AFUE Savings Calculator

About the Calculator, AFUE, HSPF & Furnace Efficiency

Want to learn more about the calculator? Do you have questions about furnace and heat pump efficiency? In the following paragraphs, we’ll talk about AFUE, HSPF, BTU, heating degree days, fuel types and more. Plus, we’ll teach you some important things to look for when choosing a new furnace / heating system for your home.

How Does the AFUE Calculator Work?

The AFUE Savings Calculator is designed to help you compare the heating costs of furnaces, boilers and heat pumps with different efficiency ratings (expressed in AFUE or HSPF) and fuel types. The calculator estimates your heating cost savings over a one, five, ten and fifteen year period.

The calculator computes how much per year you are spending on heating your home based on three primary factors:

• Your current heating system’s fuel type,
• its AFUE rating or HSPF rating
• and its BTU input capacity or its tonnage.

The calculator then compares your current heating method with a new one. You can choose to compare your furnace with furnaces of the same or different fuel types and AFUE ratings. You can also compare your heating costs with those of heat pump systems of varying efficiency ratings. Go ahead and try out several combinations — the results may surprise you!

In order to estimate heating costs as accurately as possible, the calculator also bases its results on several secondary factors, such as:

• The price of heating fuel in your area (natural gas, heating oil, propane or electricity)
• The number of heating degree days in your area
• A 70° Design Indoor Temperature and a Design Outdoor Temperature suitable to your region.
• A C-Factor which accounts for heat from other sources.

What is AFUE?

When researching furnaces, you can’t help but encounter the acronym AFUE. As stated above, AFUE stands for Annual Fuel Utilization Efficiency. It is a percentage that shows how efficiently a given furnace or boiler is able to use its fuel.

AFUE can be compared to SEER (Seasonal Annual Efficiency Ratio), which is the efficiency rating system for air conditioners. In both SEER and AFUE, a higher number indicates a more energy efficient unit. But actually, AFUE is easier to understand than its AC rating counterpart. While SEER is expressed in a number whose meaning is difficult to decode, AFUE is represented by a straightforward percentage.

For instance, a furnace with 80% AFUE is 80% efficient. This means that 80% of the fuel it consumes is converted to heat energy that warms your home. The other 20% finds its way up the flue – wasted.  A 90% AFUE furnace uses 90% of the fuel to heat your home, wasting only 10%.

It’s only possible to achieve 100% efficiency (0% waste) with an electric furnace, although some natural gas and propane furnaces do come close.

What is HSPF?

HSPF — Heating Season Performance Factor — is used to express an air source heat pump’s operating efficiency. Like AFUE, the higher the number, the more efficiently the heat pump operates. But unlike AFUE, understanding HSPF is not as straightforward.

Newer heat pumps typically have HSPF ratings between 8 and 13. The number represents the total heat output (in BTU) of a heat pump during a heating season divided by the electricity (in watt-hours) consumed by the heat pump during the same season. For example, a heat pump with a 10 HSPF rating outputs 10 BTU of heat energy per watt hour of electricity consumed.

On the surface, HSPF is difficult to compare with AFUE. Direct comparison requires converting HSPF into a percentage. We won’t go into the math here, but once HSPF is converted to a percentage like AFUE, we discover that heat pumps are 150%-300% efficient. This means a heat pump can generate 150-300% more heat energy compared to the amount of electrical energy they use.

Why are heat pumps so efficient? This is due to the heat pump’s ability to extract heat from the air and move it into your home, rather than creating heat energy from a fuel source (like gas).

It is important to note that heat pumps function best in temperatures above 32 degrees Fahrenheit. Thus, it is typical – especially in cold climates – to combine a heat pump with backup heat source, like an electric or gas furnace. Using two heat sources will ensure that your home is always toasty warm, but you can still enjoy some of the heat pump’s energy cost savings.

Since heat pumps are capable of both heating and cooling, they also have a SEER rating. The current national minimum efficiency standard for heat pumps is 14 SEER / 8.2 HSPF.

It’s also necessary to note that geothermal heat pumps use a different efficiency rating system than air source heat pumps, and are not represented in the AFUE Savings Calculator.

How Do I Find My Furnace, Boiler or Heat Pump’s Efficiency Rating?

All newer furnaces are required to display their AFUE rating on the cabinet. Likewise, all heat pumps will have a yellow label displaying their HSPF rating. For a split system heat pump, the label is typically on the outdoor unit. If you can’t find the efficiency rating, check the user manual, or locate your unit’s model and serial number and contact the manufacturer or your local HVAC contractor.

Higher AFUE Doesn’t Always Mean Lower Operating Costs

AFUE and HSPF provide a convenient way to compare the energy efficiency of different furnace and heat pump models. However, just because a furnace or boiler has a higher AFUE, or just because a heat pump is more efficient than a furnace, doesn’t mean it will cost you less to operate. In order to gain a more accurate picture of operating costs, you need to consider the fuel type.

The Relationship Between Fuel Types and Furnace Operating Costs

The AFUE Savings Calculator uses the following fuel types for comparison:

• Natural Gas
• Heating Oil
• Propane
• Electricity (forced air or air source heat pump)

As you can imagine, both the price and the availability of each of these fuel types varies from location to location. Also, the pricing is based on different units of measure. The calculator accounts for both of these factors. For example, try setting your Current Heating Method on the calculator as follows:

• Fuel Type: Natural Gas
• AFUE Rating of 87%
• BTU Input of 80,000

Then, set the New Heating Method to:

• Fuel Type: Electric (Forced)
• AFUE Rating of 99%

You’ll notice that even though the electric furnace is 99% efficient, it will cost you more per year to operate than the less efficient natural gas furnace.

This is because the cost of natural gas is lower than the cost of electricity. (There may be areas in the United States where this is not the case. But when you try this experiment comparing different fuel types and AFUE/HSPF ratings you will quickly understand the point we’re making.) That being said, you may still come away cheaper using electricity vs. natural gas when you install a heat pump, due to their highly efficient use of electricity.

Thus, efficiency alone is not an indicator of operating costs. You would be pretty disappointed if you replaced your natural gas furnace with an electric furnace hoping to save money in heating costs. The only time you can rely on AFUE as a sole comparison is when comparing two furnaces with the same fuel type. A 100% AFUE electric furnace will cost less to operate than a 90% AFUE electric furnace. For all other comparisons, always consider the fuel type and related costs.

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.

A Note on Tonnage

Heat pumps, air conditioners and some furnaces are measured in tons. Far from being a weight measurement, one ton equals about 12,000 BTU.

Input vs. Output BTU

As we’ve already established , not all heat that a furnace creates is usable. Some of a fuel a furnace consumes is wasted. This means that a furnace with an advertised 100,000 BTU does not actually create that much useable heat for your home.  In most cases, this number refers to input BTU. If the furnace in this example also has a 90% AFUE, this means that the actual output BTU is 90,000. This is the number you need to pay attention to because the output BTU is the actual amount of heat the furnace will provide to your home.

What Size of Furnace (or Heat Pumps) is Best for My Home?

This brings us to the question of sizing. Proper sizing of a furnace or heat pump is important to your comfort and energy efficiency. If your system is too large, it will cost more than it should to operate because it will constantly cycle on and off. Plus, larger furnaces and heat pumps require larger ducts, which can create problems with heat loss. However, if your system is too small it will not provide enough heat to keep your home comfortable.

What size is best for your home? This is a question you should ask a professional HVAC contractor. You can get a general idea of furnace / heat pump sizing based on climate and square footage, but in order to get an accurate measurement you have to consider many more details. The quality and amount of insulation, which direction your home faces, whether or not your home has an open floor plan, and many other factors will indicate exactly how much heating your home requires, known as its heating load. A heating load calculation takes these factors into consideration and tells you your optimal furnace / heat pump size.

A load calculation will give you the amount of heat energy required to heat your home, in BTU. This number should closely correspond to the output BTU (see above) of the furnace you select (or the tonnage of your desired heat pump). Keep in mind that the size listed by most manufacturers on their furnace models is the input BTU. You’ll need to multiply this number by the AFUE percentage to get the output BTU.

If you’re having a furnace or heat pump installed for the first time, having a professional HVAC contractor or Energy Rater do a load calculation is highly recommended. Even if you already have a furnace or heat pump, especially if you’ve made recent modifications to your home (like upgrading your windows or insulation, or building an addition), a load calculation might reveal that your system is over- or under-sized. This discovery can save you money on heating costs and help you and your family stay comfortable.

Does the Calculator Account for Climate?

Heating degree days and design temperature allow the calculator to adjust its findings based on your climate. It would be absurd to assume the same annual heating costs for an identical furnace (or heat pump) installed in a home in Fairbanks, AK and a home in Atlanta Georgia.

Efficiency aside, it can be generally assumed that the colder it is outside, the more energy your heating system will have to use to maintain a certain indoor temperature.  The more days of cold weather per year, the more often the heating system will run; and the more it runs, the more fuel it will consume.

This is where heating degree days and design temperature come in.  Behind each of these factors lie some complex equations, but we’ll spare you the gory details. Here are the basics:

• Heating degree days represent the number of days in a year that your home needs heating. Heating degree days help the calculator adjust its energy cost estimations based on your local climate. These “days” don’t directly relate to the 365 day calendar year. A heating degree day is a representation of how long and by how many degrees the outdoor temperature on a given day is below a universal base temperature of 65 degrees F*. For example, if the outdoor temperature were to remain a constant 60 degrees for 24 hours, then this would count as 5 HDD (heating degree days).
• Indoor Design Temperature refers to the temperature we try to maintain inside our homes. The calculator uses a constant of 70 degrees. Outdoor Design Temperature is not constant. It varies based on weather and climate. That’s why the Design Outdoor Temperature is an average temperature asserting that 99% of the time, the temperature in your region is equal to or above this temperature. As the difference between the outdoor and indoor design temperature increases, so does the amount of heating required. This is another way that the calculator adjusts its results for climate and region.

This is of course just a basic explanation. If you want to learn more about heating degree days and design temperature, head over to Energy Vanguard to read some excellent blog posts on these subjects.

*The annual heating degree days are averaged over a period of 30 years leading up to 2017. If the years in between 2017 and today were significantly warmer or colder, there will be variations in accuracy.

What is C-Factor?

Your heating system is not your home’s only source of heat. Lighting, your TV, your appliances, sunlight shining on your roof, even the people in your home give off heat. The calculator uses C-Factor to correct for this internal heat gain, so that the calculator can give you a more accurate projection of your heating fuel costs. The 0.77 C-Factor is a default value that accounts for the additional heat sources present in an average US household.