Measure Overview
A process boiler is a type of boiler with a capacity (rated maximum input) of 300,000 Btu per hour (Btu/h) or more that serves loads other than space conditioning and service water heating related to human occupancy. A boiler stack economizer is a heat exchanger that recovers heat from boiler flue gas and transfers it to boiler feedwater or a combination of boiler feedwater and make-up water.
This proposed code change would require boiler stack (non-condensing) economizers on process boilers with capacities at or above 10 million British Thermal Units per hour (MMBtu/h). The requirement would apply to all new process boilers, including replacement boilers and boilers in additions to existing facilities, with the following exceptions:
- Boilers with stack temperatures below 340°F at their lowest firing rate without an economizer.
- Boiler systems designed to burn biomass1 from facility processes or biomass produced from waste material produced at the facility, such as woody biomass, digester gas, landfill gas, and animal fat.
- Boilers employing stack heat recovery, such as a heat exchanger that serves an industrial heat pump or process drying application.
- Indoor replacement boilers at existing facilities with roof clearances (distance between the boiler outlet connection to the stack and the ceiling) less than 88 inches for boilers with an input capacity at or below 25 MMBtu/h; and less than 116 inches for boilers with an input capacity above 25 MMBtu/h.
The Statewide CASE Team is considering an additional exception for low-use boilers. Boilers used in oilfield production and municipal power generation are often not in buildings and therefore not expected to be within the scope of Title 24, Part 6.
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This measure page will be updated as the 2028 code cycle progresses. For questions or suggestions, email [email protected]. Please include the measure name in the subject line.
Scope of Proposed Code Change
| Building Type(s) | Construction Type(s) | Type of Change | Updates to Compliance Software | Third Party Verification |
| Nonresidential | New construction, additions, alterations | Mandatory | No updates | No changes to third party verification |
Justification for Proposed Change
Most steam boilers lose 20% or more of their input fuel energy in the form of combustion exhaust. A stack economizer, which is is a heat exchanger that recovers heat from boiler flue gas, can recover a significant fraction of this waste heat by pre-heating the boiler feedwater with heat from the hot boiler flue gas. This reduces load and thermal stress on the boiler, lowers fuel consumption, and extends boiler useful life.
The use of boiler stack economizers has been listed in Department of Energy literature as a best practice since at least the early 2000s. Despite its cost-effectiveness, this measure often goes unimplemented because of its first cost and the general lack of owner and operator awareness of the energy benefits. In general, boiler system vendors and contractors are highly familiar with this technology, and it is widely available. Stack economizers can be installed in various configurations, including roof boxes, making them feasible in most facilities.
The use of a stack economizer is typically the highest-impact action that sites can take to reduce their boiler natural gas usage, saving 2-3% of fuel energy by preheating boiler feedwater. Preliminary savings calculations and assumptions can be found in the stack economizer savings calculations document.
Additional benefits of this proposed code change include job creation in the manufacturing and installation of stack economizers, as well as improved local air quality. Many industrial facilities are located near Low- and Moderate-Income (LMI) housing, which is disproportionately exposed to lower air quality. This proposal would reduce photochemical smog in these communities.
Relevant Documents
CASE Reports
Round Two Utility-Sponsored Stakeholder Meeting Materials
Round One Utility-Sponsored Stakeholder Meeting Materials
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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.