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Nonresidential High Efficiency Boilers and Service Water Heating

Measure Description 

This proposal is making four recommendations: 

Gas Boiler Systems

The proposed prescriptive requirements for gas boiler systems would raise the minimum thermal efficiency of gas-fired hot water boiler systems for space heating to a weighted-thermal efficiency of 90 percent. This change impacts gas hot water boiler systems of capacities between 1 and 10 million Btu/h installed in newly constructed nonresidential and high-rise residential buildings. Boilers within the same building but on separate loops are not considered to be a part of the same system. Weighted thermal efficiencies are calculated based off the input each boiler provides to the total system capacity. For instance, if there are two individual boilers that are a part of a system, one with a capacity of 1 million Btu/h and efficiency of 95 percent and the other with a capacity of 500,000 Btu/h and efficiency of 90 percent, the weighted thermal efficiency is 93.3 percent. Space heating boilers installed in individual dwelling units would not be included in the calculation of thermal efficiency. Additionally, individual gas boilers with input capacity less than 300,000 Btu/h would not be included in calculations of the total system input or thermal efficiency. Table 5 below shows what system and individual level thresholds trigger this requirement for gas boilers.

This measure would also add two requirements for the hot water distribution system that aim to optimize condensing operation for boilers in the scope of the measure:

  1. Space heating coils and heat exchangers are sized so that under design conditions the return temperature of hot water to hot water boilers is 120°F or less.
  2. Hot water space heating systems is designed so that under all conditions the return water entering the boiler(s) must be 120°F or less, or flow rates for supply hot water that recirculates directly into the return system must be no greater than 20 percent of the design flow of the operating boiler.

The proposal includes a trade-off wherein boiler systems do not need to meet the thermal efficiency and hot water distribution 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.

Gas Service Water Heating Systems

The proposed code change would require gas service hot water heating systems of capacity 1 million Btu/h or greater to have a minimum weighted thermal efficiency of at least 90 percent. The revisions to the prescriptive requirements would impact newly constructed nonresidential buildings and high-rise residential buildings. Water heating systems that serve individual dwelling units would not need to comply. Individual gas water heaters with input capacity less than 100,000 Btu/h are also excepted from efficiency and input-capacity calculations. Multiple gas service hot water heating units must also meet this requirement if their combined input capacity is above 1 million Btu/h. These systems can meet the requirement if the water-heating input provided by the individual equipment provides an input capacity-weighted average thermal efficiency of at least 90 percent.

The proposed code change would include an alternative prescriptive pathway in which 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 or site recovered energy.

The table below shows what system and individual level thresholds trigger this requirement for gas service water heating and gas boilers.

Some service water heating systems can be used to provide space heating in addition to service water heating. These integrated units must comply with the proposed thermal efficiency requirements for service water heating systems.

System and Individual Unit Thresholds of Proposed Requirements
System Type System Threshold (Btu/h) Minimum Individual Unit Threshold (Btu/h)
Gas Boilers ≥ 1 million and ≤ 10 million 300,000
Gas Service Water Heating ≥1 million 100,000

Oxygen Concentration

Heat exchange in boilers is more efficient when oxygen concentrations are within the optimal range. Existing code requirements that address excess stack-gas oxygen concentrations in commercial and process boilers result in boilers operating at higher levels of thermal efficiency. The proposed changes would adjust the stack-gas oxygen concentration requirements for all newly installed process boilers. The proposal would establish an oxygen concentration requirement of 3 percent for process boilers with input capacity of at least 5 million Btu/h or greater. This change also simplifies the code so all process boilers that are subject to oxygen concentration requirements must maintain a concentration of 3 percent or less. Previously, the concentration requirements varied by system capacity and a 5 percent level of oxygen concentration was required for process boilers with input capacities between 5 and 10 Million Btu/h. For process boilers, the Statewide CASE Team recommends a clarification that the combustion air volume be controlled only by flue gas concentration, eliminating the option to control based on firing rate. Currently, commercial boilers with steady state full-load thermal efficiency of 85 percent or higher are not required to meet oxygen concentration requirements. This exception would be updated to 90 percent to better reflect the market for high efficiency products. This oxygen concentration exception at 90 percent efficiency will also be added process boilers.

Relevant Documents

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

CASE Reports

Round Two Utility-Sponsored Stakeholder Meeting Materials

Round One Utility-Sponsored Stakeholder Meeting Materials

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Final CASE Report Now Available

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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