The benefits of using central heat pump water heater systems as they contribute to decarbonization and building electrification.
Introduction to central heat pump water heaters (HPWH)
Many building codes are pushing designers and developers to shift their projects from typical natural gas fired water heating systems to all electric designs. States, cities and local jurisdictions are banning natural gas and electric resistance heating elements for new construction and encouraging the adoption of heat pump water heaters for domestic use. These water heaters have many inherent benefits, but when they are integrated into a centralized design they can really stand out against the competition.
With the aggressive goals set forth for decarbonization and electrification of buildings, there is a greater need for the adoption of these centralized systems. Furthermore, even for high efficiency natural gas water heaters, utilities are phasing out or have already phased out energy efficiency incentives for these designs. This document will outline the advantages, challenges and some basic design principles of these systems and how a design team can support their clients to lead the change in the marketplace.
According to Ecotope’s 2022 Market Characterization of the domestic water heating market, they have found that less than 20% of existing building stock utilizes centralized heat pump water heating systems in multifamily buildings.1
Systems vs. Components
Heat pump water heaters are a common device and are readily available even for smaller residential applications. The challenge, and greatest energy saving opportunity, comes when looking at a centralized system that must supply hot water to a large facility such as multifamily applications. The current practice is to either install a large centralized natural gas boiler system or individual instantaneous water heaters. As mentioned above, the decarbonization goals are forcing new projects to move away from natural gas thus eliminating the potential for standard practice. Heat pump water heaters are all electric devices that use the refrigeration cycle to heat water while rejecting the heat to the surrounding air, thus are the fundamental component to the system.
What are the benefits?
- Proven high efficiency design
- Potential lower cost with avoidance of natural gas infrastructure
- Able to support time-of-use rates and grid flexibility (load shifting)
- Some are plug and play or skid mounted designs
- Options to utilize low GWP refrigerants
What are the challenges/constraints?
- Increased electrical consumption and costs
- Lack of system sizing knowledge
- Back up water heating solutions, what happens without power
System coefficient of performance (SysCOP)
As projects get larger there are several different system layouts that are recommended to minimize loss, maintain desired temperatures, and meet the requirements for larger buildings. When designing a centralized system, it is important to look at the heat pump as well as other items such as storage tanks, mixing valves, and temperature maintenance tanks. With this new way of looking at water heating systems, our system efficiency now replaces the individual unit coefficient of performance with a System Coefficient of Performance (SysCOP) which looks at all the combined performance of all the required equipment.
The Northwest Energy Efficiency Alliance is one of the leaders in this space. They have outlined seven key different piping configurations that are qualified under their Advanced Water Heating Guide v8.0.
These systems fall under two categories, Single-Pass and Multi-Pass. Single-Pass designs are best suited to large temperature lift scenarios, for example 80˚-90˚F. They typically result in a higher SysCOP with reduced required heat capacity and storage tank volumes. Multi-Pass systems typically raise the water temperature 10˚F per pass and are seen more in temperature maintenance scenarios.
1. Single-pass primary HPWH system without HW circulation
In this design the HPWH, or multiple in parallel, draws their cold water supply from the bottom of the storage tank and returns the hot water to the top. This water is then pulled through a mixing valve to be distributed to the tenants a desired temperatures.
2. Single-pass primary HPWH system with HW circulation returned to primary storage
The next step up in this design is to incorporate hot water recirculation to the storage tank. Comparing to the previous diagram, the only difference is in the warm return water in the storage tank. By adding in the recirculation loop the end point use always has hot water. Within the first design, you might have to wait for the hot water to get to the end point.
Re-introducing the recirculation loop above the cold water inlet helps to maintain that tank stratification and the high delta T that a single pass unit thrives off of.
3. Single-pass primary HPWH system with series temperature maintenance tank (Swing tank)
A temperature maintenance tank or swing tank can be added as well. Here the second tank is used to combine the hot water return and supply separating it from the cold water supply. These maintenance tanks will typically utilize an electric resistance heating element to maintain the desired temperature.
The main design benefit is that the swing tank handles small maintenance loads (like the recirculation loop loss) so there is no need to run the heat pump water heater for a small load, which would likely be at a low Delta Temperature resulting in poor efficiency and increase general wear and tear on the unit by kicking it on/off for such a low load.
4. Single-pass primary HPWH system with parallel temperature maintenance tank & multi-pass HPWH
The final single-pass primary system is very similar to the previous but now utilizes a multi-pass HPWH in parallel for the maintenance tank.
The benefit of introducing a Multi-pass system is to swap out the electric resistance heater for the swing tank with a HPWH. From design 3, the swing tank iheater is handling the recirculation load which is relatively small and low DT. The Multi-pass system likes low delta temperatures, which aligns well in this scenario. Of course introducing another HPWH is going to be more expensive than an electric resistance.
5. Multi-pass integrated HPWH system without HW circulation
This design is for simple installations and features two integrated units. These units are multipass and have integrated storage tanks. Integrated units may be a good fit for space constrained projects.
6. Multi-pass integrated HPWH system with HW circulation returned to primary storage
Similar to Design 2, a recirculation loop may be added. Providing the benefits of on demand hot water greater system efficiency.
7. Multi-pass primary HPWH system with HW circulation returned to primary storage
Multi-pass systems do not need to use integrated units. This design demonstrates how to layout a design with a separate hot water storage tank
Sizing the system
A key part of the design process is to appropriately size the equipment with the building loads. As we have added more components and advanced control strategies it is critical that each component is selected to work together. A common tool for this is to use Ecosizer developed by Ecotope. Ecosizer is used for multifamily buildings and can help you 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.
Many manufacturers will also have their own design tools that can be used to layout and/or size the appropriate equipment. It is crucial to work with the manufacturer or their representative to ensure the system is designed for maximum efficiency.
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 centralized heat pump water heating systems. The inducements are proportional to the therms savings from an established baseline.
For a project to be eligible for inducements, they must:
- Enroll in CEDA
- Be located within the SCE, SoCal Gas, PG&E or SDG&E territory
- Designed to one of the above mentioned piping configurations
- Provide documentation on the sizing methodology and HPWH rated COP
- Provide shop drawings
- Participate in on-site verification and data logging of the system