SCAQMD Rules 1146 and 1146.1
If your facility runs a commercial or industrial boiler with a heat input capacity above 2 million BTU per hour, the compliance picture looks different from what most building owners are hearing about.
The regulations currently generating the most attention, including the January 1, 2026 deadline you may have seen in industry communications, apply to smaller units under Rule 1146.2.
For units above 2 MMBtu/hr, a separate and more complex set of rules applies: SCAQMD Rules 1146 and 1146.1.
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SCAQMD Rules 1146 and 1146.1: What Applies Today and What Is Still Proposed
Two things are true about those rules at once, and confusing them causes expensive mistakes. The current NOx limits are law, they have been enforceable for years, and most of their compliance dates have already passed. The amendments are not law, they are still in working group, and no rule language has been published. This article separates the two.
For background on the smaller-unit rule that applies to water heaters and boilers at or below 2 MMBtu/hr, see our article on SCAQMD Rule 1146.2 explained.
What Rules 1146 and 1146.1 Cover
Rules 1146 and 1146.1 regulate NOx emissions from industrial, institutional, and commercial boilers, steam generators, and process heaters with a Rated Heat Input Capacity greater than 2 MMBtu/hr.
Rule 1146 covers units at or above 5 MMBtu/hr and sorts them into groups by size and fuel type. Rule 1146.1, covers equipment above 2 and below 5 MMBtu/hr. Together they reach approximately 1,892 permitted units across the South Coast AQMD jurisdiction.
The facilities most directly affected include hospitals, hotels, large multifamily complexes, universities, manufacturing plants, food processing facilities, and any commercial or industrial operation that runs steam systems or large-scale hot water production.
If your facility has boilers that require an AQMD permit to operate, these rules apply to you.
Where the Current Rules Stand
The current NOx limits come from the December 7, 2018 amendment, which lowered concentration limits across most equipment categories. Rule 1146 has been amended once since, on December 4, 2020. Rule 1146.1 has not.
Current NOx Limits Under Rule 1146 (Units ≥ 5 MMBtu/hr)
| Equipment Category | NOx Concentration Limit |
|---|---|
| Group I — Natural Gas (≥ 75 MMBtu/hr) | 5 ppm |
| Group II — Gaseous Fuels (20 to under 75 MMBtu/hr), standard | 5 ppm |
| Group II — Legacy Fire-Tube Boilers | 7 ppm |
| Group II — Other Legacy Units | 9 ppm |
| Group III — Gaseous Fuels (5 to under 20 MMBtu/hr), standard | 9 ppm |
| Group III — Fire-Tube Boilers | 7 ppm |
| Atmospheric Units | 12 ppm |
| Thermal Fluid Heaters | 12 ppm |
| Units Fired on Landfill Gas | 25 ppm |
| Units Fired on Digester Gas | 15 ppm |
| All Units Fired on Gaseous Fuels (general) | 30 ppm |
| All Units Fired on Non-Gaseous Fuels | 40 ppm |
Current NOx Limits Under Rule 1146.1 (Units > 2 and < 5 MMBtu/hr)
| Equipment Category | NOx Concentration Limit |
|---|---|
| Natural Gas Units (excl. fire-tube boilers, atmospheric units, thermal fluid heaters) | 9 ppm |
| Fire-Tube Boilers, Natural Gas | 7 ppm |
| Atmospheric Units | 12 ppm |
| Thermal Fluid Heaters | 12 ppm |
| Units Fired on Landfill Gas | 25 ppm |
| Units Fired on Digester Gas | 15 ppm |
| All Other Units (general) | 30 ppm |
All NOx concentration values are corrected to 3 percent oxygen on a dry basis, averaged over a 15-consecutive-minute period. Alongside the NOx limits, both rules set a carbon monoxide limit of 400 ppm on the same reference basis.
Two points on the Group definitions in Rule 1146 are easy to miss. Groups II and III are written around gaseous fuels generally, excluding digester and landfill gases, rather than natural gas alone. And every unit operated at a school or university falls into Group III regardless of size, as long as it is 5 MMBtu/hr or larger, because Groups I and II exclude them.
The legacy rows apply to units that held a lower NOx limit before the December 7, 2018 amendment. Which row your unit falls into depends on the limit written into its permit at that time, so the permit is the document to check, not the nameplate.
Which Compliance Dates Have Already Passed
Most of the compliance dates attached to these limits are behind us. Table 1146-1 sets them at September 5, 2008 for the 30 and 40 ppm categories, January 1, 2013 for Group I, January 1, 2014 for atmospheric units and part of Group II, January 1, 2015 for Group III and the biogas categories, and December 7, 2018 for the tightened fire-tube and thermal fluid heater limits. Thermal fluid heaters that previously sat above 20 ppm had until January 1, 2022.
December 7, 2033 is not a general deadline. It is a backstop, and it reaches a defined set of legacy equipment:
- Group II fire-tube boilers that held a limit of 9 ppm or less and greater than 5 ppm before December 7, 2018
- Group III fire-tube boilers that held a limit of 9 ppm or less before that date
- Thermal fluid heaters that held a limit of 20 ppm or less
- Group III natural gas units permitted before September 5, 2008 that held a limit of 12 ppm or less and greater than 9 ppm
- Low fuel use units complying through the tune-up option
The first three categories apply at non-RECLAIM facilities, to units installed, modified, or permitted before December 7, 2018. Facilities that were in RECLAIM as of January 5, 2018 follow the schedule in Rule 1100 rather than these dates.
For those units the requirement lands on December 7, 2033, or at the point 50 percent or more of the unit’s burners are replaced, whichever comes first. Rule 1146.1 carries the same structure for its own fire-tube and thermal fluid heater categories.
These are not proposed limits. They are the current enforceable requirements. Units are actively being replaced and retrofitted across the jurisdiction to meet them. Outside the backstop categories above, a unit still operating above the limit written into its permit is out of compliance today, and no 2033 date protects it.
It is worth noting that at 5 to 9 ppm for most natural gas-fired units, Southern California already operates under some of the lowest NOx concentration limits for commercial boilers anywhere in the world.
Where the Amendments Stand
The South Coast AQMD opened a process to amend both rules in late 2024, driven by Control Measure L-CMB-02 in the 2022 Air Quality Management Plan, which calls for replacement or retrofit of commercial boilers and process heaters as part of the roadmap to the 2015 federal ozone standard by 2037.
That process is unfinished, and the distinction matters because a good deal of published commentary treats it as settled. As of August 2026, four working group meetings have been held: November 2024, April 2025, February 2026, and June 2026. No proposed rule language has been published, no draft staff report has been released, and no public hearing has taken place.
A hearing has been targeted for late 2026.
One point from the working group material is worth carrying, and it is directional rather than specific. Staff’s current assessment centers on burner technology rather than electrification, and on encouraging zero-emission equipment through incentives rather than requiring it. Anyone telling you that AQMD is about to mandate electric boilers above 2 MMBtu/hr is working from an earlier draft.
Nothing changes for your facility until the Governing Board votes. We follow the material as it is published and will update this page once rule language exists.
Hydrogen Is Not a NOx Solution
Hydrogen boilers appear regularly in vendor material and trade press as a zero-emission option. For NOx they are not. Hydrogen burns at a substantially higher flame temperature than natural gas, roughly 3,400 degrees Fahrenheit, and thermal NOx formation rises with flame temperature. Burning hydrogen removes carbon from the exhaust, not nitrogen oxides. AQMD staff reviewed it during this rulemaking and is not treating it as a separate NOx control technology.
If a proposal crosses your desk that presents hydrogen conversion as an AQMD compliance strategy, that is the question to ask about it.
What This Means for Your Facility
Waiting on the amended rule is the wrong instinct, and the reasons have nothing to do with what that amendment might eventually say.
The current limits are already enforceable, and this is where most exposure actually sits. A facility that has not checked its units against the limits written into their permits may be out of compliance right now, years past the applicable date. That risk exists today, and no pending rulemaking touches it.
The December 7, 2033 backstop is triggered by burner replacement, not only by the calendar. Replace 50 percent or more of a unit’s burners and the obligation lands at that moment. Any burner work planned in the next several years needs to account for the limit it will pull the unit into, which turns an unplanned repair into a decision worth making deliberately rather than under pressure.
The work that pays off now is an equipment audit: what units you run, their sizes and fuel types, the NOx limit written into each permit, and where each one sits against the compliance dates above. That answers your position under enforceable law today, and it is the foundation for whatever the amended rule requires when it arrives.
Porter Boiler’s combustion and burner service work includes emissions assessments and burner retrofits for large commercial and industrial boilers across Southern California. Our boiler installation team handles new unit procurement and installation for facilities planning ahead of their compliance timelines.
How Rules 1146 and 1146.1 Relate to Rule 1146.2
It is worth being precise about where the boundary between these rules sits, because it comes up frequently in facilities that operate equipment in multiple size ranges.
Rule 1146.2 covers units at or below 2 MMBtu/hr: commercial water heaters, small boilers, and process heaters. Its zero-emission requirements are already in effect, with the first deadline having passed on January 1, 2026 for new buildings. For a full explanation of Rule 1146.2 and its compliance schedule, see our article on SCAQMD Rule 1146.2 explained.
Rules 1146 and 1146.1 cover units above 2 MMBtu/hr. Rule 1146.1 applies to units above 2 and below 5 MMBtu/hr; Rule 1146 applies to units at 5 MMBtu/hr and above. Their current NOx limits are in effect, with most compliance dates already passed and a December 7, 2033 backstop for certain legacy equipment, and the next round of requirements is still being developed through the Working Group process.
If your facility has both small and large units, both rule tracks apply simultaneously, with different compliance schedules and different technical requirements.
Where Porter Boiler Fits In
Porter Boiler Service has been working on large commercial and industrial boiler systems in Southern California for over 65 years. We follow the AQMD rulemaking process closely because it directly affects the work our clients need to do and the timeline they need to plan around.
If your facility runs boilers above 2 MMBtu/hr and you want to understand your current compliance status under the current NOx limits, assess your equipment and replacement timeline, or begin evaluating technology options for the coming requirements, we can help.
Sources: South Coast AQMD Rule 1146 (amended December 4, 2020) and Rule 1146.1 (amended December 7, 2018). South Coast AQMD 2022 Air Quality Management Plan, Control Measure L-CMB-02. South Coast AQMD Proposed Amended Rules 1146 and 1146.1, Working Group Meeting materials, November 2024 through June 2026, available at aqmd.gov.