Why Are Electric Furnaces 100% Efficient (And Does It Matter)?
Furnace manufacturers measure efficiency using a rating known as the Annual Fuel Utilization Efficiency (AFUE) ratio. Depending on age and features, natural gas, propane, and oil furnaces will have various AFUE ratings. Typical "mid" or "standard" efficiency modern furnaces may have AFUEs as low as 80%, while higher efficiency models may get into the high 90s.
However, if you're comparing furnaces that use different fuel types, you may notice an oddity: all-electric furnaces will have an AFUE rating of 100%. Does this mean that electric furnaces are inherently more efficient? Surprisingly, the answer is "yes," but whether or not their higher efficiency matters can be a more complex question.
What Does AFUE Measure?
There are some complexities with how manufacturers measure AFUE, but the basics are relatively simple to understand. When you run your furnace, it takes a certain amount of energy as input. That energy comes from gas, heating oil, or propane for combustion furnaces. For electric furnaces, it's mains-supplied electricity.
The AFUE measures how much energy the furnace can convert into heat for your home. For example, a condensing gas furnace with an AFUE of 95% will extract 95% of the heat energy from the natural gas combustion process. The remaining 5% of the energy will exit through the flue as waste heat. Essentially, all combustion furnaces work by extracting heat from the combustion exhaust stream.
On the other hand, electric furnaces use a technology known as resistive heating. In other words, an electric heating element works by being a poor conductor of electricity. Electricity passes through a high-resistance element, producing a substantial amount of heat. Since all of the electrical energy that enters the heating element turns into heat, electric furnaces have an AFUE of 100%.
Will an Electric Furnace Save You Money?
While there's no question that electric furnaces convert more of their input energy into heat, the question of which furnace will cost less to run is far more complex. The cost of operating a furnace depends not only on its efficiency but also on its fuel cost. If natural gas is much cheaper than electricity in your area, a theoretically less efficient gas furnace may still be much cheaper to operate.
To make an accurate comparison, you'll need to compare energy unit prices for your area. Your climate can also be a factor since you'll see a more pronounced difference in total heating costs if you need a larger furnace. Ultimately, these calculations can be complex, so the best option is to work with your heating contractor to determine the most cost-effective option for your area.
Speak to a residential furnace contractor to learn more.