your home.  For furnaces that are ten years old or more, it’s advisable to consider replacing your existing furnace.  This is especially true if you’ve noticed an increase in the amount of fuel or electricity being consumed by your furnace every month.  But, if your furnace is in good repair, less than 10 years old, and has an AFUE rating of more than 80%, it’s likely not worth spending the money to have a new furnace installed.


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