What is Furnace Efficiency?

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 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.