What Is the Measure?
A semi-central HPWH system provides efficient domestic hot water for commercial and multifamily buildings served by heat pump water heater (HPWH) systems in a semi-central configuration. Semi-central HPWH configurations utilize two or more central HPWH plants located throughout a building, each serving multiple commercial hot water end-uses or dwellings.
With the need to reduce greenhouse gas emissions and combat climate change, designers and developers are shifting their projects from typical natural gas-fired water heating systems to all-electric designs, aligning with California’s decarbonization goals established in the California Global Warming Solutions Act of 2006 [Assembly Bill 32 (AB 32)], which outlines a comprehensive, multi-year program to achieve carbon neutrality by 2045.
Sizing the System
A key part of the design process is to appropriately size each semi-central HPWH plant to serve the intended domestic hot water loads for specific areas of the building. Early engagement with architects is essential for collaborating with the plumbing engineer to reserve adequate space for each semi-central HPWH plant throughout the building, minimizing hot water distribution and potentially eliminating the need for recirculation. This design approach can potentially lower construction costs compared to typical unitary or large central HPWH systems by eliminating the need for multiple individual heat pumps and ancillary equipment in typical unitary designs. It also reduces overall installed piping compared to large distributed central systems. However, it requires early involvement with design teams to set aside space and locate hot water end-uses appropriately.
A common tool for sizing semi-central HPWH systems is Ecosizer developed by Ecotope. Ecosizer is used for commercial and multifamily buildings and can help determine the equipment size and capacities. The tool will look at several factors including apartment unit quantities, design temperatures, schematic layout, and can even look at load shifting strategies.
What Makes It Different?
While semi-central systems tend to be identical in parts to a fully centralized system, the design of a building with multiple smaller central plants has the potential to reduce overall system size and reduce the amount of distribution piping in a building. Water recirculation systems are essential for providing relatively instant hot water at the faucet. Fully centralized systems often require extensive distribution piping, but semi-central systems require less piping as heat pumps are strategically placed near their end-uses. This minimizes pump energy use for water distribution and improves overall system efficiency by reducing the energy needed for hot water recirculation. Therefore, semi-centralized systems can reduce recirculation pumping energy and piping thermal energy loss which is typically maintained by electric resistance swing tanks.
In addition, compared to an individual HPWH, a semi-central system will serve at least two or more dwelling units or commercial domestic hot water end-uses from one plant.
Key Benefits of Semi-Central HPWH Systems
- Highly efficient, pulling heat out of the air at an efficiency of 300% to 500% depending on the system and climate
- Saves space and reduces the amount of installed distribution piping in a building
- Potentially reduces heat losses typically seen in recirculation piping, minimizing or eliminating temperature maintenance loads
- Reduces recirculation pumping energy
- Increases energy efficiency and decreases carbon emissions
- Flexible to install as smaller units are placed in various locations within building
What Are the Challenges/Constraints?
- Ensuring the strategic placement of hot water end-uses and multiple smaller units to minimize distribution piping can be complex and requires early engagement with architects
- Due to the use of multiple plants, installation may require more work and potentially lead to higher cost
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Sources:
Association for Energy Affordability, Considerations for Selecting a Configuration
Energy Trust of Oregon, Central Heat Pump Water Heater Design Guide
California Energy Commission, Getting to All-Electric Multifamily Zero Net Energy
Construction
National Renewable Energy Laboratory Heat Pump Water Heater Guide for Small Businesses
Plumbing & Mechanical, Heat Pump Water Heaters Poised for Growth in Commercial Markets
