Furnace AFUE: A Guide to What It Is and Why It Matters

November 14, 2022
Savings in Seattle, WA

When looking to replace your furnace or install a new unit, the two most important things to focus on are the size of the furnace and its energy-efficiency rating. The size or strength of a furnace is measured in BTUs (British Thermal Units), which refers to how much heat it can produce in an hour. How large of a furnace you need is dictated by both the size of the home and the local climate.

As such, furnace size is something you don’t have much of a choice in since you need to be sure that the unit is the appropriate size to heat your home effectively. The energy efficiency of the furnace is an area where you do have a lot of choice, and this will determine how much it costs to heat your home. Furnace efficiency is measured in AFUE (annual fuel utilization efficiency), and here is everything you need to know about how AFUE is measured and why it matters.

How AFUE Is Calculated

One of the biggest misconceptions about AFUE is that it refers to how much energy the furnace uses, which isn’t exactly true. If you compare two furnaces that produce the same number of BTUs, they will always use the same amount of energy when they are running regardless of their AFUE ratings. Instead, AFUE refers to the percentage of heat created by the furnace that it utilizes. An easier way to think about it is by considering AFUE to be an expression of how much energy the furnace wastes.

Electric furnaces always have an AFUE of 100%, which means that all of the electricity the unit uses is converted into heat. Gas furnaces can range anywhere between 80% and 98.5% AFUE. While electric furnaces are technically more energy efficient, they are still more expensive to run due to the higher cost of electricity. They are also not nearly as effective at heating and, as such, aren’t well suited to colder northern climates. For this reason, we will focus mostly on gas furnaces so that you can better understand exactly what AFUE means.

If a furnace has an AFUE of 80%, this means that 80% of the gas it uses is converted into heat energy. The remaining 20% of the heat the furnace creates is wasted. This waste occurs due to some of the hot combustion fumes escaping out of the exhaust flue before the furnace can fully utilize the heat. A 90 AFUE furnace captures 90% of the heat energy it creates, while a 98.5 AFUE furnace will utilize all but 1.5% of the heat it creates when burning gas.

Again, it is important to note that all of these units will still burn the same amount of gas in the same time span. The difference is that the higher AFUE units can more effectively utilize the heat created by burning the gas. As a result, these units will produce more heat in the same time span and thus effectively raise the indoor temperature more quickly.

An 80 AFUE furnace may need to run for 30 minutes or so to raise the temperature to the point where the thermostat signals it to shut off. The same size furnace with a higher AFUE will burn just as much gas whenever it runs, but it may only need to run for 15 minutes to raise the temperature by the same amount. As a result, the higher-rated unit will use less gas overall since it will always run for less total time throughout the day.

AFUE is an extremely important consideration as the higher the rating is, the more effectively and efficiently the unit will heat. This can help to ensure that the temperature remains more consistent and also help to eliminate issues with some parts of the home always staying too hot or too cold. Although higher AFUE units obviously cost more upfront, they will usually be cheaper in the long run due to the decreased energy waste. Higher AFUE units will also boost your overall comfort due to the aforementioned reasons.

Minimum AFUE Ratings

All furnaces in the U.S. are required by law to be at least 80 AFUE as this is the minimum efficiency standard set out by the Department of Energy. The Department of Energy has proposed a change to this rule that would require all furnaces to be at least 95 AFUE. If the change passes, this new rule would go into effect in 2029.

The proposed change would eliminate all conventional furnaces and eventually force all buildings to switch over to a high-efficiency condensing furnace. To understand why this is, we’ll now take a closer look at the different types of furnaces and how furnace type and AFUE are related.

Comparing Conventional and Condensing Furnaces

Gas furnaces can be lumped into two broad categories — conventional units and high-efficiency condensing units. Conventional furnaces range between 80 and 89 AFUE, and all condensing furnaces have a minimum AFUE of 90%. Both types of furnaces produce heat in the same way, but condensing furnaces have an additional component that allows them to make use of more of the heat that they create.

To produce heat, gas furnaces burn either natural gas or propane inside a closed combustion chamber. This chamber is separated from the rest of the furnace and ductwork by a heat exchanger. The heat exchanger works to absorb the heat created inside the combustion chamber. This process causes the heat exchanger to become extremely hot.

In order to utilize this heat, the HVAC blower fan draws cold air in through the return vents and ducts. This cold air is forced over the heat exchanger, and all of the heat energy stored in the exchanger naturally flows into the cold air. This process instantly heats the air, and the blower then circulates the hot air throughout the supply ducts and vents to warm the building.

The one issue with this process is that the heat exchanger can only absorb heat so quickly. As a result, the combustion fumes always retain some latent heat when they exit through the exhaust flue, and this is where the energy waste comes from.

Condensing furnaces minimize heat loss and energy waste by using a secondary heat exchanger. This allows the unit to absorb the majority of the remaining heat energy from the combustion gases before they flow out through the flue. The primary heat exchanger still captures close to 90% of the heat energy. The hot combustion gases then flow into another chamber where the secondary heat exchanger then captures most of the heat energy, and the cold combustion gases then flow out through the flue.

This process absorbs so much heat from the combustion gases that they become cold enough for condensation to begin to form inside the furnace, hence the name condensing furnace. To prevent water damage or leaks, condensing furnaces have a condensate drain pan that collects all of the water that forms inside the unit. This water then flows out of the drain pan through the condensate drain line and usually into a nearby floor drain.

If you have any questions about AFUE, Brennan Heating & Air Conditioning is here to help. We are a full-service HVAC company specializing in furnace maintenance, repairs, and installation, and we also work on and install air conditioners, heat pumps, ductless mini-splits, and hydronic heating systems. We also offer air duct cleaning, home energy consultations, and a range of water heater and electrical services for customers throughout the Seattle area. To learn more about AFUE and your options for furnace replacement, give us a call today.

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