Answering Questions about Furnace Efficiency Ratings (AFUE)

Three lit furnace burnersOne of the most important services that we provide our customers is energy audits and home performance assessments. This is the “whole-house” approach to reducing utility bills, where our team finds spots where a home is wasting power and then offers solutions.

One place where we frequently find that a house is expending more energy than necessary is the heating system. Most households use a furnace for their heat, and older furnaces have much lower efficiency than newer ones. (Older furnaces will also lose efficiency because of basic wear and tear from age.) Our installers can replace an older furnace with one that has a higher AFUE rating, which is the standard measure of furnace energy efficiency.

What Exactly Does AFUE Mean?

The simple answer is that it stands for annual fuel utilization efficiency. The more complex answer is that it’s a percentage that represents how much of a furnace’s fuel supply converts into actual heating power. For example, a gas furnace with an AFUE of 85% would change 85% of the natural gas it burns into heat for a house, and lose 15% of it to exhaust gases.

Who Regulates AFUE Ratings?

The U.S. Department of energy is responsible for setting the minimum AFUE standards. The ENERGY STAR program, in conjunction with the Environmental Protection Agency and the DOE promotes products with improved efficiency and better performance.

How High Are AFUE Ratings Today?

Furnace technology advancements have made new models more efficient than ever. Older furnaces often score AFUE ratings between 56% and 70%. These furnaces use continuous pilot lights and heavy heat exchangers. Current mid-efficiency systems that use electronic ignitions, small-diameter flue pipes, and lighter heat exchangers have 80% to 83% AFUE. The current high-efficiency systems use advances such as second heat exchangers to draw out even more heat from the combustion gases and sealed combustion chambers to achieve ratings of 90% to 98.5% AFUE. The improvement just from 70% to 85% can make a tremendous difference in household energy use.

Does a Furnace Always Perform at its AFUE Rating?

No, it doesn’t—and this is an important part of understanding how to gain the best energy efficiency in a home. AFUE is measured over a year, which means a furnace on average will work at the listed efficiency. Sometimes it will be less, at other times more.

Outside factors affect a furnace’s efficiency is as well, and this is where the whole-house approach to energy use is helpful. A furnace can lose large amounts of the heat it produces to leaky and poorly constructed ductwork. Gaps in the heat envelope of a home will create drafts that will seriously reduce how effectively a furnace operates. A high AFUE rating is no guarantee of actual energy-saving performance.

This is why you need to exercise caution when it comes to a new furnace installation or replacement: simply picking the most expensive high-efficiency system with the most impressive AFUE rating doesn’t ensure that you are really getting the best furnace for your needs. Always look to HVAC professionals, particularly professionals who are familiar with the whole-house approach to energy savings.

If you are planning on a new furnace installation, we strongly recommend an energy audit done as part of it. You’ll end up with a new heating system that can truly perform at the promise of its efficiency rating.

Comfort Central, Inc. offers exceptional home performance assessments in Asheville, NC and throughout Western North Carolina.

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