Does Colorado's NOx rule affect a replacement boiler?
Colorado's new NOx standards (HB23-1161, in effect since January 1, 2026) largely preclude conventional atmospheric burners.
Ultra-low-NOx atmospheric units do exist — they're common in water heaters — but in the residential boiler class, ultra-low-NOx has in practice meant sealed premix designs, so the realistic replacement path is a condensing unit.
Boilers under the rule
The rule is disputed for residential boilers.
The statute's "water heater" definition (heats water to 210°F or less at 160 psig or less for use outside the device) can reach a typical hydronic boiler, and the bill's final fiscal note refers to "water heaters, boilers, and certain furnaces."
Some trade sources still say boilers are exempt. That point is unsettled.
We lean on what local suppliers carry, and we recommend erring on the conservative side and budgeting for a low-NOx boiler.
A 2026 law (SB26-002) exempts propane-fueled appliances from these emission standards until Jan. 1, 2031; natural-gas units are still covered. (Signed June 2, 2026, effective Aug. 12, 2026.)
The limits
Since Jan 1, 2026, new gas heating appliances sold in Colorado must meet ultra-low-NOx limits: 10 ng/J for water heaters at or below 75,000 BTU/hr input, and 14 ng/J from 75,000 to 2,000,000 BTU/hr, the residential boiler class if the rule reaches it. Conventional atmospheric models do not meet 14 ng/J.
ENERGY STAR certification counts as compliance under the statute, and ENERGY STAR boilers are condensing units.
Why condensing boilers vent differently
Condensing boilers vent in PVC, CPVC, or polypropylene rather than metal or masonry — those materials suit the much lower exhaust temperatures and don't degrade in the condensate that forms as a consequence of them.
The existing chimney is retired in favor of sealed sidewall or roof venting, with piped combustion air and a condensate drain.
That change, more than the boiler itself, drives the installed cost of an atmospheric-to-condensing swap.
Related
General information, not project-specific advice — that's what the visit is for.
Last reviewed 2026-11-01. Accurate to the best of our knowledge as of that date. Things change — that's why the date is there.