Rules and quoting
Ultra low NOx water heaters: the rules, the deadlines, and how to quote them
Ultra low NOx gas water heaters are required in parts of California, in Utah and in Colorado, and two California air districts are moving to zero-NOx. Here is what each rule says, which dates are final, and what changes on the quote.
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If you replace gas water heaters in Southern California, the Bay Area, Utah or Colorado, the heater you install has to meet a NOx (nitrogen oxides) limit set by the local air rules. Most of the time that means an ultra low NOx model. And in two California air districts, the rules are moving past ultra low NOx to zero-NOx, which in practice means electric or heat pump units for some jobs.
This guide sets out what "ultra low NOx" means, where it's required, which zero-NOx dates are final and which are still being debated, and what it changes on your quotes. Every rule below links to the agency's own page.
Air district rules change. The Bay Area board votes on November 4, 2026 on moving its zero-NOx dates, and a court challenge to the South Coast rule for larger heaters is still pending. We checked everything on October 17, 2026; check the district's page before you quote.
What "ultra low NOx" means
NOx limits for gas water heaters are written in nanograms of NOx per joule of heat output (ng/J). The common tiers:
| Tier | Limit | Typically applies to |
|---|---|---|
| Ultra low NOx, small heaters | 10 ng/J | Gas water heaters with an input of 75,000 Btu/h or less (most residential tanks) |
| Ultra low NOx, larger heaters | 14 ng/J | Over 75,000 Btu/h up to 2,000,000 Btu/h (high-input tanks, gas tankless, small boilers) |
| Low NOx | 40 ng/J | The older limit, still used in Texas for water heaters |
| Zero-NOx | 0 | No combustion: electric resistance or heat pump water heaters |
Manufacturers say so on the spec sheet. A. O. Smith's ProLine ultra low NOx sheet says its burner meets South Coast Rule 1121 at "less than 10 ng/J," Bradford White's sheet says its models meet the "10 ng/J NOx limit," and Rheem's product pages say "SCAQMD Rule 1121 compliant." Under Rule 1121, the model number, date of manufacture and certification status must be on both the carton and the nameplate, so check the label before you install.
Where ultra low NOx is required
| Where | Rule | What it requires |
|---|---|---|
| South Coast AQMD (Orange County and the urban parts of Los Angeles, Riverside and San Bernardino counties) | Rule 1121 | Gas water heaters under 75,000 Btu/h: 10 ng/J (or 15 ppmv at 3% O2), phased in from 2006 to 2008 by tank size and vent type. Applies to manufacturers, distributors, retailers and installers. |
| South Coast AQMD | Rule 1146.2 | Larger units up to 2,000,000 Btu/h, including gas tankless: 14 ng/J (or 20 ppmv) until the zero-NOx dates below |
| Bay Area AQMD | Regulation 9, Rule 6 | 10 ng/J for 75,000 Btu/h or less; 14 ng/J from 75,001 to 2,000,000 Btu/h since 2013; zero-NOx dates below |
| Utah | R307-230 | 10 ng/J for 75,000 Btu/h or less, 14 ng/J up to 2,000,000 Btu/h, for heaters sold or installed on or after July 1, 2018 |
| Colorado | HB23-1161 | Sales rules for new water heaters from January 1, 2026. Rheem lists Colorado among the states its ultra low NOx line is built for; we couldn't open the signed act to confirm the limits. |
| Texas | 30 TAC 117.3205 | Statewide, but at 40 ng/J for water heaters: low NOx, not ultra low |
Other California districts have their own rules. Rheem's regulatory flyer lists San Joaquin Valley (Rule 4902), Sacramento, San Diego, Imperial, Yolo-Solano and Mojave Desert, with dates from 2010 to 2026. We took those from the manufacturer, not from each district, so confirm with the district before you rely on them.
The zero-NOx deadlines
South Coast: small tanks stay gas, tankless doesn't
In 2025, South Coast AQMD proposed amendments to Rule 1121 (and Rule 1111 for furnaces) that would have phased in zero-emission sales targets. On June 6, 2025, the board voted 7 to 5 not to approve them. Rule 1121, last amended in 2004, is still the rule: a 10 ng/J gas tank under 75,000 Btu/h remains a legal replacement.
Rule 1146.2 is different. Amended on June 7, 2024, it sets a zero-emission limit for larger units in three phases:
| Phase | Covers (examples) | New buildings | Existing buildings |
|---|---|---|---|
| I | Gas tankless (instantaneous) water heaters up to 200,000 Btu/h; other units above Rule 1121's range up to 400,000 Btu/h | January 1, 2026 | January 1, 2029 |
| II | Larger tankless units, pool heaters, units over 400,000 Btu/h | January 1, 2028 | January 1, 2031 |
| III | High-temperature units | January 1, 2029 | January 1, 2033 |
What it means for a residential plumber in the South Coast area:
- New construction: since January 1, 2026, a typical gas tankless water heater can't go into a new building.
- Replacements in existing homes: a gas tankless that meets the 14 ng/J limit can still be installed until January 1, 2029. During that window, these units carry the label "If Installed in South Coast AQMD: For Installation and Use in Existing Buildings Only."
- No forced change-outs in homes: the rule's age-based limits don't apply to residential structures (up to four families). It bites when a unit is sold or installed.
- Emergency replacements: after the deadline, if a heater fails and the zero-emission option needs an electrical upgrade, a seller or installer may rent the homeowner a compliant gas unit for up to six months, reported on the district's Compliance Portal within 72 hours.
The rule was challenged in court. On July 2, 2026 the Ninth Circuit held that federal appliance law doesn't preempt it, and the plaintiffs asked the full court to rehear the case on August 13, 2026. The rule also requires a technology review by June 2027.
Bay Area: 2027 now, maybe 2028
The Bay Area AQMD adopted zero-NOx standards on March 15, 2023. As adopted, water heaters of 75,000 Btu/h or less manufactured after January 1, 2027 must be zero-NOx, and larger ones (up to 2,000,000 Btu/h) after January 1, 2031. The district says the rules apply only to new appliances, when the current one burns out, and don't require anyone to replace a working heater.
Those dates may move. On October 2, 2026 the district published proposed amendments for a board hearing on November 4, 2026: small water heaters would become subject to zero-NOx on January 1, 2028 instead of 2027, larger ones on January 1, 2033 instead of 2031, and new exemptions would cover cases like low-income owners and installs limited by space or electrical capacity. Tanks under 30 gallons would stay at 10 ng/J, because no zero-NOx models that size exist yet. Until the board votes, the 2027 date is the law.
Statewide: not yet
The California Air Resources Board is working on "Clean Space and Water Heater Standards," but as of its FAQ updated July 13, 2026, it says the regulation "has not been finalized" and hasn't gone to its board. The December 2025 concept would limit emissive water heaters to 60% of sales in 2030, and CARB says it wouldn't require people to replace working equipment.
What changes on the job
The good news: the ultra low NOx tank heaters we looked at install like the standard atmospheric models they replace. The manufacturers' spec sheets and manuals we read list no special venting or gas supply for the ultra low NOx versions:
- Venting: Bradford White's 3x4 draft diverter takes a 3-inch or 4-inch vent up to 40,000 Btu/h, and 4-inch above that. A. O. Smith's ProLine ultra low NOx model is atmospheric with a 4-inch flue.
- Gas supply: A. O. Smith's installation manual caps the gas supply at 14 inches of water column.
- Service: the same manual recommends 24 inches of clearance in front for service, and warns that in rooms with lint, dirt or oil "it may be necessary to periodically clean the flame-trap." That's a reason to add a check to your maintenance visits.
Everything else is the normal water heater checklist: relief valve discharge, pan, expansion control, straps, garage height, valves and the permit. Our guide to water heater code requirements goes through each item.
How to quote it
- Price the right model. Ultra low NOx models are their own SKUs. If your price book still has a standard-NOx tank for jobs in a ULN area, update it. We didn't find a published price difference, so use your supplier's current price, not a rule of thumb.
- Ask about the building. In the South Coast area, "new building" versus "existing building" decides whether a gas tankless is even an option. Write the answer on the quote.
- Offer the zero-NOx option early. Where the deadline has passed or is coming, quote a heat pump water heater (see our heat pump water heater installation guide) alongside the gas option, with the electrical work as its own line (circuit, panel capacity, condensate). The customer sees the real choice, and you aren't rewriting the quote on site.
- Keep the label. Take a photo of the nameplate showing the certification. It answers the inspector's question and the customer's.
If you sell gas tankless, our tankless water heater installation guide covers the gas line, venting and condensate lines to put on that quote, and our water heater replacement quote checklist covers the permit.
Plumbo builds the water heater quote from your own price book, so the ultra low NOx model, the heat pump option and the electrical work come out at your prices, not from memory. Plumbo is on the App Store and Google Play. Try the 60-second demo →
Questions plumbers ask
Are gas water heaters banned in California?
No, not statewide. In South Coast AQMD, a 10 ng/J gas tank under 75,000 Btu/h is still allowed after the 2025 vote, while gas tankless is out for new buildings since 2026 and for existing ones from 2029. In the Bay Area, small water heaters made after January 1, 2027 must be zero-NOx under the current rule, which the board may move to 2028.
Is an ultra low NOx water heater required everywhere in California?
No. It depends on the air district. South Coast and the Bay Area require it, and Rheem's flyer lists several other districts. Ask the district or the building department where the job is.
What is the difference between low NOx and ultra low NOx?
The limit. Low NOx is usually 40 ng/J (Texas still uses it for water heaters). Ultra low NOx is 10 ng/J for heaters of 75,000 Btu/h or less and 14 ng/J for larger ones.
Do ultra low NOx water heaters need special venting?
Not the atmospheric tank models we checked from A. O. Smith and Bradford White: they vent like standard tanks. Always follow the installation manual of the exact model.
This guide summarizes air district rules for planning and quoting. It isn't legal advice. Rules and dates can change; check the district's current rule before you install.
Sources
- South Coast AQMD, Rule 1121 (amended September 3, 2004), checked October 17, 2026.
- South Coast AQMD, Proposed Amended Rules 1111 and 1121 (June 6, 2025 vote), checked October 17, 2026.
- South Coast AQMD, Rule 1146.2 (amended June 7, 2024: Tables 1 to 4, residential provisions, emergency replacements, labeling), checked October 17, 2026.
- South Coast AQMD, News release on the June 7, 2024 amendments (technology review by June 2027), checked October 17, 2026.
- U.S. Court of Appeals for the Ninth Circuit, Rinnai America v. South Coast AQMD, No. 25-5129 (July 2, 2026), and the petition for rehearing en banc (August 13, 2026), checked October 17, 2026.
- Bay Area AQMD, March 15, 2023 adoption and Building Appliances, checked October 17, 2026.
- Bay Area AQMD, Notice of public hearing and staff report on proposed Regulation 9, Rule 6 amendments (October 2, 2026), checked October 17, 2026.
- California Air Resources Board, Clean Space and Water Heater Standards FAQ (updated July 13, 2026), checked October 17, 2026.
- Utah Administrative Code, R307-230-5 and R307-230-2, checked October 17, 2026.
- Colorado General Assembly, HB23-1161, checked October 17, 2026.
- Texas Commission on Environmental Quality, NOx rules for water heaters, and 30 TAC 117.3205, checked October 17, 2026.
- Rheem, Ultra Low NOx regulatory flyer and Professional Classic Ultra Low NOx product page, checked October 17, 2026.
- A. O. Smith, ProLine Ultra-Low NOx spec sheet and installation manual, checked October 17, 2026.
- Bradford White, Spec sheet 1113, checked October 17, 2026.