TL;DR
Furnace Efficiency
An 80% furnace wastes about 20% of its fuel up a metal flue and is being phased out, but it is still a solid choice in some homes. 96% and 97% furnaces waste only 3 to 4% of their fuel and vent through PVC pipe. The right choice often depends on your existing setup, how long you plan to stay, and your budget.
Furnace efficiency is the biggest choice you'll make on a new furnace, and the MoJo HVAC price calculator lets you compare 80%, 96%, and 97% side by side for your home. The right choice often depends on your existing setup. This guide explains what each option really means, so you can choose with confidence.
What AFUE Means
AFUE (Annual Fuel Utilization Efficiency) is the share of your gas that becomes heat inside your home. An 80% furnace turns 80 cents of every gas dollar into heat and sends the other 20 cents up the flue. A 96% or 97% furnace wastes only 3 to 4 cents. If you're curious about the basics, here's how a furnace works.
The 80% Furnace: The Old-School Standard
The 80% furnace with a metal flue is the standard, old-school setup in a lot of Denver homes. It's still a great furnace, but it's being phased out because it wastes about 20% of its fuel going out the flue. It's also the simplest and least expensive to install, especially when you're replacing another 80% furnace and can reuse the existing venting.
You can keep an 80% furnace and still improve your efficiency and comfort by moving from a single-stage unit to a two-stage or variable-speed one. Our single-stage vs two-stage vs variable-speed guide explains how that works.
The 96% and 97% Furnaces: High Efficiency
96% and 97% furnaces are condensing furnaces. A second heat exchanger pulls extra heat out of the exhaust before it leaves your home, so only 3 to 4% of the fuel is wasted. For the same amount of heat, that's roughly 17% less gas than an 80% furnace. Over a long Denver heating season, the savings on your monthly energy bills add up.
High-efficiency furnaces also come in two-stage and variable-speed versions, for quieter, more even comfort on top of the efficiency. Because the exhaust is cool enough to condense, they vent through PVC pipe and need a drain for the condensation.
Switching From 80% to High Efficiency
Moving from an 80% furnace to a 96% or 97% means new PVC venting, usually out a side wall or through the roof, plus a condensate drain. If your water heater shares the old metal flue with the furnace, it may need its own venting update once the furnace no longer uses that flue. None of this is unusual, and it's all something we check before you get a final price.
How to Choose
Keep an 80% furnace if new venting would be difficult in your home, you may move in a few years, or upfront cost matters most. Consider upgrading to two-stage for better comfort.
Choose 96% for the best balance of cost and savings in most Denver homes.
Choose 97% if you plan to stay for years and want our top tier: the highest efficiency, the lowest monthly bills, and the quietest, most even comfort we offer.
Most homeowners land somewhere in the middle, and that's usually the right call. Without good information, it's easy to get pushed into a premium system you don't need, or to grab the cheapest option and live with the trade-offs for the next fifteen years. Our approach is to educate first and recommend second: explain the options plainly, then recommend what we would choose in your shoes.
See Your Price for Each Option
Select more than one efficiency level in the MoJo HVAC price calculator and you'll see them side by side, with an estimated total and monthly payment for each. Prefer to talk it through? Call or text 720-807-4050 or contact our team. We'll answer every question and recommend what fits your home and budget, never more than you need.
Sources & References
U.S. Department of Energy - Furnaces and Boilers, 2023
ENERGY STAR - Furnaces, 2024
Frequently Asked Questions
How can I tell if I have an 80% or a high-efficiency furnace?
Look at the vent pipes. A metal pipe going up through the ceiling usually means an 80% furnace. White plastic (PVC) pipes coming out the side of the furnace usually mean a 90% or higher furnace. The label inside the furnace door will also list the AFUE.
Can I still install an 80% furnace?
Yes, 80% furnaces are still sold and installed today. They're being phased out as efficiency standards tighten, so it's worth comparing the long-term cost before you choose.
Is a 97% furnace worth it over a 96%?
The efficiency gap between 96% and 97% is small. The choice usually comes down to the comfort features that come with the top tier and how long you plan to stay in the home.
Does a high-efficiency furnace need different maintenance?
Mostly the same, with one addition: the condensate drain needs to stay clear. We check it during every tune-up.
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