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SCAQMD Proposed Amended Rule 1146.2 – Control of Oxides of Nitrogen (NOx) from Large Water Heaters, Small Boilers, and Process Heaters

June 20, 2024
image of 3 large white water heaters

*The information presented in this article is based on the SCAQMD Proposed Amended Rule 1146.2 language as of June 7, 2024. Please verify current guidelines and regulations independently.

The South Coast Air Quality Management District (SCAQMD) has proposed amendments to Rule 1146.2 that affect industrial and other facilities.  These changes aim to regulate nitrogen oxide (NOx) emissions from large water heaters, small boilers, and process heaters rated less than or equal to 2 million British thermal units (Btu) per hour.  As of June 7, 2024, the proposed rule establishes emission limits and compliance dates for existing equipment in a phased schedule based on equipment type, with a transition plan towards a zero-emission standard beginning in 2026.

Emission Limits and Compliance Dates

Rule 1146.2 categorizes large water heaters, small boilers, and process heaters into Type 1 and Type 2 units based on their heat input capacity.  Type 1 units have a capacity of 400,000 Btu per hour or less, while Type 2 units range from 400,000 to 2,000,000 Btu per hour.

The proposed rule amendments aim to gradually phase out existing units and transition toward a zero-emission standard for future units through a scheduled compliance process.  This process involves three phases of compliance based on equipment type, rating, and unit age.

Simply put, as equipment gets older, it will be expected to operate at zero emissions; once it reaches a certain age limit, based on its category, it will fall out of compliance.  Unit age will be determined by its original manufacturing date, which can be validated through several means such as purchase invoices or nameplate information.  Any unit age that is unable to be verified will be assumed at end of life by January 1, 2025.

The proposed limits for both unit types are listed in Table 1 below.

Table 1: Proposed Emission Limits1
Equipment Category NOx Emission Limit CO Emission Limit
Type 1, Excluding Pool Heaters 20 ppm @ 3% O2 N/A
Type 1 Pool Heaters 55 ppm @ 3% O2 N/A
Type 2 Units 20 ppm @ 3% O2 400 ppm @ 3% O2
1. Per Table 1 of Draft PAR 1146.2 posted June 7, 2024.

The phases by equipment type are listed in Table 2 below.

Table 2: Proposed Compliance Schedule and Unit Age1
Equipment Category Compliance Schedule Compliance Date Unit Age
Type 1 Unit2 Phase I New Buildings: January 1, 2026
Existing Buildings: January 1, 2029
Instantaneous Water Heater ≤200,000 Btu/hr3 New Buildings: January 1, 2026
Existing Buildings: January 1, 2029
Instantaneous Water Heater >200,000 Btu/hr3 Phase II New Buildings: January 1, 2028
Existing Buildings: January 1, 2031
Type 1 Pool Heater New Buildings: January 1, 2028
Existing Buildings: January 1, 2031
Type 2 Unit4 New Buildings: January 1, 2028
Existing Buildings: January 1, 2031
Type 1 High Temperature Unit5 Phase III New Buildings: January 1, 2029
Existing Buildings: January 1, 2033
Type 2 High Temperature Unit5 New Buildings: January 1, 2029
Existing Buildings: January 1, 2033
1. Per Tables 2 and 3 of Draft PAR 1146.2 posted June 7, 2024.
2. Referring to a Type 1 or 2 unit that is not a Pool Heater, High Temperature Unit, or Instantaneous Water Heater.
3. Instantaneous water heater refers to a tankless water heater with rated heat capacity <2,000,000 Btu that heats water only on demand.
4. Referring to a Type 2 Unit that is not a High Temperature Unit or Instantaneous Water Heater.
5. High Temperature Units are defined as units designed to produce steam or water above 180 degrees Fahrenheit.

Retrofit and Burner Replacement

Operators can choose to modify current units with a certified retrofit kit and demonstrate, through an independent source test, that these upgrades meet the standards set out in the proposed rule.

Additionally, if a unit has its burner replaced or modified before the compliance deadline and before reaching maximum unit age allowed for its category, it will be required to meet the NOx and CO limits outlined in Table 1.  The unit’s age is based on date of manufacture and the installation of a new burner does not represent a new unit under the amendments being discussed.

Recordkeeping Requirements

The proposed rule amendments will require equipment owners to maintain certain records, which include:

  • Records of the manufacturer’s and/or distributor’s written instructions, maintenance activity, and fuel usage for a period of no less than 3 years;
  • A copy of a government-issued document that grants permission to initiate a construction project (which can determine the eligibility of a new building or an existing building in compliance with this rule);
  • A copy of the manufacturer’s or distributor’s manual or invoice;
  • Documentation of the rated heat capacity of the unit; and/or
  • If the unit is modified, there needs to be documentation showing the new heat capacity of the modified unit, signed by the licensed person modifying the unit. This documentation  should also describe the changes that were made, dates of modifications, and a calculation of rated heat input capacity.


The proposed rule amendments also allow for several exemptions for specified situations, such as:

  • Low-use exemptions for units installed before the rule’s adoption, meeting Table 1 emission limits:
  • Units rated 1,000,000 to 2,000,000 Btu/hr using under 3,000 therms/year;
  • Units rated 400,000 to 1,000,000 Btu/hr using under 2,000 therms/year;
  • Low-use exemption for Type 2 units manufactured before January 1, 2000, using under 9,000 therms/year; and
  • Units in residential structures and small businesses may also get an exemption, if registration and recordkeeping requirements are met.

Alternative Compliance Options

The proposed rule also includes a few specific alternative compliance options for unique situations such as multiple units, utility delays, instantaneous water heaters, leased property extensions, and construction delays.

Have questions or need support?  Yorke Engineering specializes in air quality and EH&S consulting and has assisted more than 1,900 customers, including a wide variety of industrial facilities and government organizations.  To learn more about how Yorke Engineering can help your company, please visit our website at or click here for the Air Quality services page on our website: Air Quality | Yorke Engineering, LLC (



For more information and detail on the SCAQMD’s Proposed Amended Rule 1146.2, please visit the SCAQMD’s website: Proposed Amended Rule 1146.2 (

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