Colorado HB23-1161 & Gas Furnaces — What You Should Know

The short version

• HB23-1161 targets NOx emissions from new residential gas furnaces sold in Colorado and pushes the supply chain toward ultra-low NOx compliant equipment.
• For many homes, the practical result is that new furnace replacements are quoted as high-efficiency (often 96%+ AFUE) systems that meet the compliance pathways installers use every day — not legacy minimum-efficiency designs.
• Existing furnaces are generally grandfathered in the sense that you are not forced to swap out a working unit solely because of this law.
• Repairs on equipment already in your home remain a normal service — the policy focus is on what gets sold as new.

Why Colorado passed furnace NOx rules

NOx contributes to Front Range ozone issues during warmer months. Residential combustion equipment is one slice of the overall picture. HB23-1161 is part of Colorado's broader effort to tighten emissions from new furnace sales so replacements pollute less than older designs over time.

Denver's geography makes ozone a particular concern. The city sits in a bowl surrounded by mountains on three sides, and temperature inversions during summer trap ground-level pollutants. Reducing NOx from new residential equipment is one part of a broader statewide air quality strategy.

What the NOx standard actually means

The standard set by HB23-1161 limits new gas furnaces to 14 nanograms of NOx per joule of heat output (14 ng/J). For context, older standard-efficiency furnaces could emit 40–80 ng/J or more. Ultra-low NOx designs use staged combustion and specific burner technology to achieve sub-14 ng/J ratings.

In practice, this means the furnace models available in Colorado after January 1, 2026 are predominantly high-efficiency condensing furnaces (96%+ AFUE) from the major brands. 80% AFUE non-condensing furnaces are effectively no longer a new-install option in Colorado, as most don't meet the NOx threshold.

What changes for Denver homeowners replacing a furnace

If your furnace needs replacement after January 1, 2026, your installer will propose a compliant high-efficiency model. Here is what that means practically:

• New venting requirements: High-efficiency condensing furnaces exhaust through PVC pipe (not metal flue), and produce condensate that must drain. Older Denver homes with metal B-vent or single-wall flue systems require new venting routes — this is typically routed through a side wall or the existing flue chase. Budget $200–$600 for venting in most older homes.
• Higher AFUE means lower gas bills: A 96% AFUE furnace converts 96 cents of every gas dollar into heat, versus 80 cents for an older 80% AFUE unit. For a typical Denver home, this can mean $150–$300 per year in gas savings depending on usage.
• Federal tax credits may apply: The Inflation Reduction Act provides up to $600 in tax credits for qualifying high-efficiency furnace installations. Ask your installer for documentation to support the credit.
• Equipment pricing: Compliant models carry a modest premium over the old minimum-efficiency options, but the gap has narrowed as manufacturers scaled production. A full furnace replacement in Denver typically runs $4,500–$9,000 installed, depending on home size, venting complexity, and brand.

What Denver homeowners should verify on a quote

1. Model numbers (indoor furnace + matching controls if communicating).
2. AFUE and whether the proposed system meets the compliance pathway your contractor is using.
3. Manual J heat load documentation — sizing matters at altitude.
4. Venting / condensate / combustion air plan for high-efficiency furnaces.
5. Permit & inspection scope (Denver metro jurisdictions vary).
6. Warranty registration responsibilities and what is parts vs labor.

Ready to compare options? Start with our furnace installation overview or installation cost guide.

Related reading

• Furnace repair vs replace (Denver guide)
• Best furnace brands for Denver homes
• Top 10 furnace brands (2026 blog)