80% Furnaces vs. 90% Furnaces: Which Should You Choose?

Published 12/11/2025

When your furnace kicks on in December and keeps running through April, every bit of wasted energy adds up. That’s why homeowners across Chicagoland are taking a closer look at how their heating systems perform. If you’re comparing an 80% furnace to a 90% model, it’s not just about the price tag. It’s about long-term comfort, reliability, and HVAC efficiency.

At Mr. Duct, we help homeowners in places like Naperville, Oak Brook, and Evanston make smart choices about furnace replacement. Whether you need a standard system or want to upgrade to high-efficiency, our HVAC company will install the right solution for your home with a focus on safety, comfort, and cost savings.

Understanding AFUE Ratings and How Furnace Efficiency Is Measured

AFUE stands for Annual Fuel Utilization Efficiency. It tells you how much of the fuel your furnace turns into heat for your home during a typical heating season. An 80% AFUE furnace converts 80 percent of its fuel into usable heat, while the other 20 percent escapes through the exhaust. A 90% furnace does a better job, turning 90 percent of that fuel into heat and wasting less.

This difference changes how much gas your furnace needs to keep your home warm. In Chicagoland, where heating services is needed from October to April, that extra efficiency really counts. Better HVAC efficiency means you get more heat from the same fuel, so you use less energy and enjoy steadier comfort indoors.

Differences Between 80% Furnaces and 90% Furnaces

Both types of furnaces use natural gas to heat your home, but they work differently. These design changes affect fuel use, installation, and maintenance. Here’s the difference between an 80% furnace and a 90% high-efficiency model:

• Fuel efficiency: An 80% furnace loses more heat through exhaust. A 90% model extracts more heat from combustion, using less gas over time.
• Heat exchangers: Standard furnaces have one heat exchanger. High-efficiency models include a second exchanger that pulls more heat from the exhaust gases before they’re vented outside.
• Combustion air: 80% models typically pull air from inside the home. 90% furnaces use sealed combustion, drawing air from outside, which helps maintain indoor air quality.
• Exhaust system: 80% furnaces vent hot gases through a metal flue or chimney. 90% furnaces release cooler exhaust through PVC piping that exits through a side wall.
• Condensate drainage: 90% furnaces produce condensation during operation that must be drained properly. 80% systems don’t create moisture and don’t require drainage lines.
• Installation needs: High-efficiency models often need new vent openings, a drain connection, and sometimes a condensate pump. Standard models can often reuse existing chimney flues.
• Upfront cost: 90% furnaces usually cost more due to added components and installation labor.
• Long-term savings: Because they use less gas, 90% furnaces generally have lower heating costs over time, especially in cold climates like Chicagoland.

Why 90% Furnaces Use Two Heat Exchangers Instead of One

A 90% furnace is more efficient because it has a second heat exchanger. After the gas burns in the first exchanger, the exhaust still has heat left. Instead of letting that heat escape, the furnace sends the exhaust through a second exchanger to pull out more warmth. This cools the exhaust so much that water vapor turns into liquid and drains away. This process saves energy that an 80% furnace would lose, which is why 90% models are called condensing furnaces.

Since the exhaust is cooler, it needs to go out through PVC pipe instead of a chimney. The furnace also needs a drain for the water and sometimes a pump to move it out. These systems use sealed combustion, bringing in air from outside to burn fuel instead of using indoor air. This setup makes your home safer, keeps indoor air cleaner, and boosts HVAC efficiency. The design is more complicated than an 80% system, but you get real savings and better comfort.

Installation Requirements for 80% vs. 90% Furnaces

Installing an 80% furnace is usually easier because it can use your existing metal flue or chimney and doesn’t need a drain. A 90% furnace needs PVC pipes through an outside wall and a drain for the water it makes. If gravity drainage won’t work, a pump might be needed. The vent pipes have to be set up with the right slope and space, which can take more time, especially in older homes. Some homes may also need new vent openings or the old chimney sealed. These extra steps make installation more complex, but they are important for safety and better efficiency. Luckily, Mr. Duct can install these systems with ease, giving homeowners peace of mind.

Cost Comparison: 80% Furnace vs. 90% Furnace

Installing an 80% furnace is usually easier because it can use your existing metal flue or chimney and doesn’t need a drain. A 90% furnace needs PVC pipes through an outside wall and a drain for the water it makes. If gravity drainage won’t work, a pump might be needed. The vent pipes have to be set up with the right slope and space, which can take more time, especially in older homes. Some homes may also need new vent openings or the old chimney sealed. These extra steps make installation more complex, but they are important for safety and better efficiency. For help installing either system the right way, schedule furnace installation with Mr. Duct and get expert service from licensed Chicagoland HVAC technicians.

Which Furnace Makes the Most Sense for Your Home?

An 80% furnace may be the better choice if you’re on a tight budget, plan to move soon, or live in a home where venting upgrades would be difficult. It provides reliable heat with lower upfront costs and fewer installation changes.

A 90% furnace is a smarter option if you plan to stay in your home long-term and want to lower your gas use every winter. With better HVAC efficiency, sealed combustion, and more consistent comfort, it’s a strong fit for colder suburbs like Evanston or Naperville.

Common Questions Homeowners Ask About 80% and 90% Furnaces

Can I install a 90% furnace in an older home?

Yes, but it may take extra work. Older homes often need new vent openings or a drain line added to support the system.

Does a 90% furnace always need a pump for drainage?

Not always. If there’s a floor drain nearby, gravity drainage usually works fine, but a pump is needed when the drain is far or above the unit.

Will a 90% furnace heat my home faster than an 80%?

No. Both can deliver the same heat output when sized correctly. The difference is that a 90% furnace uses less fuel to do the job.

Professional Furnace Installation and Service From Mr. Duct

Mr. Duct installs and services all major furnace brands, like Goodman, Lennox, Trane, and others. Whether you’re replacing an old 80% unit or moving up to a high-efficiency system, our licensed HVAC technicians will install your new furnace safely and properly. Our HVAC company is also a certified Goodman heating and cool products dealer, carrying various quality products for homeowners. If you want better HVAC efficiency and comfort, contact Mr. Duct today to schedule service or call us for more information.