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Nonresidential High Efficiency Boilers and Service Water Heating

Measure Description

This CASE Report will include three submeasures: high efficiency boiler systems, high efficiency service water heating equipment systems and oxygen trim control. This will update the boiler efficiency and service water heating system requirements to harmonize with the requirements proposed for Addendum BC of ASHRAE Standard 90.1-2016 and section 7.5.3 of ASHRAE Standard 90.1-2016 respectively. To harmonize with the ASHRAE proposal, this CASE Report will proposal increases to the minimum thermal efficiency for high capacity boilers (1,000,000 – 10,000,000 Btu/hr) to a weighted-thermal efficiency of 90 percent (90%Et), while section 7.5.3 requires the same 90% Et for gas service water heating systems.

This CASE measure will harmonize with the design requirements for the hot water distribution systems for large boiler systems in Addendum BC as shown below.

  • Hot water return entering the boiler(s) be 120oF or less the temperature difference between the hot water supply leaving the boiler and the return to the boiler be 40oF or more.
  • Establish flow rate requirements for supply hot water that recirculates directly into the return system to ensure flow rates are slow enough that condensing occurs. The tentative requirements are that flow rates be no greater than a) 20 percent of the boiler system hot water flow rate at design conditions AND b) 5 percent above the sum of the minimum flow rates of operating boilers.

The ASHRAE proposal includes an exception that boiler systems do not need to meet the equipment efficiency or hot water distribution system design requirements if 25 percent of the annual space heating requirement is provided by site-solar energy, site-recovered energy, or heat recovery chillers or where half or more of the design heating load is served using perimeter convective heating, radiant ceiling panels, or both. The Statewide CASE Team will explore these exceptions and whether they would apply for California.

Additionally, the oxygen trim control measure proposes that newly installed commercial and process boilers with an input capacity of 5 MMBtu/h (5,000,000Btu/h) and greater shall maintain excess (stack-gas) oxygen concentrations at less than or equal to 3 percent by volume on a dry basis over firing rates of 20 percent to 100 percent. Combustion air volume shall be controlled with respect to firing rate or measured flue gas oxygen concentration. Use of a common gas and combustion air control linkage or jack shaft is prohibited. Commercial boilers with steady state full-load thermal efficiency of 85 percent or higher are exempt from this requirement. The Statewide CASE Team will explore if it is feasible and cost-effective to lower the excess oxygen requirements below 3 percent and/or expanding the requirement to boilers with input capacity lower than 5 MMBtu/h.

Provide Feedback

Participate in the Utility-Sponsored Stakeholder meeting for this topic on October 15th, 2019! View the event here.

This measure description will be updated as research is developed to support the 2022 code cycle. For questions or suggestions, email Include the measure name in the subject line.


Relevant Documents

Measure proposals, supporting documents, and other outside references will be made public as they become available.


Give Us Your Feedback

The Statewide CASE Team values input from all stakeholders engaged in the Title 24, Part 6 code change process. We encourage the open exchange of code change comments and concerns.

  • Use the form above to provide feedback on this measure.