80 vs. 90 Furnace: Understanding Efficiency Differences

When it comes to heating and cooling, the phrase “energy efficiency” is thrown around an awful lot. It’ often accompanied by numbers, such as 80% and 90% — but what does it all mean? Here is a brief explanation of the difference between an 80% and a 90% furnace.

What Is Furnace Efficiency?

Furnace efficiency is how well a furnace converts energy from fuel into heat. It is measured by annual fuel utilization efficiency (AFUE). This ratio compares a furnace’s annual heat output to its total annual fuel consumption. The AFUE percent shows how much energy goes to heating your home versus how much is wasted.

The Real Difference Between 80% Efficiency and 90% Efficiency

Though both an 80% and 90% efficient gas furnace will heat your home, there are some significant differences that you may want to think about if you are considering a new furnace in the future. With an 80% efficiency furnace, you are only receiving 80% of the heat that you pay for, while 20% is wasted. With a 90% efficiency furnace, you receive 90%, meaning only 10% is wasted.

The 10% difference between the two furnaces might not seem large, but in the winter when you are consistently using your furnace, that 10% can really add up.

Cost Implications of Choosing Between 80% and 90% Furnaces

Choosing an 80% furnace comes with lower upfront costs. With a 90% furnace, the initial price point is higher, but the long-term savings could be significant. The more energy-efficient your furnace is, the lower your monthly energy costs could be, making a high-efficiency furnace well worth the investment.

Venting Differences

The other difference between an 80% efficiency furnace and a 90% efficiency furnace is how the furnace is vented. An 80% efficiency furnace has high exhaust temperatures due to the open combustion process the furnace uses to heat your home. The hot gases must be vented through the chimney via a chimney liner. 

A 90% efficiency furnace vents at much lower temperatures thanks to a closed combustion process. A sealed combustion chamber retains heat and a secondary heat exchanger helps capture additional leftover heat before it escapes, resulting in less waste. It also leads to lower exhaust temperatures, meaning the gases can be vented through PVC pipes on the side of the home.

Enjoy the Benefits of a High-Efficiency Furnace

All in all, the major difference between the two furnace types is how the 90% efficiency furnace can save more money and energy than the 80% efficiency furnace, all while keeping your home comfortable throughout the winter.

FLAME Heating, Cooling, Plumbing & Electrical is dedicated to helping you get the most out of your furnace investment. Our team can help you select the right unit for your home and perform quick and seamless installations. We also offer comprehensive repair and maintenance services to ensure your furnace keeps your home warm and energy-efficient.

Contact us today to learn more about our furnace services!

Last Updated on March 11, 2025 at 10:30AM EST