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Multifamily Domestic Hot Water

Measure Description

This CASE Report for multifamily domestic water heating will include four submeasures: central heat pump water heating, hot water distribution, solar thermal water heating, and multifamily drainwater heat recovery.

Central Heat Pump Water Heating

This measure will develop a prescriptive compliance pathway for central domestic water heating systems with heat pump technology serving multiple dwelling units by leveraging existing software algorithm development, lab testing, and design guide development. The Statewide CASE Team will perform additional market readiness research across a broad group of stakeholders and develop code and compliance manual language.

Central heat pump water heating (HPWH) is a key design strategy to decarbonize domestic water heating. Several successful designs have been implemented and are operational in Washington and California in field demonstrations. However, energy savings are highly dependent on design and not guaranteed. Heat pump water heaters require low entering water temperature to operate at high efficiencies. Design variables that must be considered include hot water draw schedules and volume throughout a multifamily building, and stratification strategies including tank sizing and piping configuration. Multiple water heaters can be designed to operate in parallel to increase overall capacity, and each heat pump water heater model has different performance characteristics.

Central HPWH is a relatively new design approach and no design guidelines exist to ensure appropriate design. This measure would integrate best practices into the code language for central HPWH design and operation to ensure energy savings. The Statewide CASE Team will gather the following information from various on-going research efforts:

  • Heat pump water heater sizing and model selection in different climates
  • Tank sizing and piping configurations that lead to beneficial stratification
  • Control methods to maintain supply water temperature, reduce cycling, and optimize defrosting
  • Location within the building and distribution piping, including impact on space heating and cooling loads.

The goal of this measure is to develop a prescriptive compliance pathway for central HPWH to encourage implementation in multifamily buildings. The Statewide CASE Team will collaborate and build upon the PG&E funded lab testing effort that include:

  • Conduct performance testing of central HPWH systems and develop central HPWH model to be included in CBECC-Res and CBECC-Com
  • Develop installation criteria for multifamily HPWH systems

There are several overlapping issues with accurately modeling central HPWH, which the Statewide CASE Team needs to coordinate with other domestic hot water (DHW) CASE topics and on-going research efforts:

  • Recirculation loop modeling accounting for multiple loops or length of loops, solar thermal, or solar PV designs.
  • Updating CBECC-Res to allow for central HPWH modeling in all multifamily buildings

Based on the outcome of the research, the Statewide CASE Team may recommend several alternate compliance pathways depending on the location, number of loops, and/or number of dwelling units being served by the central HPWH system.

Hot Water Distribution

The 2013 Multifamily Central DHW and Solar Water Heating CASE Report [1] estimated that 33 percent of hot water generated at the water heater is lost in the recirculation loop to ambient space. As a result of the 2013 CASE Report, currently Title 24 prescriptively requires demand control recirculation, as well as a minimum of two recirculation loops in multifamily buildings that have nine or more dwelling units.

The current requirement for 2 loop recirculation systems has faced adoption challenges including inconsistent interpretations of meaning of the requirement and challenges establishing appropriate baseline and proposed systems in the California Compliance Simulation Engine (CSE).  In addition, the 2 loop requirement was developed based on research on low-rise multifamily that is not directly applicable across all multifamily buildings types (low-rise, mid-rise, and high-rise).

The 2022 multifamily hot water distribution CASE measure will replace the current 2 loop requirement while reducing the performance budget for multifamily DHW recirculation systems with the following prescriptive requirements.

  1. Increased Insulation: This measure involves increasing pipe insulation requirements in Table 120.3-A for the temperature range of 105F-140F. The CASE team has identified three options to increase pipe insulation requirements: (a) increase insulation on all pipes including recirculation loop and runout pipes, (b) increase insulation only on recirculation loop pipes, (c) or increase insulation on specific portions of recirculation loop pipes such as large supply headers. Option (a), (b), or (c) will be chosen based on cost effectiveness, feasibility, and compliance enforcement considerations.
  2. Pipe Insulation Verification: This measure involves adding a requirement for field verification of pipe insulation quality. Field verification would confirm installation of code minimum pipe insulation meeting California Plumbing Code (CPC) and Title 24 Part 6, including insulation on all fittings and valves, and thermal isolation at pipe hangers. This measure builds on the current low-rise pipe insulation credit (PIC-H) and extends it to become a prescriptive baseline for all multifamily buildings.

The 2022 multifamily hot water distribution CASE measure will also add the following alternative compliance options or alternate prescriptive paths:

  1. CPC Appendix M pipe sizing: This measure will add at least one alterative prescriptive compliance path that includes pipe sizing based on CPC Appendix M (CPC 2019, UPC 2018). CPC appendix M is a performance-based pipe sizing calculation procedure that typically results in smaller pipe sizes than standard practice sizing that reduce first costs and distribution system heat loss. An alternative prescriptive compliance path would include a requirement for Appendix M sizing instead of other DHW requirements such as solar hot water, drain water heat recovery, or increased pipe insulation, and would result in similar or lower TDV energy use compared to the primary prescriptive baseline.  The primary prescriptive baseline model assumes standard practice pipe sizing based on CPC (fixture units, hunters curve, etc.).
  2. Clustered Design without Recirculation: This measure will support use of clustered design without recirculation as a compliance option in the performance approach. The CASE team has identified through interviews and CASE research that distributed water heaters serving multiple dwelling units (clusters) without recirculation loops is growing in popularity. CSE currently has the capability of modeling clustered systems without recirculation, but there is no mention of the option in Title 24 Part 6 and limited guidance in the Alternative Calculation Method (ACM) Reference Manual. This measure will add code language and clarify and align code and ACM requirements for clustered design without recirculation.

The measure includes modifying the prescriptive baseline, adding alternative prescriptive pathways, refining field inspection procedures, and updating the Alternative Calculation Method (ACM) Reference Manual and Reference Appendices.

Because there is limited data readily available on these sub measures, the CASE Team will first assess the potential magnitude of savings of each of these measures through spreadsheet calculations or other modeling, in coordination with subject matter experts.

Solar Thermal Water Heating

This submeasure will investigate ways to expand solar thermal water heating requirements for new high-rise and low-rise multifamily buildings for individual/central gas water heaters and individual/central heat pump water heaters. The Statewide CASE Team will evaluate energy savings and cost-effectiveness of the following solar water heating measures.

  • Increase the prescriptive solar savings fraction (SSF) requirement of solar thermal water heating for central gas water heating systems.
  • Add requirement of solar thermal water heating for individual gas water heaters, if feasible and cost-effective.
  • Develop prescriptive solar water heating (SWH) requirement for central heat pump water heaters. Evaluate option for installing an energy-equivalent PV system if solar water heating is not cost effective for this system type.
  • Evaluate an option to combine solar thermal system with individual heat pump water heater, if feasible and cost effective.
  • Revise performance approach to include an hourly model of solar thermal energy production, for improved accuracy.

Multifamily Drainwater Heat Recovery

This measure would prescriptively require drain water heat recovery (DWHR) in all multifamily buildings for all hot water system types. This CASE topic builds on a DWHR measure for low-rise residential in 2019-Title 24 as a compliance credit and a prescriptive alternative path for gas storage water heaters, heat pump water heaters, and solar water-heating systems. [1]

DWHR is an energy-saving technology used to reduce the amount of energy needed by a water heater or fixture to heat incoming water to the required temperature. The technology utilizes a heat exchanger in the drain line to pre-heat cold water supplied to the cold-water side of a water heater and/or fixture. The device can be installed in either an equal flow configuration (with preheated water being routed to both the water heater and the shower) or an unequal flow configuration (with preheated water directed to either the water heater or shower). DWHR comes in horizontal and vertical configurations, and both are included in this measure.

The 2019 CASE Report introducing DWHR as a compliance credit and prescriptive alternative path found that DWHR was cost-effective for low-rise multifamily buildings in all climate zones, assuming four residential units share one DWHR unit (and not accounting for access panels or water meters), and would likely be cost-effective for high-rise multifamily buildings with a similar configuration. The unequal flow configuration is more common in multifamily buildings with central water heaters to reduce pipe lengths.

Key costs that were not considered in the 2019 CASE measure include:

  • Access panels that comply with California Plumbing Code (CPC) Appendix L and facilitate alterations.
  • Additional water meters necessary per Senate Bill (SB) 7 [2] which subsequently led to 2019 CPC 601.2.1.

Prescriptive requirements for DWHR may be dependent on hot water distribution type (central or by unit), heat recovery installation (equal or unequal flow), and DWHR location (in drain line serving multiple dwelling units or in drain line serving one dwelling unit). The measure may allow for exceptions for dwelling units that are slab-on-grade and may also allow for tradeoffs with other hot water compliance credits, such as compact hot water distribution. ​

Relevant Documents

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

Submeasure Summaries

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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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