What is the Measure?
Domestic hot water (DHW) systems often account for a significant share of a building’s total energy use. While equipment efficiency, such as that of water heaters, is frequently prioritized, the design of the plumbing distribution system is often overlooked. High-performance hot water distribution systems incorporate efficient layouts, smart controls, and optimized piping to reduce waste, enhance comfort and increase the performance of heat pump water heaters (HPWHs).
Why Sizing Matters: From Legacy to Appendix M
Traditional sizing methods, like Hunter’s Curve or Uniform Plumbing Code (UPC) tables, tend to oversize hot water piping. Oversized systems have longer pipe runs, increased heat loss, and slower hot water delivery. In contrast, Appendix M of the 2022 California Plumbing Code offers a modern sizing method based on actual flow needs, allowing for smaller diameters and shorter runs. This method, approved for statewide voluntary use in 2024, better reflects real-world demand but must still be used carefully to ensure adequate performance.
Benefits of Appendix M-based sizing:
- Reduces energy loss and hot water wait times
- Enables the use of HPWHs by lowering peak demand
- Promotes material and labor savings in new construction
Design Considerations
High-performance systems prioritize compact design. Shorter pipe runs, fewer branches, and centralized water heating reduce heat loss and increase responsiveness.
Key strategies:
- Group hot water fixtures near each other.
- Minimize total developed length of piping.
- Insulate all hot water pipes, including recirculation loops.
- Consider pipe material and thermal properties when selecting.
Key Control Components
Right-sizing alone isn’t enough. High-performance distribution systems also require modern control strategies to ensure hot water is delivered safely, efficiently, and consistently. Three essential components of this approach are:
- Master Mixing Valves (MMVs): These blend hot water from the heater with cold water to deliver a safe, consistent outlet temperature—typically around 120°F. MMVs are especially critical in systems storing water at elevated temperatures (e.g., 140°F) for Legionella prevention to protect from scalding water and reduce temperature fluctuations at fixtures.
- Thermostatic Balancing Valves (TBVs): Installed in recirculation loops, TBVs automatically regulate flow based on return temperature. This ensures balanced heat distribution across all branches, minimizes wait times, and eliminates the need for manual balancing, which is static and often inaccurate.
- Variable Volume Circulating Pumps (VVCPs): When paired with demand controls or return temperature sensors, variable speed pumps can adjust flow based on real-time system conditions. This reduces pump energy use, limits unnecessary recirculation during low-demand periods, and helps maintain optimal temperatures with less heat loss— improving both efficiency and user comfort. These are particularly useful in multifamily and commercial buildings with varying demand throughout the day.
When properly configured with demand controls, temperature sensors, and safe operating parameters, VVCPs can be used effectively—even in systems with Legionella risk considerations. Together, MMVs, TBVs, and VVCPs support energy savings, water conservation, and occupant comfort while ensuring system safety.
Return Loop Sizing for High Performance
In DHW systems, especially those utilizing recirculation, the length of return piping loops plays a major role in determining system efficiency, hot water delivery times, material costs, and pump sizing. To maintain a high-performance distribution system, industry’s best practices recommend:
- For systems with one return loop, the hot water return piping must not exceed 160 feet of total developed length.
- For systems with multiple recirculation return loops, no single return loop should exceed 160 feet of total developed length.
The total developed length refers to the actual pipe run, measured along its centerline, and includes both the straight pipe segments and the added equivalent lengths of all fittings, such as elbows, tees, and valves. This measurement is crucial for accurately determining pressure drop, recirculation pump sizing, and heat loss potential across the system.
How to Calculate Total Developed Length
To properly estimate total developed length during design:
- Measure all straight sections of the return loop piping.
- Add equivalent lengths for fittings, valves, tees, and elbows using standard values that account for pressure loss (e.g., a 90° elbow might add 5-6 feet, depending on diameter).
- Sum the total: Total Developed Length = Straight Pipe + Equivalent Lengths of All Fittings.
Limiting total return loop lengths to 160 feet is a best-practice threshold that promotes performance, efficiency, and long-term occupant satisfaction. To achieve this, engineers must take an active role in early design collaboration, influencing fixture layout, equipment room placement, and vertical riser coordination. Through proactive engagement with architects, structural engineers, and project stakeholders, engineers can create cost-effective, code-compliant, and high-performance DHW systems that meet both technical and architectural goals.
When to Consider this Measure
Designing a high-performance distribution system is most effective when incorporated into new construction buildings. This way, sizing, layout, and piping decisions can be made early in the design process. While it is possible to implement this measure in retrofits, doing so may require additional planning, coordination, and cost. This measure is especially beneficial in buildings with high hot water demand, such as high-rise multifamily housing, hotels, and dormitories.
Pairing Considerations
One of the most effective pairings is with HPWHs. HPWHs offer high efficiency water heating by using ambient air to heat water, and when combined with a high-performance distribution system, their effectiveness is maximized. This is because the system helps retain the heat during distribution, reducing energy loss and improving delivery.
Additionally, low-flow fixtures are another ideal complement. These fixtures reduce both water and energy use. When paired with a well-designed distribution system, featuring insulation, reduced pipe sizes, and shorter runs, they can maintain acceptable wait times and prevent cooling in the pipe. This improves both efficiency and occupant comfort.
DWH Distribution System Configurations
There are five different typical structured plumbing layouts to implement in design for DHW distribution systems that include hot water recirculation based on the Northwest Energy Efficiency Alliance (NEEA) in their Advanced Water Heating Specification v8.0 or later. The systems being:
1. Single-pass primary HPWH system HW circulation returned to primary storage
In this design the HPWH, or multiple units in parallel, draws cold water from the bottom of the storage tank and returns heated water to the top. The hot water is then pulled through a mixing valve to be distributed to tenants at the desired temperatures. This design also incorporates a hot water recirculation loop, which returns warm water to the storage tank. By introducing the recirculation loop above the cold-water inlet, the system maintains tank stratification and the high delta T that single-pass units need for optimal performance. This setup ensures hot water is always readily available at the point of use, reducing wait times for tenants.
2. Single-pass primary HPWH system with series temperature maintenance tank (Swing tank)
A temperature maintenance tank, or swing tank, can be added as well. The second tank separates the hot water return and supply from the cold-water supply. Typically, these tanks use an electric resistance heating element to maintain the desired temperature.
The primary benefit of this design is that the swing tank handles small maintenance loads, such as losses from the recirculation loop, preventing the need to activate the HPWH for minor demands. This reduces wear and tear and avoids running the unit at a low delta T, which can lower efficiency and increase unnecessary cycling.
3. Single-pass primary HPWH system with parallel temperature maintenance tank & multi-pass HPWH
The final single-pass primary system closely resembles the previous design but now incorporates a multi-pass HPWH in parallel with the maintenance tank.
The advantage of the multi-pass system is that it replaces the electric resistance heater in the swing tank with a HPWH. In the previous design, the swing tank heater handled the recirculation load, which is relatively small and involves a low delta T. The multi-pass system thrives in low delta T conditions, making it ideal for this application. However, adding another HPWH will be more expensive than using an electric resistance heater.
4. Multi-pass integrated HPWH system with HW circulation returned to primary storage
This design is suited for simple installations and features two integrated, or otherwise called packaged, multi-pass units with built-in storage tanks. These integrated units are ideal for space-constrained projects, offering a compact solution that combines heating and storage in one system. A recirculation loop is added, offering the benefits of on-demand hot water and improved system efficiency.
5. Multi-pass split-system HPWH system with HW circulation returned to primary storage
Multi-pass systems aren’t always in the form of integrated or packaged units. This design illustrates how to configure a system with a separate hot water storage tank and heat pump, providing flexibility in layout.
For the most accurate setup tailored to your specific product, consult the manufacturer’s documentation provided with your equipment. It’s also recommended to work with a qualified professional to determine the optimal configuration for your equipment and project requirements.
Northwest Energy Efficiency Alliance. (n.d.). Advanced water heating specification (Version 8.1). Advanced Water Heating Specification.pdf – neea.org
What are the Benefits?
- Reduces pipe heat loss, pump run time, and hot water waste through rightsizing and minimizing loop lengths.
- Supports energy-efficient technologies like HPWHs by enabling higher storage temps and smoother delivery with MMVs and VVCPs.
- Shorter wait times at fixtures due to TBVs and compact layouts.
- More stable and consistent hot water temperatures via MMVs and well-balanced loops.
- Minimizes risk of scalding (with MMVs) and legionella (with 140°F storage + thermostatic control).
- Avoids under-heated or stagnant return lines through dynamic circulation control and proper loop design.
- TBVs and VVCPs reduce manual balancing errors, maintenance needs, and post-occupancy complaints.
- Better long-term reliability through automated, responsive system components.
- Aligns with Title 24, LEED, CALGreen, and electrification policies.
- Improves EUI (Energy Use Intensity) and supports grid-friendly HPWH integration.
What are the Challenges?
- TBVs, MMVs, VVCPs, and smart controls add upfront equipment and installation costs.
- Education is needed to shift perception of TBVs, VVCPs, and loop limitations as “overdesign” rather than performance optimizations.
- Compact piping layouts, accessible valve locations, and mechanical room and riser chase sizing require early, coordinated design decisions across plumbing, mechanical, electrical, architectural, and structural teams. This coordination helps avoid routing limitations and conflicts with framing or other building systems.
- Outdated code references and cost-driven decision-making during design and construction lead to continued reliance on oversizing and manual balancing.
- Advanced commissioning and control tuning require more technical labor and specialized knowledge than conventional systems. In particular, systems with VVCPs must be carefully controlled to maintain thermal hygiene.
Qualifications for CEDA Inducements
This measure supports building high-performance HPWH distribution systems as an add-on to qualifying central or semi-central HPWH projects. It is primarily applicable to multifamily buildings, lodging, hotels/motels, dormitories, and residential care facilities.
For a project to be eligible for inducements, it must meet the following requirements:
- Be located within the SCE, SoCal Gas, PG&E or SDG&E territory
- Enrolled in CEDA
- Include a qualifying central or semi-central HPWH system serving the building’s primary DHW needs
- Distribution system must be sized using Appendix M, for group R-1 and R-2 occupancy projects (as defined in the 2022 California Building Code)
- Projects with multiple riser hot water circulation systems must use VVCP controlled to vary the pump speed based on system demand (i.e. operates with differential pressure or temperature control)
- Exemption: In multiple dwelling unit buildings with concerns over legionella growth and the need to maintain temperatures at 120°F the requirement for demand control pump speed may be exempted
- Install self-actuating TBVs to control the system flow at each riser
- TBVs installed in more than one DHW supply riser, shall be accessible and located after the last supply branch from the supply riser, in the direction of flow, and be set to a maximum of 120°F
- Provide an MMV installed on the central or each semi-central heating plant’s hot water supply outlet header leading to the recirculation loop
- Provide DHW return loop lengths no longer than 160 feet of total developed length
- Provide equipment submittals stamped and approved by the responsible engineer of record
- Provide equipment-cost information
- Select products listed on the NEEA Advanced Water Heating Specification (AWHS) Qualified Products List v8.0 or later
- Participate in on-site verification and possible data logging of the system
Notes:
- Project may be selected by PG&E for a future case study
- Measure requirements are subject to change; this guide reflects information available as of June 2025—for the most current measure requirements, contact CEDA@willdan.com
Contact us today to enroll and build resiliency into your project.
Resources:
- U.S. Environmental Protection Agency. (n.d.). Efficient hot water distribution: A best practices guide for new residential construction. Guide for Efficient Hot Water Delivery Systems pdf – epa.gov
- International Association of Plumbing and Mechanical Officials. (n.d.). Water demand calculator—California edition. WATER DEMAND CALCULATOR CALIFORNIA – Water Efficiency and Sanitation Standard for the Built Environment – iapmo.org
- TRC. (2023, August). 2025 Title 24 CASE report: Multifamily domestic hot water – Final. Multifamily Domestic Hot Water.pdf – title24stakeholders.com
- Northwest Energy Efficiency Alliance. (n.d.). NEEA. Advancing Energy Efficiency in the Northwest. Advancing Energy Efficiency in the Northwest- neea.org
