Colorado’s Low-NOx Furnace Law: What Changed on January 1, 2026

By the team at Homegrown Heating and Air Conditioning.

The Colorado low-NOx furnace law is real, it took effect January 1, 2026, and it does not require you to replace a working furnace. That last point is why most people land on a page like this. Below is what the statute says, where the number comes from, and what it does to the price of a replacement in Northern Colorado.

What the Colorado Low-NOx Furnace Law Actually Restricts

The rule comes from House Bill 23-1161, signed June 1, 2023 and codified in the Colorado air quality statutes. Section 9 of the act created a new emissions standard for heating appliances, and that standard turned on at the start of 2026.

The operative language in the text of House Bill 23-1161 as signed into law reads that on and after January 1, 2026, a person shall not manufacture, distribute, sell, offer for sale, lease, or offer for lease in Colorado any of the covered new products unless emissions do not exceed the stated limits.

Read that list of verbs carefully. It covers manufacture, distribution, sale and lease. It does not cover installation or ownership, and says nothing about equipment already in a house.

The limit for furnaces is 14 nanograms of NOx per joule. The statute sets it directly, so there is no separate agency rule to look up. For water heaters the limit is 10 nanograms per joule up to 75,000 BTU per hour and for any power-assisted unit, and 14 nanograms per joule above that.

NOx is defined in the statute as the sum of nitric oxide and nitrogen dioxide, the combustion byproducts that drive ground-level ozone. That is the Front Range air quality problem the legislature was aiming at.

Which Furnaces Are Covered, and Which Are Not

The scope is narrower than most summaries suggest. A fan-type central furnace falls under the standard when it requires single-phase or three-phase electric supply, is used for comfort heating, and has a rated heat input capacity under 175,000 BTU per hour. For combination heating and cooling units, the cooling rate must also be under 65,000 BTU per hour.

That covers essentially every residential gas furnace in Northern Colorado.

The statute then carves out four categories:

• Products already held in inventory in Colorado on the effective date of the standard.
• Products installed in manufactured homes at the time of construction.
• Products designed expressly for recreational vehicles.
• Products that do not burn fossil fuels, which is why heat pumps and electric furnaces are outside this entirely.

The inventory exemption explains a lot of what homeowners saw during 2026. Non-compliant stock sitting in Colorado warehouses on January 1 could still be sold legally, so availability and pricing shifted gradually through the year rather than overnight. That inventory is largely gone now, which is why quotes written this autumn look different from quotes written last winter.

The Part Everyone Gets Wrong: This Is Not the 95 Percent AFUE Rule

Two separate rules, two separate clocks. Confusing them is the most common error in contractor content on this topic.

	Colorado low-NOx rule	Federal AFUE standard
What it limits	NOx emissions, 14 nanograms per joule	Efficiency, expressed as AFUE
Current requirement	In effect since January 1, 2026	80 percent AFUE minimum
Next change	None scheduled	95 percent AFUE for units manufactured on or after December 18, 2028
Applies to	Sale, lease and distribution in Colorado	Manufacture nationally
Source	Colorado statute	10 CFR 430.32(e)

The federal minimum efficiency for a residential gas furnace is 80 percent AFUE today. The 95 percent figure applies to units manufactured on or after December 18, 2028, and that compliance date is itself under an active Department of Energy petition. Anyone telling you that 95 percent is required right now is wrong.

More to the point: an 80 percent AFUE furnace can comply with the Colorado NOx limit. Emissions and efficiency are different measurements. Low-NOx burner designs have been on the market for years because California adopted comparable limits well before Colorado did, and compliant models exist across the efficiency range.

There is one wrinkle worth knowing. The statute includes a safe harbor: equipment certified to the current ENERGY STAR program for its type is deemed to comply. ENERGY STAR gas furnaces for the northern region require 97 percent AFUE, so the simplest compliance route happens to be a high-efficiency unit. That is a shortcut manufacturers may use, not a requirement placed on you.

What It Does to a Replacement Quote in Northern Colorado

Three things, and none of them is a mandate.

The model list is shorter. Distributors carry fewer compliant SKUs than they carried in 2025, and low-NOx burner assemblies cost more to build. That pressure lands in the equipment line of the quote.

It stacks on work this market already required. Every gas furnace installed around 5,000 feet needs its gas input adjusted for altitude by changing burner orifices, sized from the manufacturer’s high altitude listing for that exact model. A new low-NOx burner design means a different orifice chart, so ask whether whoever is quoting you has installed that model here before. Our furnace installation work starts with the model number.

The timing collides with a rebate deadline. Xcel Energy’s rebate for a 95 percent or higher AFUE furnace replacing a non-condensing unit is $225, and it ends January 1, 2027. If you were already leaning toward a condensing furnace, that is a reason to decide this heating season.

If you are repricing anyway, put a heat pump next to the furnace quote. Heat pumps are outside the NOx rule because they burn nothing, and we are a registered Colorado Energy Office contractor, so we claim the Colorado heat pump tax credit and pass the discount through on your invoice. For an air-source system that is a $1,000 credit with at least $333 off the price. You file nothing.

Do You Have to Replace a Working Furnace?

No. Nothing in the statute reaches equipment that is already installed.

There is no inspection requirement, no retirement date, and no penalty attached to owning an older furnace. Keep it running, maintain it, and replace it when it makes sense for you. When it does fail, the furnace you buy will be compliant, because that is what will be legal to sell you.

The decision that matters is repair versus replace, and it turns on heat exchanger condition, the age of the unit, and what a repair costs against remaining life. That is a measurement question, not a regulatory one, and it is where our furnace repair diagnostics start.

A Customer Who Replaced Systems Before He Had To

Some homeowners get ahead of the decision instead of waiting for a failure. One of ours replaced his systems on his own schedule and put them on a maintenance plan afterward, which is the cheapest version of this story.

Gil Moon summed up the standard he holds a contractor to in five words: “It’s the SERVICE that counts”. He went on to write that his family had chosen us “to replace our systems and signed up for their continuing maintenance program.”

That is the pattern we recommend when a furnace is near the end and a rule change is moving prices. Plan it, pick the equipment on its merits, and avoid deciding at 11 p.m. with no heat in the house. Read more customer reviews from Northern Colorado homeowners.

Get a Quote That Separates the Rules From the Sales Pitch

If you have been told you must replace a working furnace, get a second opinion before you spend anything. Contact Homegrown Heating and Air Conditioning for a furnace evaluation anywhere in Northern Colorado, seven days a week, 7 a.m. to 7 p.m. We are licensed and insured throughout Northern Colorado.

Questions Northern Colorado Homeowners Ask

Does the Colorado low-NOx furnace law mean my furnace is now illegal?

No. The statute restricts manufacturing, distributing, selling and leasing new equipment in Colorado. It places no requirement on a furnace already installed in your home, and there is no deadline by which you must remove one.

What is the actual emission limit for a furnace?

Fourteen nanograms of NOx per joule. The number is written into the Colorado statute itself rather than into a separate agency regulation, and NOx is defined there as the sum of nitric oxide and nitrogen dioxide.

Am I required to install a 95 percent AFUE furnace in Fort Collins now?

No. The federal minimum is 80 percent AFUE, and the 95 percent standard applies to units manufactured on or after December 18, 2028. It is a separate federal rule with its own timeline and no connection to the Colorado emissions limit. Local code in Fort Collins does not change that.

Why did my quote go up compared to what my neighbor paid last year?

Partly because the compliant model list is shorter and low-NOx burner designs cost more to build, and partly because non-compliant inventory that was legal to sell in Colorado during 2026 has now cleared. Equipment cost is only one line, so ask for the labor, permit, venting and altitude setup lines separately.

Does the rule apply to boilers, water heaters or wall furnaces?

Water heaters are covered, at 10 nanograms per joule up to 75,000 BTU per hour and 14 above that. The furnace standard is written for fan-type central furnaces under 175,000 BTU per hour input, which is a defined term and does not automatically capture every heating appliance in a house.

Does a furnace replacement still need a permit in Northern Colorado?

Yes, in every jurisdiction we work in, and that has not changed. Replacing a fuel-fired appliance also brings Colorado’s carbon monoxide alarm requirement into play. We pull the permit, handle the inspection and verify alarm placement as part of the installation.