Low-NOx limits by air district
The district limit decides which equipment can be permitted at all, so it is the first number in every selection. Here is where the major California districts stand, by rule and size tier.
The tightest mandatory limits
Los Angeles, Orange County, and the urban Inland Empire. All limits are ppm NOx at 3% O₂. Amended through June 2024, and the larger tiers are in active rulemaking again, so confirm the current rule text before permitting.
- Rule 1146 · 5 MMBtu/hr and up
- Group III (5 to under 20 MMBtu/hr): 9 ppm on natural gas, 7 ppm for firetube boilers. Groups I and II (20 MMBtu/hr and up): 5 ppm, except units at schools and universities, which stay in Group III at any size. Atmospheric units and thermal fluid heaters: 12 ppm. Non-gaseous fuels: 40 ppm. Units still on legacy limits comply by December 7, 2033, or at 50% burner replacement, whichever comes first.
- Rule 1146.1 · over 2 to under 5 MMBtu/hr
- On natural gas: 9 ppm generally, and 7 ppm for firetube boilers, which is most of what this size range actually is. Atmospheric units and thermal fluid heaters: 12 ppm. Other fuels: 30 ppm. Carries the same December 2033 legacy provision as Rule 1146.
- Rule 1146.2 · 2 MMBtu/hr and under
- Natural-gas units: 20 ppm (14 ng/J) to sell or install today. The June 2024 amendment phases in zero-emission standards: new construction starting 2026, existing buildings from 2029, with later phases running to 2033 for higher-temperature units, each triggered when a unit reaches its replacement age. The Ninth Circuit upheld the rule in July 2026.
Two regimes, split at 2 MMBtu/hr
The zero-NOx headlines here apply to small gas appliances, not to commercial and industrial boilers above 2 MMBtu/hr. District rulemaking page.
- Regulation 9, Rule 7 · over 2 MMBtu/hr
- 30 ppm for units above 2 and up to 5 MMBtu/hr; 15 ppm above 5; 9 ppm at 20 MMBtu/hr and up, with 15 for load-following units; 5 ppm at 75 and up. Registration applies from 2 MMBtu/hr, and a full permit to operate at 10 and up. No zero-NOx requirement applies to commercial and industrial boilers in this range.
- Regulation 9, Rules 4 and 6 · small gas appliances
- Zero-NOx sale standards adopted in 2023 for appliances up to 2 MMBtu/hr: small water heaters from 2027, furnaces from 2029, larger water heaters from 2031. Amendments pending in late 2026 would move the first date, so treat it as unsettled until the board votes.
The lowest numbers anywhere
The valley's 2.5 ppm target eliminates whole classes of burner. Ultra-low-NOx designs or SCR are the compliant set. District rule list.
- Rules 4306 and 4320 · over 5 MMBtu/hr
- Drive existing units down to 2.5 to 5 ppm, with an annual-fee alternative for units that stay on higher limits.
- Temporary units (TREU)
- A rental replacing a permitted unit that is down can operate without its own permit under an application shield: file the application within seven days of installation, keep the unit on site for under 180 days in any twelve months, and stay within the replaced unit’s heat input and emissions.
The limit leads the selection
In California the emissions limit is established first, and the equipment conversation happens inside the compliant set. At 5 to 9 ppm, standard low-NOx burners are out entirely; sub-9 ppm designs are standard catalog offerings on current burner lines, and sub-5 is achievable on some. A rule tightening under otherwise sound equipment is the most common trigger for a burner retrofit.
For temporary plants, the district is frequently the critical path rather than the equipment: rental boilers generally need their own permit or a pre-permitted unit, and the San Joaquin Valley's 180-day host-permit provision is the exception worth knowing about.
San Diego, Ventura, and Sacramento run their own boiler rules; we quote against the current text per project. Every figure on this page was verified against the district rule texts on August 13, 2026. Rules move. This page is orientation, not compliance advice: we confirm the governing rule as part of every quote.
Which limit applies to your project?
Tell us the district and the unit. We'll come back with the limit that governs and the equipment that can be permitted under it.
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