If your furnace is 15 or 20 years old, this is worth reading before it breaks. Finding out mid-January that the replacement you expected doesn't exist anymore is no fun.
What Does the Law Require?
Every new gas furnace sold in Colorado now has to take one of two paths:
- Ultra-low NOx: it meets a strict limit on nitrogen oxide emissions (14 nanograms per joule).
- ENERGY STAR: it's certified efficient, which since July 31, 2026 means 97% AFUE or better. (It was 95% for the first half of 2026.)
Most of the standard 80% furnaces we used to install meet neither, so they're off the table. (Sellers were allowed to sell off stock they already had, but new stock has to comply.)
Why Can't I Just Replace My 80% Furnace With Another One?
Sometimes you can. But three things get in the way:
- Price. The ultra-low NOx 80% furnaces cost us 50% or more than the standard 80% furnaces did in 2025.
- Size. As of November 2026, nobody makes an ultra-low NOx 80% furnace above 100,000 BTU input. At 80% efficiency, that's about 80,000 BTU of actual heat. If your house needs more than about 80,000 BTU, that's a tight fit. If it needs 95,000 to 100,000 BTU or more, a high-efficiency furnace is your only furnace option.
- Width. To our knowledge, none come in an A cabinet, the narrow 14.5-inch size. If your furnace is in a tight attic, a crawlspace or a closet with a small access hatch, a newer 80% furnace sometimes just won't fit through the opening.
What About a High-Efficiency Furnace?
High-efficiency furnaces (95 to 97% AFUE) are, relatively speaking, as affordable as they've been in a long time. A budget high-efficiency model can even cost less than an ultra-low NOx 80% furnace. They also save money on gas over time.
The catch is venting. Most high-efficiency furnaces need two connections to the outside, one bringing air in and one sending exhaust out, run in 2- or 3-inch PVC pipe. That usually means cutting a couple of holes in the side of your house and finishing them with a termination kit, so it looks tidy rather than like two pipes sticking out of the wall. They also need a drain for the water they produce.
What If I Have a Heat Pump or Want One?
With a dual fuel system, the furnace does much less work. Above a switchover of 15 or 20 °F, the heat pump does the heating and the furnace mostly just runs the blower and the controls. So paying extra for furnace efficiency matters less. If a compliant 80% furnace exists in your size and fits your space, it can make sense. See our dual fuel guide for more.
How Should I Plan Ahead?
The first question is simple: is a compliant furnace made in your size, and will it physically fit where your current one sits? We can answer that on a walkthrough, before anything breaks, so you can budget for the right option instead of deciding in a hurry on a cold night.
Furnace Rules FAQ
Do I Have to Replace My Furnace Because of This Law?
No. It applies to new furnaces being sold. You can keep using and repairing the one you have.
Is a High-Efficiency Furnace Worth It?
Often, yes, especially for bigger homes, or if you don't plan to add a heat pump. The main extra cost is venting.