AFUE Rating For Furnaces: How To Calculate AFUE Savings?

AFUE rating stands for ‘Annual Fuel Utilization Efficiency’. This is a key energy-efficiency rating for furnaces, and it’s most commonly used when evaluating the energy efficiency of gas furnaces.

In the most basic sense, the AFUE rating tells us what percentage of fuel (natural gas, propane, oil) is converted into heat.

Here are two examples that illustrate this:

  1. Gas furnace has 80 AFUE: That means that 80% of energy in the fuel is converted into heat.
  2. Gas furnace has 94 AFUE: That means that 94% of energy in the fuel is converted into heat.

In the 1950s, the AFUE rating of an average gas furnace was 50. With the advances in technology – condensing units, primarily – the modern gas furnaces can have an AFUE rating over 90. You can read more about these high-efficiency gas furnaces here

The energy efficiency is average on an annual scale; hence we get an annual fuel utilization efficiency. This is because the efficiency can vary a bit throughout the heating season.

We’ll look at how AFUE rating is calculated based on the gas and oil energy capacity. We’ll also look at how the new 94 AFUE rating gas furnaces are more efficient than 20-year-old 80 AFUE rating ones. 

Quick example: 94 AFUE furnace will burn $1,000 amount of natural gas while 80 AFUE furnace will have to burn $1,175 worth of gas for the same heating effect.

How Is AFUE Rating Calculated?

To calculate the annual fuel utilization efficiency, we need to know how much energy natural gas and oil contain. Here are the numbers:

  • 1 cubic meter of natural gas contains 36,000,000 J of energy. 
  • 1 barrel of oil contains 6,120,000,000 J of energy. 

A furnace that utilized natural gas should, at 100% efficiency, produce 36,000,000 J of energy. However, the utilization of natural gas is not 100%.

Here is how the AFUE rating is calculated:

AFUE Rating = Output Energy / Input Energy (based on 100% efficiency)

In the 1950s, an average 50 AFUE rating gas furnace would extract only 50% or 18,000,000 J of energy from every cubic meter of natural gas.

In the 2020s, the modern 94 AFUE rating condenser gas furnace can extract 94% of energy or 33,840,000 J of energy. 

Here is an example of this calculation:

Output Energy = AFUE Rating * Input Energy

In the case of 94 AFUE furnace using 1 cubic meter of natural gas, this is:

Output Energy = 0,94 * 36,000,000 J = 33,840,000 J

In effect, the gas furnaces have become so advanced that they can extract up to 15,840,000 J of energy more from every 1 cubic meter of natural gas.

80 vs 94 AFUE Rating Furnaces Compared

Today, most people either have:

  • 20-year-old gas furnaces with 80 AFUE rating, and are looking to buy a new one.
  • First-time owners who are choosing between cheaper older 80 AFUE  vs expensive modern 94 AFUE furnaces.

It makes sense to calculate how good 80 AFUE vs 94 AFUE furnaces are. The price difference between the units can be up to $1,000. Does the higher AFUE rating justify the additional cost? The following section is a bit calculation-heavy.

An average household will need about 100 million BTUs (British Thermal Units) per heating season. That equates to 105,500 billion J (Joules). 

Now let’s look at the price of natural gas. According to the EIA, the average residential price of 1,000 cubic feet of natural gas in February 2020 was $9,12. That is 28,32 cubic meters. Here is the summary:

  • 80 AFUE rating furnace produces 792,960,000 J burning $9,12 of fuel.
  • 94 AFUE rating furnace produces 958,348,800 J burning $9,12 of fuel.

For an average home and an average heating season, here are the total 1-year costs of running a gas furnace:

  • 80 AFUE furnace will burn through $1,213.37 worth of natural gas.
  • 94 AFUE furnace will burn through $1,003.98 worth of natural gas.

That means that, all things equal and using average heating needs, 94 AFUE furnace will save $210.61 per year compared to 80 AFUE furnace.

In 20 years (average furnace lifespan), you’re looking to save more than $4,000 just on natural gas.

Essentially, paying $1,000 more for a high-efficiency 94 AFUE rating is genuinely a smart investment for your home.

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