AFUE Savings Calculator (Heating System)

Would a more energy efficient furnace or heat pump save you money?


Similar to a SEER Rating, an AFUE rating communicates the energy efficiency of a furnace or boiler system. More specifically, it shows what percentage of the consumed energy is converted into usable heat. The higher the number, the more efficient the system.

Keep going if you want to know the answer to these questions:

  • What are the total operating costs of my current system?

  • Keeping all else the same, what would I save by changing to a more efficient system?

  • Would a change in energy source (for instance, electric to gas) help me save even more?

Find out now by using our AFUE Savings Calculator!


AFUE stands for Annual Fuel Utilization Efficiency.

Intrigued by the calculator? Let's learn more.

How Does the Calculator Work?

Our Savings Calculator helps you compare different types of heating systems by both efficiency ratings and fuel types. It estimates your heating costs over a variety of periods to help you determine your total return on investment.

For the calculator to work, we will need to know:

  • How your heating system is currently fueled,

  • its current energy rating (either AFUE or HSFE),

  • and its tonnage or BTU capacity.

With this information, the calculator can compare other methods of heating. We can dig in even deeper though, getting more accurate with more information. Some other factors include:

  • Your areas fuel costs,

  • average use in your area,

  • regional heating information,

  • accounting for heat from other sources.

De-Mystifying the Acronyms.

When researching furnaces, you'll find a bit of an alphabet soup. AFUE and HSPF are the most prominent - and most important.

AFUE stands for Annual Fuel Utilization Efficiency, which is a percentage efficiency rating given to a furnace or boiler. This is similar to SEER, which is found on air conditioners in that the higher the number, the more efficient it is.

Unlike SEER, AFUE is actually straightforward to understand. A furnace that with an 80% AFUE means 80% of the fuel used is converted to heat energy. The rest of the fuel is burned away and wasted. We obviously want as little waste as possible, especially with rising energy costs.

Does a 100% fuel efficient furnace exist? Yes, but only in electric furnaces which are often the most expensive to operate. Natural gas and propane furnace options will offer the next highest efficiency ratings.

What about HSPF? Heating Season Performance Factor expresses the efficiency of an air source heat pump, rather than a furnace or boiler. Again, the higher the number the more efficient but the logic isn't as straightforward.

Most often, you'll see an HSPF rating between 8 and 13, numbers which represent the total heat output in BTU during the heating season divided by the electricity consumed during that same period. So, its an equation that demonstrates the energy usage instead of a percentage.

If you were to convert HSPF equations into AFUE you'd find percentages over 100% - like 130%-150%. How is this possible? Well, since a heat pump pulls warmth out of the air and pumps it into your home it is not just burning fuel to create heat.

Heat pumps are a good option for South Carolina, since it rarely drops below freezing. Really cold environments just don't do well with heat pumps because there isn't enough warmth in the air. Sometimes homes will use more than one type of heating system, in order to stay warm while also taking advantage of a heat pumps efficiency.

Heat pumps also have a SEER rating, since they do both heating and cooling, so look for both on the big yellow sticker!

Oh, if you are interested in or have a geothermal heat pump they use yet another efficiency rating. Unfortunately, this is not included in our savings calculator. However, geothermal heating and cooling offers incredible efficiency savings - feel free to reach out if you are interested in these systems!

The Full Picture of Operating Costs

These rating systems are useful for determining how efficient a unit is, but they don't give you all the information you need about the costs of operation. For that, we have to factor in fuel costs. Natural gas, heating oil, propane, and electricity are all available fuel types for heating.

Prices of these fuel types vary by location and vary a good bit based on their unit of measure. These nuances sound small but they can make an incredible difference over the lifetime of your heater.

For these reasons, you'll often find that a less efficient gas heater costs less to run than a 100% efficient electric heater. Feel free to try several different scenarios for your homes heating system configuration.

About BTU

Each model of a particular furnace often comes in several different sizes. A furnace’s size is measured in BTU, which stands for British Thermal Unit. This is a unit of heat energy equal to the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. A match, for instance, has about 1 BTU, while furnace capacities equal tens of thousands of BTU. BTU tells you how much heat per hour your furnace is capable of producing from the fuel you put in.

Accounting for Climate

Our calculator accounts for climate in two important ways, heating degree days and design temperature. Obviously, its much harder to heat a home in Vermont than one in South Carolina. The colder the outdoor temperatures, the harder your system will work to maintain temperature in your home.

Consider heating degree days, which describes how much of the year your home needs to run heat. It's not based on calendar days, but instead how long and by how many degrees your unit has to control the temperature.

While outdoor temperature is inconstant, we hope to maintain a constant temperature inside. We use the terms indoor and outdoor design to temperature to determine the difference between the average temperature outside and the indoor temperature you desire. The bigger that difference, the more heat your home requires.

The Last Factor

There are other things in your home that put off heat. Your appliances, the lights in your home, and even the people all give off some heat. That's the C-Factor! We use this in the calculator to help give a more accurate representation of your fuel costs. Our default value for this is .77, if you want to go with an average.

20 views0 comments

Recent Posts

See All

Want to know how much you could save by switching to a higher efficiency air conditioner? Every HVAC unit has a SEER rating, designated by its efficiency. The higher the SEER number you see, the LESS