Introduction to Semi-Central Heat
Pump Water Heaters (HPWHs)
In response to reducing emissions and our reliance on fossil fuels, designers and developers are increasingly moving from traditional natural gas water heating systems to all-electric designs. HPWHs are gaining popularity as states and local jurisdictions ban natural gas and electric resistance heat elements in new constructions, driven by their superior energy efficiency, which results in lower operating costs and reduced environmental impact. Semi-central HPWH systems utilize two or more plants comprised of multiple HPWHs and storage tanks to provide domestic hot water (DHW) to the entire building. For this type of design, smaller HPWH plants are strategically placed throughout a building, each serving multiple locations. Semi-central configurations for HPWHs offer many efficiency and energy savings benefits including minimizing recirculation heat losses and reducing pump energy. With aggressive decarbonization and electrification goals, driven by California’s targets set forth in the California Global Warming Solutions Act of 2006 [Assembly Bill 32 (AB 32)], which aims to achieve carbon neutrality by 2045, the adoption of these systems is essential. As higher efficiency options become available, incentives for high-efficiency natural gas water heaters are being phased out. This document outlines the benefits, challenges, and basic design principles of semi-centralized HPWHs, helping design teams lead this market transition.
What Are the Benefits?
- Highly efficient depending on the system and climate, transferring heat from the air
at an efficiency of 300% to 500% - Eliminating recirculation can improve overall system efficiency since warm water return temperature no longer impacts single-pass HPWH unit performance
- Minimizing or eliminating recirculation piping can substantially reduce pumping energy
- Closer proximity to water fixtures helps reduce water waste due to long hot water waiting periods
- Provides greater service flexibility, as shutting down one loop for maintenance does not impact the rest of the building
- Reduces the amount of installed distribution piping in a building
- Minimizing or eliminating recirculation piping can reduce thermal energy losses and temperature maintenance loads, potentially avoiding the need for supplemental heating equipment
- Increases energy efficiency and decreases carbon emissions
- Provides installation flexibility, as smaller units can be placed in different locations within the building.
What Are the Challenges/Constraints?
- Ensuring the strategic placement of hot water end-uses and multiple smaller plants 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
When to Consider the Measure?
Semi-central HPWH configurations are particularly beneficial for buildings with large footprints, such as multifamily buildings, where the last hot water fixture at the end of the loop is significantly distant from the water heater plant. In these types of buildings, long recirculation lines are needed to maintain hot water availability immediately at the faucet to minimize water waste. Maintaining large hot water recirculation lines requires significant amounts of pumping energy and temperature maintenance to offset the thermal energy losses associated with large piping networks. Typical solutions to reduce recirculation losses
includes installing individual HPWHs with homeruns within dwelling units, however this becomes impractical since individual water heaters quickly consume too much room in valuable rentable space. Compared to individual HPWH systems, semi-central systems serve at least two or more dwelling units or commercial hot water end-uses from one plant. This approach is beneficial where central systems can be too costly or complex for the building.
The 2022 CA Energy Code (T24) §170.2 covers prescriptive compliance requirements for water heating systems serving multifamily buildings.1 There are currently no T24 recirculation system requirements for non-residential buildings. In accordance with T24 §170.2(d)2G HPWH systems serving multifamily buildings only require recirculation systems when serving more than 8 dwelling units and there is an exception stating that DHW recirculation is not required for HPWH systems serving multifamily buildings with 8 or fewer dwelling units. Since semi-central HPWH systems typically eliminate or minimize recirculation loops due to closer proximity to DHW fixtures, the energy code appears to support the adoption of semi-central HPWH system designs with the potential to set the standard for future development.
1 California Energy Commission, 2022 Building Energy Efficiency Standards
Compared to individual HPWH systems, semi-central systems serve at least two or more dwelling units or commercial hot water end-uses from one plant. This approach is beneficial where central systems can be too costly or complex for the building.
System Configurations
Ensuring semi-central HPWH systems operate to their fullest potential, it is imperative designers pay close attention to the overall system specification, design, and layout. Designing HPWHs like traditional gas water heater systems can lead to poor HPWH system performance and equipment operational issues. When designing semi-central HPWH systems, engineers must determine the type of HPWH system configuration, water circulation, and temperature maintenance.
The specific system configuration to use for a given building depends on numerous factors, including climate, building type and size, equipment availability, and cost. Below are some key factors to consider when selecting a system configuration. Additionally, examples of possible configurations, as outlined by the Northwest Energy Efficiency Alliance in their Advanced Water Heating Guide v8.1, provide practical guidance.
These configurations fall under two main categories: Single-Pass and Multi-Pass. Single-Pass designs are best suited to large temperature lift scenarios, such as 80˚-90˚F. They typically result in a higher SysCOP with reduced required heat capacity and storage tank volumes. Multi-Pass systems, on the other hand, are more commonly used in temperature maintenance scenarios, where water temperature is typically raised 10˚F per pass.
2 Northwest Energy Efficiency Alliance, Advanced Water Heating Specification Version 8.1
1. Single-Pass Primary HPWH System without HW Circulation
a. In this design the HPWH, or multiple units in parallel, draw cold water from the bottom of the storage tank and return heated water to the top. The hot water is then pulled through a mixing valve to be distributed to tenants at the desired temperatures
2. Single-Pass Primary HPWH System with HW Circulation Returned to Primary Storage
a. The next step up in this design is to incorporate hot water recirculation to the storage tank. The key difference from the previous diagram is the warm return water in the storage tank. By
incorporating a recirculation loop, hot water is always readily available at the point of use, unlike the first design where a delay might occur. Re-introducing the recirculation loop above the cold-water inlet helps maintain tank stratification and the high delta T that a single pass unit needs for optimal performance.
b. Key Factors to Consider:
- Ensuring the recirculation return temperature remains cooler than 120°F to maintain system efficiency and prevent equipment stress.
- Leveraging the ability of single-pass equipment to handle sustained high inlet water temperatures efficiently, which is critical for reliable operation in this design
3. Single-Pass Primary HPWH System with Series Temperature Maintenance Tank (Swing Tank)
a. 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 heat pump water heater 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.
b. Key Factors to Consider:
- Use if single pass equipment cannot run efficiently with high inlet water temperatures.
- Consider if temperature-maintenance heat loss is low or if the circulation pump can be turned off when the building is not in use (note: not recommended in most multi-family buildings)
4. Single-Pass Primary HPWH System with Parallel Temperature Maintenance Tank & Multi-Pass HPWH
a. The final single-pass primary system closely resembles the previous design but now incorporates a multi-pass HPWH in parallel for 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.
b. Key Factors to Consider:
- Use if single-pass equipment cannot run efficiently with high inlet water temperatures.
- Ideal if temperature-maintenance heat loss is high
5. Multi-Pass Integrated HPWH System Without HW Circulation
a. This design is suited for simple installations and features two integrated 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.
6. Multi-Pass Integrated HPWH System with HW Circulation Returned to Primary Storage
a. Similar to Design 2, a recirculation loop can be added, offering the benefits of on-demand hot water and improved system efficiency.
b. Key Factors to Consider:
- Single-pass equipment is not available
- High recirculation return temperature.
7. Multi-Pass Integrated HPWH System with HW Circulation Returned to Primary Storage
a. Multi-pass systems don’t require integrated units. This design illustrates how to configure a system with a separate hot water storage tank, providing flexibility in layout.
Design Considerations
Tank Satisfaction:
To increase the energy efficiency of the system, it is best practice to introduce cold water at the bottom of the tank. Similarly, mildly warm water from the temperature-maintenance loop should be returned to the bottom of the tank to help maintain optimal stratification and improve overall efficiency.
Hydraulic Buffering:
HPWHs and mixing valves both operate more effectively when water pressure and temperature are stable or change slowly. Providing hydraulic separation between the mixing valve and HPWH helps maintain steady temperature and pressure for each component. This hydraulic separation, or buffering, applies to the HPWH inlet and outlet, the mixing valve, and the domestic cold water make-up pipe.
Temperature Sensor Location:
Care should be taken when considering temperature sensor locations in both single-pass and multi-pass systems. In stratified storage tanks, cycle times and effective storage capacity depend on the placement of the on/off sensors.
Single-Pass Heat Pump Recommendations:
- For return-to-primary, include a variable speed pump or thermal balancing valve controlled to limit return water temperature to below the activation temperature of the heat pumps and/or adjust the on temperature to a value meaningfully hotter than the circulation loop return temperature.
- For return-to-primary, increase the primary tank storage volume to account for reduced
thermal storage efficiency due to the mixing temperatures within the tank. - A fully developed specification for the primary storage tanks, recirculation pump controls, swing tank, control system, any back-up heat, and all parts and pieces of a fully functional multifamily water heating system need to be provided by the manufacturer to produce reliable, repeatable results.
- Include guidelines for the minimization of temperature-maintenance losses in the design of distribution piping.
- In the absence of any electric resistance backup elements, include an extra heat pump in design sizing to provide redundancy and reliability.
Multi-Pass Heat Pump Recommendations:
- Multi-pass heat pump systems do not require dedicated temperature-maintenance tanks. The multi-pass heating nature of the heat pump makes them suitable for return-to-primary configurations. However, reduced effective storage volume must be taken into consideration in sizing of the storage tanks under return-to-primary configurations. The implications for design are that the tank will still be stratified and the bottom will remain warm, but the water may not reach a high enough temperature for full usability.
- Multi-pass heat pumps are appropriate for use in return-to-secondary, parallel tank configurations if the equipment can sustain operation with high entering water temperatures. Always verify heat pump specifications ensure compatibility before selecting.
- Storage sizing should account for partial stratification and not assume fully mixed storage as this will lead to unnecessarily over-sized storage.
Installation Considerations
Location:
A key consideration when installing a HPWH for the first time is the additional space required for the physical dimensions of the unit. Different HPWH models have different dimensions, so it is best practice to check the requirements of the selected HPWH make/model. Heat pump water heaters may require 6 feet and 6 inches of height clearance to account for air filter clearance, and a 3-foot diameter to provide clearance for the drain pan, T&P valve, and any other connections. The HPWH should be positioned so the exhaust outlet is at least 8 inches distance from any wall, door, or ceiling.
Air Volume:
To ensure efficient operation, a HPWH should be installed in a sufficiently large room, or be properly vented. Manufacturers typically require access to a minimum of 450 or 700 cubic feet of free air space where the water heater is installed, along with ample space to allow installation and service. An 8-ft by 12-ft room with an 8-ft ceiling, for example, provides sufficient volume. If the HPWH location does not have sufficient air volume, there are three options: 1) install with a fully louvered door or one with top and bottom grills, ensuring the room has at least 700 cubic feet of volume; 2) use a dual-duct system (both intake and exhaust); or 3) use a single-duct system (either intake or exhaust) with a permanent opening, such as a grill, providing at least 20 square inches of intake/exhaust air. Designers should consult the manufacturer’s installation guide for recommendations specific to each model.
Condensate Drainage:
Heat pump water heaters produce approximately 0.5 to 1.5 gallons of condensate per day, requiring a condensate drain line and drain. Unlike condensing gas water heaters, which produce acidic condensate as a combustion byproduct, there are no special piping or treatment requirements for HPWH condensate other than to pipe the condensate water to a drain. Condensate drain lines are based on gravity moving the water to the drain. Therefore, the drain line should be sloped downward away from the condensate discharge port on the HPWH. Condensate should be drained to a floor drain, trench drain, mop sink, hub drain, standpipe, utility sink, laundry sink, or other drain location approved by the Authority Having Jurisdiction to prevent unsanitary conditions and potential health hazards. Condensate should not be drained directly to drain-waste-vent (DWV) piping, nor the safety water pan under the heater.
Pairing/Integration Considerations
Semi-central HPWH systems integrated with high-performance distribution systems can further address the disadvantages of distributed piping losses and poor compatibility between the warm return temperature of recirculated hot water that affects HPWH performance. Although semi-central systems focus on eliminating hot water recirculation, some additional efforts can be considered to further enhance overall system efficiency and performance, including:
System Sizing:
- Group R-1 and R-2 projects using semi-central heat pump water heating are required to size the distribution system using Appendix M of the Uniform Plumbing Code. 3
3 International Association of Plumbing & Mechanical Officials, 2024 Uniform Plumbing Code
Variable Volume Circulation Pump:
- Multiple riser hot water circulation systems shall use a variable volume circulation pump controlled to vary the pump speed based on system demand (operates with
differential pressure 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.
Thermostatic Balancing Valves (TBVS):
- Install self-actuating TBVs to control the system flow at each riser
- Install TBVs in each main return line
- TBV installed in more than one DHW supply riser should be accessible, located after the last supply branch from the supply riser in the direction of flow, and set to a maximum of 120°F
Require Master Mixing Valves (MMV):
- The MMV must be installed on the central heating plant hot water supply outlet header leading to the recirculation loop
Return Loop:
- For systems with one return pipe loop, hot water return piping that does not exceed 160 feet total developed length
- For systems with multiple recirculation return pipe loops, no return pipe may exceed 160 feet of total developed length
Who is Eligible for Inducements?
- In continued efforts to increase electrification and decarbonization of buildings and the grid, CEDA is offering inducements for the installation of semi-centralized heat pump water heating systems. The inducements are proportional to the therms savings from an established baseline.
- The qualifications for CEDA inducement include:
- Products selected shall be listed on the AWHS Qualified Products List (NEEA) v8.0 or later. 4
- Design shall meet the requirements of the Advanced Water Heater Specification v8.0 (NEEA 2022) or later. 5
- Engineered semi-central HPWH system drawings shall be provided identifying which of the configurations the system aligns with from one or more of the qualified NEEA piping configurations. 6
- System shall be sized using an Ecotope Ecosizer or the manufacturer’s alternative sizing tool with documentation of the method
4 https://neea.org/our-work/advanced-water-heating-specification#resources
5 https://neea.org/img/documents/Advanced-Water-Heating-Specification.pdf
6 https://ecosizer.ecotope.com/sizer/
