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What Is A High Efficiency Furnace, And What Does AFUE Really Mean?

Does a high AFUE or the designation as a high efficiency furnace really mean that particular heating system will be better than someone else’s older system? To answer this, first let’s have a look at a definition of the terms.

AFUE Explained

On the energy.gov site, AFUE is explained as follows:

“A central furnace or boiler’s efficiency is measured by Annual Fuel Utilization Efficiency. AFUE is a measure of how efficient the appliance is in converting the energy in its fuel to heat over the course of a typical year.

“Specifically, AFUE is the ratio of annual heat output of the furnace or boiler compared to the total annual fossil fuel energy consumed by a furnace or boiler. An AFUE of 90% means that 90% of the energy in the fuel becomes heat for the home and the other 10% escapes up the chimney and elsewhere.”

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High Efficiency Furnace Defined

This is a bit more vague because there are electric, oil and gas (natural gas and propane) furnaces available; all are considered high efficiency despite operating at AFUEs varying as much as 15.5%.

  • Generally, for natural gas and propane furnaces, an AFUE above 90% is high efficiency. There are models that go as high as 98.5%.

  • Oil furnaces are advertised as high efficiency even when AFUE is as low as 83%, with averages going between 83 and 86.5%.

  • Electric furnaces are generally considered high efficiency and AFUE goes between 95 and nearly 100%.

Why AFUE Can be Misleading

Of course, if you are replacing an inefficient furnace with a more efficient one, you will without question experience greater economy and performance. But even though actual measurement at the furnace is a high percentage, one has to consider the entire system. The energy.gov site also states:

“AFUE doesn’t include the heat losses of the duct system or piping, which can be as much as 35% of the energy for output of the furnace when ducts are located in the attic.”

Also, when speaking of oil, natural gas and propane furnaces, you have to factor in the efficiency of the electric motor running the blower. Older style motors are not nearly as efficient as new ones. A particularly efficient type has been developed that exceeds all others in performance. The ECM, or electrically commutated motor (simply put, a motor operating without the need of brushes), operates with a variable speed, giving only what is necessary to maintain optimum conditions rather than running on a full-on or full-off basis that ends up wasting energy.

For electric furnaces, an AFUE of 95-100% ignores a major flaw in reasoning. Electricity is a secondary power source. Its true efficiency is vastly affected by the primary source used to generate the electricity. Power generation around the country depends on diesel, coal, hydroelectric, nuclear and even wind and solar sources. All of these have their own efficiencies, and imply some loss in transmission and handling. If you are environmentally concerned, you may not like the hidden price in pollution and waste of resources.

Furnaces: Pay Attention to the Whole Picture

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Finally, the home’s overall energy efficiency may affect things more than the furnace. Insufficient insulation or thermally inefficient doors and windows and other features may be squandering whatever maximum efficiency your furnace is capable of.

Definitely invest in high efficiency, but don’t overlook the fact that a high AFUE is only part of the picture. Contact a competent HVAC professional to advise you on your overall home energy efficiency and any local considerations as to which furnace may best meet your needs.

Creative Commons AttribuCreative Commons Attribution: Permission is granted to repost this article in its entirety with credit to Alley and Co. Heating/Air and a clickable link back to this page.

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