What is Furnace Efficiency?

Margo Upson
Margo Upson

An estimated 35 million homes in the United States (US) are heated with natural gas through forced-air heating. Unfortunately, most homes lose 30% of this heat before it even reaches the living areas. The heat escapes through the flue and other areas of the heating system. In addition, many older furnaces burn twice the fuel necessary to provide heat. These furnaces have a very low level of efficiency.

Furnace efficiency involves the difference between how much heat a furnace provides and the amount of energy it uses.
Furnace efficiency involves the difference between how much heat a furnace provides and the amount of energy it uses.

Furnace efficiency measures the amount of heat produced compared to the amount of fuel burned. The percentage of fuel that a furnace turns into actual heat is called Annual Fuel Utilization Efficiency (AFUE.) A furnace is then labeled as low-, mid-, or high-efficiency. Low-efficiency models, under 78% AFUE, are older, and not generally for sale. Mid-efficiency models start at 78% and go up to 90% AFUE. Anything above that level is considered high-efficiency. In 1992, the US Department of Energy set the minimum AFUE level on a new furnace at 78%.

A furnace’s AFUE rating is printed on the appliance’s yellow Energy Guide sticker, placed in a prominent location. This number is just for the furnace itself, not for the entire heating system. Having a very high furnace efficiency won’t help with the leaks in the vents. Also, the size of the home may alter the efficiency of a furnace. It is important to buy a furnace that is big enough for the space that it is heating, but not too big. A furnace that is too large will turn on and off more often than it should, and waste a lot of fuel.

Just because a furnace is old does not mean that it needs to be replaced for a newer model. There are many ways to improve the efficiency of older furnaces. One way is to clean the ducts regularly, as clean ducts allow more heat to come through. Air filters should be changed every three months. Check any ducts for holes, and have them patched up. Hire a professional to clean and inspect the furnace. Often times, a professional will know a few tricks to safely improve furnace efficiency without having to replace the whole system.

Furnace efficiency is one of the most important factors to consider when trying to reduce heating costs. Having a highly efficient furnace can save thousands of dollars over the lifespan of the furnace, usually 20 years. A furnace operating with an AFUE percentage over 90 can save homeowners around $10,000 US dollars during its lifespan. This can make quite a difference in annual heating bills.

Margo Upson
Margo Upson

Margo has a varied academic background, which has involved everything from psychology and culinary arts to criminal justice and education. These wide-ranging interests make her an ideal wiseGEEK writer, as she always enjoys becoming an expert on new and unfamiliar topics.

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Discussion Comments


I am kind of skeptical about the cost saving claims that people make when advertising high efficiency gas furnaces. I know that you can save a lot on your utility bill, but buying a new furnace is expensive as well.

Once you factor in the installation cost you are looking a pretty sizable chunk of change. So has anyone installed on of these things and seen real, measurable savings? Is it worth it?


Is natural gas always the most efficient fuel to heat a furnace? I have heard lots of good things about wood stoves and electric heating is available as well.

I am not an expert in this area at all I am just curious. We all seem to have natural gas furnaces. Maybe there is a better way.


Having an efficient furnace can go a long way to reducing your heating bills in the winter. I live in upstate New York and we get some pretty brutal winters. The furnace is running at full capacity all winter long.

I had mine replaced last year from a model that was about 25 years old to a brand new high efficiency furnace. I have seen my heating bills drop by 40% and my house is warmer than ever. I wish I would have done this years ago.

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