What is a central heat pump water heater?
Central heat pump water heaters (CHPWHs) are service ("domestic") hot water heaters, serving showers, sinks, washing machines, etc. Central heat pump water heaters serve systems with larger loads than a typical single-family home.
Central HPWHs in Multifamily Buildings
Residential HPWHs in Commercial Buildings
A commercial building such as a hospital may be served by a CHPWH system if it uses a lot of domestic hot water, or it may use a residential-type heat pump water heater if it's an office building that doesn't require much domestic hot water.
Central HPWH Systems
Located on a roof, in a garage or a mechanical (or "boiler") room, the equipment in a central system is more easily reached for maintenance and operations adjustments, and offers central access to the outside of the building if needed for a source of heat, etc. In a large multifamily building, maintaining HPWHs in a central location will be easier on the occupants in the event that changes or adjustments are needed at each HPWH some point after the building is fully occupied.
On the other hand, central systems create challenges not encountered by residential heat pump water heaters. The biggest challenge is increased complexity for design, installation, testing, and operation of an new, relatively unfamiliar built-up system. A built-up system is made up of components selected, connected, and coordinated by a project design engineer rather than the manufacturer.
Here is a brief description of the increased complexity of a central HPWH system.
A commercial heat pump water heater--like a residential heat pump water heater--transfers heat out of the ambient air and transfers it into domestic hot water. The evaporator portion of the heat pump is often located outside on the roof or in an open garage, where it can take heat out of the air without excessive temperature drops. On the other hand, I've seen a project where a very large CHPWH was plopped down into a basement mechanical room then hooked up to the central domestic hot water system. When the system was turned on, after a while the CHPWH went off on alarm because the temperature of the mechanical room had rapidly dropped so low the CHPWH couldn't operate. The designer was taken off the project and someone else began to figure out how to duct outside air into and out of the mechanical room at flow rates sufficient to provide the necessary heat source without undue drop in temperature. This project was memorable because the mechanical room was nowhere near an outside wall. But the general pattern of discovery has happened on other projects, in other mechanical rooms or garages where the air flow rate around the CHPWH were part of the design, but not sufficient, so provision for increased air flow needed to be added after initial project completion to assure reasonable performance.
- In large buildings there's normally a recirculation loop added to the hot water piping, to assure that the occupant doesn't have to wait a long time for hot water to reach an outlet once the faucet it turned on, particularly during periods of low hot water use.
- Normally, to keep down the cost of the heat pumps, there are large hot water storage tanks included in the system to allow hot water to be stored during periods of low hot water use. Then during peak periods of hot water use, an amount of hot water can be provided using the storage tanks that could not have been met by the heat pumps alone. Storage tanks, often 300 gallons each or more, need to avoid mixing of hot and cold water so that when the water is moved from the tank to the building it is hot rather than tepid.
- Thermal stratification can be achieved from the simple fact that hot water is lighter than cold water, or in a large a number of tanks are piped in series with hot water being delivered from the CHPWHs at one end, so a plug of hot water moves horizontally through the tanks.
- When the hot water is removed from the tanks, the flow direction reverses, serving water to the building from the hottest tank, and filling up the coldest tank at the other end with city water. After the storage tanks, the hot water often passes through a water tank that has electric resistance heaters to bring the water up to setpoint when the CHPWHs plus storage tanks cannot.
- Finally, the water goes into the hot leg of a central thermostatic mixing valve (or several thermostatic mixing valves in parallel), that mixes that hot water with cold water to attain the temperature setpoint for water delivered to the recirculation loop. In the recirc loop there may be another electric resistance heater for "temperature maintenance" of the recirc loop during periods of low hot water use. This is by no means the only configuration used. But it gives you a sense of the complexity.
- Complexity is further added by the fact there may be mixing valves--thermostatic or manually adjusted--at the plumbing fixtures themselves, and sometimes undocumented, to avoid excessively hot water supply to a shower, etc.
- For a built-up system, in addition to selecting the equipment (valves, pipes, heaters, storage tanks, etc), the design engineer is supposed to include in his or her design a control sequence explaining what setpoints and logic are to be used to coordinate the selected components to produce a stated outcome.
- The operation and setpoints for the the HPWHs themselves is normally tracked carefully by a controller. The operation of electric resistance hot water heaters on the same system is sometimes only tracked by a dedicated controller on the front of the heater, or sometimes not at all. Likewise, the operation and setpoints of the central thermostatic mixing valves are sometimes tracked by a central controller, but sometimes they aren't. Although temperatures through the system are sometimes centrally monitored, pressures resulting from the multiple pumps going on and off--and often at variable speeds--often are not. The direction of flow, likewise, is often unmonitored in the portions of the systems that have bidirectional flow.
- When the backup or temperature maintenance is provided by a natural gas boiler or boiler, on some such projects the boilers produce operating temperatures too high for compatibility with heat pumps.
Once a new type of built-up system has been around for enough time, the design and manufacturing worlds begin to converge on a set of standard practices that are relatively tried and true, which allows the designer of record on each project to represent the owner's interest while pulling together design documents that require a reasonable level of performance from the manufacturers and contractors. In a world of rapid innovation, a built-up system may have sparce design documents and a strong dependence on the ability of the manufacturer to figure out how to coordinate the project design. Or there's a shift toward design-build, an approach that's efficient when it goes well, but may delay the realization by the owner that something is going wrong until late in the project. Either way, the normal process through which the design engineer coordinates selection of components and their integrated operation through a controls sequence is weakened.
Commissioning Agent. To promote quality control in commercial HPWH systems, it's desirable at the least to hire an independent commissioning agent thoroughly experienced in commercial heat pump water heater systems to review every step of the project, and to test the system's controls and performance both before occupancy and after occupancy. In the early phases, the commissioning agent needs to make sure that the design documents, whether design-building or not, include a clear, complete and reasonable sequence of operation and system schematic. In the early phases, the project should also include a clear statement of when the electric resistance heat will be needed, and what type of control system interface will be available for commissioning and owner operation, offering remote read-only access to trends of important information defined in the design documents. Allow the commissioning agent to access the system before the pipes are insulated, to help determine whether the check valves and balancing valves are working as intended, and the direction and relative quantities of flow are correct where flows merge passively. In the commissioning agreement, include a test of the control sequence via trend logs after the project is fully occupied, and preferably during cold weather. The proper use of the storage tanks may need tuning as occupancy grows, and the full use of the heat pump capacity when the building is fully occupied, during cold weather, will be best put to the test once the building is fully occupied. Review of trends during cold weather will provide information on how well the defrost and freeze protection are working, as well as any problems with excessive draw-down of the ambient temperature for interior installations.
System Delivered On a Skid By the Manufacturer. One of the major CO2 HPWH manufacturers, Sanden, is addressing this problem by delivering its new generation of commercial HPWHs on a skid, where all piping, controls, storage, and electric heaters are selected, assembled, and programmed by the manufacturer. For systems with several 300 gallon storage tanks, this is remarkable. We won't know for a while how well the approach is going to work, but the idea is a very good one.
Pay for Performance Contracting. The wise building owner will pay for a central heat pump hot water system under a pay for performance contract. Such an installation will need to have a dashboard for measuring the system (not just the heat pump) energy efficiency, as well as any parameter considered critical for performance, such as ability of the system to maintain a roughly constant and reasonable supply water temperature and provide a decontamination cycle if necessary to suppress Legionella. Ideally the system providing these measurements would be provided by an independent third party. A Pay-for-Performance contract would pay at least some portion of the contract amount based on actual system efficiency and ability to avoid Legionella and deliver water at a reasonably constant specified temperature after full occupancy and for a long enough period to create an annualized calculation. The purpose of a pay for performance contract is to place pressure on the parties involved in the project to use the heat pumps productively after they are installed. Without such pressure, any problems with the installation are likely to result in maintenance of safety and comfort by depending heavily on simple, reliable electric resistance heaters.
(The long term solution for a complex new technology is to stick around long
enough that it can be fine tuned by a succession of projects until a
strong approach becomes familiar to the design, manufacturing, testing,
and operations world. At that point people converge on a standard practice that becomes familiar and can be carried from one project to the next. This process speeds up if permanent monitoring
of performance becomes a standard part of each installation, so that all
parties begin to know which systems are working well, and why. If such
monitoring is too expensive, which it may be, dissemination of
understanding slows down.)
Because the temperature of the water reaching the occupant is controlled for safety and comfort, and any problems in this area are readily reported to building operators by occupants, the challenges listed here do not normally lead to problems for the occupant. However, these systems often offer back up natural gas or electric resistance heaters that end up being used more than anticipated if the integration of the system isn't working properly with the heat pump water heaters. The disadvantage of complexity thus falls on
owners who invest in the (relatively expensive) heat pumps because of their energy efficiency
agencies offering financial incentives based on energy efficiency
building code officials that require use of heat pump water heaters for electrification in response to global warming, presumably representing the interest of the society at large
5/29/25