Colorado 97% AFUE Furnace Rule
Colorado homeowners have had a lot of information to sort through this year about the new furnace rules. If you have been following along, here is one specific update worth knowing about: on July 31, 2026, the ENERGY STAR efficiency threshold for furnaces in Colorado moved from 95% AFUE to 97% AFUE.
Before anything else, two reassurances. Gas heating is still available in Colorado. And if you already have a furnace in your home, this change does not require you to do anything at all.
Quick Recap: What HB23-1161 Requires
Colorado passed HB23-1161 in 2023. As of January 1, 2026, any new gas furnace sold or installed in the state has to take one of two paths: it must be ENERGY STAR rated, or it must meet Ultra-Low NOx standards, which cap nitrogen oxide emissions at 14 nanograms per joule. A furnace needs to satisfy one path, not both.
Existing systems are not affected. There is no deadline for homeowners to replace equipment they already own. For the full background, see what Colorado’s Low NOx law actually means for you and our overview of the 2026 Colorado HVAC and water heater standards.
What Changed on July 31, 2026: 97% AFUE
Colorado sits in the ENERGY STAR North region. ENERGY STAR is an EPA and Department of Energy program that recognizes high efficiency equipment, and it revises its criteria periodically.
From January 1 through July 30, 2026, a gas furnace in our region needed at least 95% AFUE to carry ENERGY STAR certification. Effective July 31, 2026, that bar rose to 97% AFUE.
One detail matters here, and it is good news: ENERGY STAR certification is tied to when a furnace was manufactured, not when it gets installed. A unit built before July 31 that met the 95% standard keeps its ENERGY STAR certification. Furnaces manufactured on or after July 31 need to hit 97% to carry the label.
The Ultra-Low NOx path did not change. That 14 ng/J limit is exactly where it was on January 1, and it remains a valid way to comply.
What This Means If You Are Replacing a Furnace
Practically speaking, the pool of qualifying equipment on the ENERGY STAR side has narrowed somewhat. Fewer models clear 97% than cleared 95%. That said, every common furnace size is represented at the 97% level, and so is every major manufacturer, so there are still options across a range of prices and features.
The change also makes the Ultra-Low NOx route worth a closer look than it may have gotten six months ago. Because the two paths are independent, an Ultra-Low NOx furnace can comply without meeting any AFUE threshold. In older homes, where adding the venting and condensate drainage a high-efficiency condensing furnace needs can mean significant extra work, that is sometimes the more sensible option.
Which path makes sense depends on your house, your ductwork, your budget, and how long you plan to stay. What matters most is working with an installer who knows what currently qualifies and will walk you through both options honestly.
Do I Have to Replace My Furnace Because of This?
No. The July 31 change applies to newly manufactured equipment, the same way the January 1 rule did. Nothing about it obligates you to touch a system that is working.
If your furnace is aging and you would rather plan a replacement than react to a breakdown in February, our guide on how to assess, decide, and compare your options is a good place to start. If your system has already failed and you are figuring out next steps, read I missed the deadline, now what?
Questions? We Are Happy to Help
Welzig has been keeping up with these standards since before they took effect, and we are glad to explain where things stand for your specific situation, whether you are planning ahead or dealing with a furnace that quit on you.
Call us at (720) 336-3777 or schedule online. We have served the greater Longmont and Boulder area since 1994.
This post is general information and is not legal advice. Standards and effective dates can change. For guidance on your specific situation, please reach out to us directly.
