In a previous EP Knows article we took a look at large scale heat pump water heaters plants where the condensing units are located remotely (typically outside) and water is stored elsewhere, inside the building. While a few manufacturers offer a similar arrangement for small scale domestic hot water generation, these split heat pump systems are typically installed at the whole building scale. For smaller jobs such as single-family apartments or homes, a more compact solution is often desirable. For these applications, a hybrid heat pump water heater may be a fit.
The core components for a hybrid heat pump water heater stay the same. A condensing unit pulls in heat from the ambient air and uses that heat to generate domestic hot water via a heat exchanger. This process is very efficient and can use as little as 25% that of an electrical resistance water heater. The main difference is that instead of locating the condensing unit remotely from the storage tank, a hybrid heat pump water heater places the condensing unit directly on top of the tank, creating a consolidated unit that can be located inside. Like their larger cousins, the hybrid heat pump water heaters often have an electric resistance element as well, for times of high peak demand. But unlike the larger systems, these units are connected to a building’s plumbing system virtually identically to an electric resistance water heater. The untrained eye may even confuse a hybrid heat pump water heater for an electric resistance heater if they didn’t know what they were looking for.
There are still design challenges that need to be considered before designing and installing a hybrid heat pump water heater. The concerns highlighted in our previous article regarding cost and carbon footprint are still valid. Conversely, low ambient outdoor air conditions are not a concern for hybrid models because these units are installed inside; but this creates the potential problem of excess cooling of the space in which the heat pump is installed. The heat pump pulls in warmer air from the space in which it is installed and puts out cooler air. If the water heater is installed within a small room, this could cause the cold air to build up and cool down the room. As the room cools down, the unit efficiency drops. For this reason, most manufacturers have requirements that these units be installed in larger rooms, often with louvered doors. Some manufacturers give the option to duct the intake, exhaust, or both to elsewhere, but this can result in a cold zone wherever the exhaust is ducted to. If there is a room that is perpetually warm throughout the year like a year-round boiler room or laundry room this cooling could be beneficial but often these areas are not proximate to the water heater and do not need to be cooled year-round.
Heat pump water heaters also create more ambient noise than a resistance electric or gas water heater which typically produce almost no sound. A hybrid heat pump water heater compressor can be at the sound level of a modern window ac unit but it varies by manufacturer and some are quieter than others. As such, if these units are going to be located next to a bedroom or office, it may require some additional sound proofing measures and in situations where louver doors are required as previously discussed, a higher ambient sound level may be inevitable.
From a retrofit standpoint, most commercially available units are 240 volts which may require some rewiring if a resistance electric or gas-fired water heater is being replaced. There are some 120 volt options out there but they tend to be smaller units that may not meet the end users requirements depending on home or apartment size and usage. Hybrid units tend to be taller as well in order to fit the compressor on top of the tank, so it is not feasible to hang them or locate them in crawlspaces or mezzanines. The compressors also generate condensate so if a floor drain is not available in the area this will need to be pumped to a drain elsewhere in the space.
Despite all these challenges, there is continued innovation in the heat pump water heater space and they retain all the old benefits of a standard tank type water heater. Water heaters can be individually controlled and installed on a one-to-one basis, allowing owners to monitor hot water usage of tenants and easily pass costs of domestic hot water on to tenants. They also reduce the amount of domestic water piping that needs to be run through buildings and can even eliminate the need for a domestic water recirculation system entirely if the water heater can be located directly proximate to apartment bathrooms and kitchens.
As the globe continues to deal with the impacts of climate change, improving building efficiency will only become more in focus for building owners, operators, and developers and hybrid heat pump water heaters are another tool to be considered.