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Single Family Variable Capacity HVAC Compliance Software Revisions

Measure Description

Since the introduction of computer models for demonstrating performance-based compliance under the Title 24, Part 6 Standards, the models have calculated heating and cooling energy use using the entered equipment efficiency and fixed assumptions for duct performance. Because variable capacity systems typically have higher performance ratings, they can be used to improve compliance. However, research by the UC Davis Western Cooling Efficiency Center (WCEC) has shown that when variable capacity systems operate at lower speeds and reduce airflow in ducts, distribution effectiveness falls, leading to a decrease in the overall system coefficient of performance (COP). The proposed measure addresses this compliance model deficiency. This measure would apply to central ducted split system air conditioners, heat pumps, and furnaces in new single family homes and townhouses only. It would not apply to variable capacity heat pumps (commonly known as mini-splits), which are dealt with separately by 2019 Title 24, Part 6 requirements (California Energy Commission 2019) 

For new homes using the performance path, the proposed measure would improve methods for modeling air distribution effectiveness when variable capacity heating and cooling systems are specified. The current (2019) Alternative Calculation Method provides for duct losses to be calculated at a fixed total airflow rate of 350 cfm per ton of cooling capacity and using the estimated temperature difference between the air in the ducts and the attic environment. The proposed software modification would adjust the airflow and distribution effectiveness as a function of the hourly load2. Energy consultants and heating, ventilating, and air conditioning (HVAC) designers for new homes would have three options for compliance when variable capacity systems are specified. These include placing ducts in the attic and allowing compliance software to calculate distribution effectiveness at reduced airflow, locating ducts in conditioned space, or installing a zone control that controls the speed of the system based on the number of zones calling. The latter two options would apply current methods for calculating distribution effectiveness used by the 2019 Title 24, Part 6 Standards.

Relevant Documents

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

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 info@title24stakeholders.com. 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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