AFUE

What Does AFUE Stand For?

Annual Fuel Utilization Efficiency

AFUE, or Annual Fuel Utilization Efficiency, measures how efficiently a furnace converts fuel to heat. In simpler terms, this metric determines how much fuel your furnace is actually turning into heat for your home.

Thus, the higher the AFUE rating, the more energy-efficient the system and the more heat entering your home. As a homeowner, you may want a high AFUE-rated furnace because it’s a good thing for the environment, your home, and your wallet.

What is a good AFUE rating?

AFUE ratings measure the energy efficiency of furnaces and some other heating systems. To better understand AFUE, it helps to understand how a furnace heats your home. To produce heat, the furnace uses a fuel source, like propane or natural gas, to make its own heat from scratch for your home.

AFUE rating can be lowered if heat escapes through a chimney, leaks out of the system, or gets produced by an inefficient burner during the heating process. Keep in mind, heat losses in the ductwork of your home are not taken into account to calculate the AFUE rating, so if your ducts are broken or not properly sealed, you could be sacrificing even more heat.

To get a little more technical, AFUE is the ratio of the furnace’s annual heat output compared to its total annual energy input. According to Energy.gov, a high-efficiency heating system has an AFUE rating of 90% to 98.5%. A mid-efficiency heating system has an AFUE rating of 80% to 83%, and any AFUE rating lower than that is considered a low-efficiency heating system.

You can find your system’s AFUE rating displayed on the Energy Guide sticker on your unit. If you can’t find the Energy Guide sticker, check your owner’s manual for the AFUE rating.

How is an AFUE rating calculated?

To figure out the AFUE rating, HVAC professionals use this calculation:

AFUE = (Total annual heat output in BTUs / Total annual energy input in BTUs) x 100

A typical furnace needs 100 million BTUs (or British Thermal Units) per heating season, so that number is always used as the total annual energy input. HVAC professionals then use sensors and other tools to determine the total annual heat output (in BTUs) for each furnace or heating system. Once they have that number, they divide it by 100 million BTUs and then multiply by 100 to find the AFUE rating as a percentage. Later, we’ll show you a few sample calculations so you can understand what those numbers really mean.

Why AFUE matters when buying a furnace

The higher the AFUE rating, the more energy efficient the furnace. For a homeowner, a high AFUE-rated furnace will not only heat your home faster, but it may also save you money while doing it. While virtually impossible to achieve, 100% is the maximum AFUE rating. This means 100% of energy is devoted to heating your home and no energy is being wasted.

To understand AFUE in terms of money, think of that 100% efficiency as being equal to a dollar spent on heating. If your furnace had a 100% AFUE rating, all of that dollar would go toward heating your home, and none of it would be wasted on escaped heat. As the AFUE rating gets lower, part of each dollar toward your heating bill starts to chip away. Check out the following AFUE calculations for two different furnaces to get a better understanding of how a high AFUE rating is saving you money. Remember that in these calculations, total annual energy input is always 100 million BTUs and the total annual heat output varies based on furnace.

AFUE Calculation Example 1:

Your furnace has a total annual heat output of 80 million BTUs.
AFUE = (Total annual heat output in BTUs / Total annual energy input in BTUs) x 100
AFUE = (80,000,000 BTUs/ 100,000,000 BTUs) x 100
AFUE = 80%

This means 80% of the energy used by this furnace is becoming heat while 20% of it is being wasted. Using our dollar analogy, that would mean 80 cents of your dollar is going to heat your home, while 20 cents is being wasted.

AFUE Calculation Example 2:

Your furnace has a total annual heat output of 95 million BTUs.
AFUE = (Total annual heat output (in BTUs) / Total annual energy input (in BTUs)) x 100
AFUE = (95,000,000 BTUs/ 100,000,000 BTUs) x 100
AFUE = 95%

In this case, 95% of the energy used by this furnace is becoming heat while only 5% of it is wasted. Using our dollar analogy, that would mean 95 cents of your dollar is going to heating your home, while just 5 cents is being wasted, a much better use of your money.

In a nutshell, a low AFUE rating is likely costing you money on your heating bill on top of not heating your home in an efficient way. But aside from AFUE, there might be other reasons your home’s not reaching your desired temperature in the winter. If you find your furnace is having problems efficiently heating your home, call in an HVAC professional to do some furnace troubleshooting and diagnose the problem.

Other factors to consider when furnace shopping

While AFUE ratings are definitely important, they aren’t the only thing to consider when investing in a new furnace. Here are a few other things to keep in mind before buying your new heating system:

• Price: Price is an important factor when deciding which HVAC system is right for your home. Depending on your location, existing system type, annual heating needs, and other factors, a different type of system may be more cost effective for your home. We recommend talking to a Trane professional to help determine the best system choice for your home.
• Type of system: Before you commit to buying a furnace, make sure that it’s the right heating system for you. Furnaces are typically found in areas that experience colder winters, since they generate heat at a rapid pace. If you live somewhere with more mild winters, another type of system — like a heat pump and air handler, or a dual fuel system — might be better suited to your home’s heating and cooling needs.
• Fuel source: Furnaces utilize propane, natural gas, or oil to produce heat. On top of that, they utilize electricity to run internal components like the blower motor that help complete the job of making heat, but no furnace relies solely on electricity to produce heat. If you’re looking for a 100% electric HVAC system to produce heat, considering an air handler would be a better choice for you. But, if you want a natural fuel source, a furnace makes sense for your home.