Colorado’s Low-NOx Water Heater Law: What Changes When You Replace a Gas Water Heater in 2026

What is Colorado’s low-NOx water heater law?

Colorado’s low-NOx water heater law comes from HB23-1161, the Environmental Standards for Appliances act signed June 1, 2023, which added Part 15 (C.R.S. 25-7-1501 to 25-7-1507) to state law. Its NOx limits, in C.R.S. 25-7-1504, have applied since January 1, 2026 to new gas water heaters sold or leased in Colorado.

Part 15 covers new fossil-fuel water heaters rated at 2 million Btu/h of heat input or less, plus certain fan-type central furnaces. The limits are in nanograms of NOx per joule (ng/J):

New gas water heater	Limit since January 1, 2026
Rated heat input up to 75,000 Btu/h	10 ng/J
Any unit with power assist	10 ng/J
More than 75,000 and less than 2 million Btu/h	14 ng/J
Certified to the most recently published ENERGY STAR version for its type	Deemed to comply

The Colorado Department of Public Health and Environment (CDPHE) implements the standard, and knowing violations carry civil penalties of up to $2,000 each.

Which water heaters does Colorado’s NOx rule cover?

New natural gas water heaters rated at 2 million Btu/h or less are covered, tank and tankless alike. Electric and heat pump water heaters are outside the rule, because Part 15 does not apply to products that do not burn fossil fuels.

Water heater	Status under Colorado’s water heater law
New natural gas tank or tankless	Covered: 10 or 14 ng/J, or ENERGY STAR certified
New propane	Exempt from August 12, 2026 until January 1, 2031
Gas unit held in Colorado inventory on January 1, 2026	Not covered; can still be sold
Electric resistance or heat pump	Outside the rule
Replacement designed and listed only for manufactured homes	Exempt until January 1, 2031, then 40 ng/J
Designed for recreational vehicles	Outside the rule
Your existing heater	No replacement required

Is there a high-altitude exemption for low-NOx water heaters?

No. Part 15 has no altitude exemption, and since August 12, 2026, NOx compliance testing must reflect normal field use in Colorado, including any high-altitude kit and a real or simulated test elevation.

CDPHE’s FAQ, updated July 2026, says that requirement makes the SCAQMD methods and approved product list “insufficient for high-altitude products.” It adds that Colorado does not currently have a list of approved models and that CDPHE will create a process for accepting compliance certifications before January 2027.

The fuel gas code requires gas flow to be adjusted for altitude above 2,000 feet (IFGC 402.2), and Fort Collins sits at about 5,000 feet, Loveland at 4,986 and Estes Park at 7,530, so altitude matters on every gas install here.

What it means when you replace a gas water heater in Larimer County

Gas is not banned. What changed is which new gas models can be sold, so check these points before you sign.

How to read the NOx rating on a quote or spec sheet

1. Get the make and full model number. Our written quotes list both, with the size, first-hour rating and UEF.
2. Look for the NOx figure in ng/J on the spec sheet, and ask for it if you only see a badge. Labels such as “Low NOx” or “Ultra-Low NOx (ULN)” vary by brand, while Colorado’s limits are numbers. Rheem’s September 2025 flyer, for example, says its ULN line meets California, Utah and Colorado requirements.
3. Match it to the right limit using the input rating in Btu/h: 10 ng/J up to 75,000 Btu/h or with power assist, 14 ng/J above that. For a power-vent or fan-assisted model, ask which limit the manufacturer says applies.
4. Or check ENERGY STAR certification to the most recently published version.
5. If it meets neither, ask which exemption applies.

We have not seen a reliable figure for how much, if at all, the rule changes water heater prices, so compare written quotes model by model.

Ask whether the model is set up for your altitude

Colorado now ties NOx testing to a model’s high-altitude setup, and altitude ratings and kits vary by model, so ask:

• Is this exact model listed for installation at my home’s elevation?
• Does it need a high-altitude kit or setting, and is that on the quote?
• Does its Colorado NOx compliance reflect that setup?

Settings change at set elevations, so homes near a manufacturer’s boundary, such as in Wellington, Masonville or Estes Park or on foothill lots, need a site check.

If your home uses propane

The NOx rule does not limit which propane model you choose until January 1, 2031, when propane models come back under the limits as the law now reads. Because the rule applies to new units, it does not require replacing a propane heater installed before then.

Heat pump and electric alternatives

Neither type is covered by the NOx rule, but another part of HB23-1161 requires new electric storage water heaters of 40 to 120 gallons (12 kW or less) sold in Colorado since January 1, 2026 to have an AHRI 1430 demand response port.

Electric tanks typically need a 240-volt circuit, while some heat pump models plug into 120 volts. See our heat pump water heater page and our gas vs electric water heater guide.

Venting and permits

If the new heater vents differently from the old one, say a power-vent or condensing model replacing an atmospheric tank, expect venting work: new plastic exhaust pipe must pass Colorado’s 5 psi pressure test, and Loveland requires an exhaust test certification. Our guide to power vent water heaters and other venting types explains each.

We pull the permit and schedule the inspection on every replacement, and every job is done by licensed plumbers covered by liability insurance. See water heater installation in Fort Collins for the code items, or start with water heater replacement in Fort Collins.