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Nonresidential High Efficiency Boilers and Service Water Heating

Measure Description 

High Efficiency Boilers 

This measure would update the boiler efficiency and service water heating system requirements to harmonize with the requirements proposed for Addendum BC to ASHRAE Standard 90.1-2019 and section 7.5.3 of ASHRAE Standard 90.1-2016 respectively. The ASHRAE proposal increases the minimum thermal efficiency for gas hot water boiler systems for space heating (1,000,000 – 10,000,000 Btu/h) to a weighted-thermal efficiency of 90 percent (90%Et). Multiple boilers are able to meet the requirement if the space-heating input all of equipment used has an input capacity-weighted average Et of at least 90 percent. 

Additionally, Addendum BC includes the following design requirements for the hot water distribution systems for large boiler systems: 

  • Hot water return entering the boiler(s) be 120oF or less, 
  • 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 design flow of the operating boiler. 

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 on site renewable 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. Exceptions also exist for space heating boilers installed in individual dwelling. Furthermore, the ASHRAE proposal notes that individual gas boilers with input capacity less than 300,000 Btu/h should not be included in calculations of total system input or efficiency. 

Oxygen Concentration Control 

The measure proposes that newly installed commercial and process boilers with an input capacity of at least 5 MMBtu/h (5,000,000 Btu/h) shall maintain 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 measured flue gas oxygen concentration.  

Service Water Heating System Efficiency

This measure would update the gas service hot water heating requirements to harmonize with section 7.5.3 of ASHRAE Standard 90.1-2016. The ASHRAE proposal requires systems with 1,000,000 Btu/h or greater shall have a minimum thermal efficiency (Et) of 90% or have a weighted Et of 90%. Multiple units may meet this requirement if the water-heating input provided by the equipment with Et above and below 90% provides an input capacity-weighted average Et of at least 90%. This measure includes combination (integrated) systems that provide both service water-heating and space heating.

The ASHRAE proposal includes an exception that service hot water heating systems do not need to meet efficiency requirements if 25 percent of the annual service water-heating requirement is provided by site-solar energy or site-recovered energy, or if water heaters are installed in individual dwelling units, or if individual gas water heaters have an input capacity not greater than 100,000 Btu/h. The Statewide CASE Team will explore these exceptions and whether they would apply for California.

Relevant Documents

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

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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.

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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.

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