While many heat pump articles, documentation and research exist, the plumbing industry still has much catching up to do compared to HVAC systems. As a result, there is a general lack of understanding of how the systems function, a perception that heat pumps are new technology, or unaware of the potential application.

The plumbing design and engineering community has been focused on improving safety and efficiency in plumbing systems for years. The industry has come a long way since that time by combining innovative technology and lessons learned. We also embraced options offering better safety, efficiency, etc. 

These last few years have highlighted the potential to use heat pumps for domestic water heating while reducing greenhouse gas emissions. One’s stance on decarbonization notwithstanding, we need to remain open to options to maximize our available resources. 

Regardless of their type, domestic hot water (DHW) heat pumps are significantly more efficient than traditional electric water heaters. They also present opportunities to recover heat from other systems that would otherwise be thrown away. So, what are the hesitations in considering heat pumps for DHW systems? 

Unproven Technology

If the assumption is that heat pumps are unproven modern technology, consider the following: Heat pumps, while not often applied to domestic water heating, have been used in HVAC applications for some time. Peter von Rittinger developed and built the first heat pump in the mid-1850s for drying salt in Austria’s salt marshes. 

In the 1950s, Harry W. Brown and William C. Webster developed the first practical heat pump, making heat pumps more accessible and efficient. It was not until 1965 that Alfred T. Rowe patented the modern heat pump. The widespread application of heat pumps led to the rapid growth and development of areas such as the arid and hot American Southwest.

Heat pump design and operation have continued to be developed since then. New technology has advanced improvements in equipment and the development of more efficient refrigerants. Today, millions of heat pump HVAC installations worldwide prove that the technology works. 

Now is the time for the plumbing industry to take advantage of this well-founded technology and consider it an option. Of course, the technology needs to match the building’s needs or program. 

Unfamiliarity With Heat Pump Operation

Heat pumps use the vapor compression cycle to transfer heat from one location to another. The electrical input into a heat pump is used primarily to operate a compressor, which provides the work input into the cycle. In HVAC applications, heat pumps can operate in either heating or cooling modes via a reversing valve. Air-conditioning units, by contrast, operate solely in cooling mode. 

Heat pumps can be air-source or water-source, which describes the source from which heat may be extracted or rejected. Consider the following for domestic water heating:

• Air-source heat pumps. These heat pumps use a compressor and an expansion valve to cool an outside air coil (while extracting heat from the air). The now-hot refrigerant passes through a heat exchanger, where it heats domestic water. The refrigerant is then compressed again to repeat the cycle. 

As the source of heat extraction is from the air, the outside air temperature greatly influences its efficiency. Less heat is available in colder air, making it harder for the heat pump to extract heat and limiting the amount of heat available.

Air-source heat pumps cannot take advantage of heat recovery by relying on air as the source. As such, they will never be as efficient as their water-source counterparts. Air-source heat pumps require additional fans to move air past the coils for extraction, leading to an increased number of components and production costs. Their primary advantage, however, is that they can be deployed independently of other systems, simplifying operations and maintenance. 

• Water-source heat pumps. As the name implies, the source of heat rejection is now water. In making DHW, water-source heat pumps can also provide beneficial cooling to other systems. The efficiency of these systems varies based on the type of fluids used and the temperatures they operate at. The larger the differences, the harder the heat pump works.

The advantage of the water-source heat pump, however, is that it can generate beneficial cooling for other systems. The cooling side of the heat pump could be chilled water, process cooling water or even ground-source geothermal. Additionally, water has a higher heat capacity than air, allowing for a much smaller heat exchanger. As no fans are needed for extraction, water-source heat pumps cost less and have a smaller footprint. 

Regardless of the heat pump system selected, hot water storage is a critical aspect of all heat pump-based DHW systems. Over the last decade, engineers have been tasked with reducing storage volumes and going for higher and higher instantaneous output boilers to minimize standby losses and drive energy efficiency. 

While this process is straightforward with instantaneous gas burners or electric heating elements, its application is challenging for heat pumps. Even with standby losses included, significant volumes of hot water storage are highly beneficial to heat pump water heater systems. 

In a heat pump application, small changes in loads will force the compressor to cycle on and off in rapid succession, a condition known as short-cycling. This will result in premature compressor failures and other problems throughout the system. Like a heat pump in your home, they operate more efficiently and last longer when they can operate for extended periods. This leads to another aspect of sizing the heat pump.

Sizing heat pumps for instantaneous demands results in large system size and electrical demand, a larger footprint and increased first cost. The integration of water storage, however, allows for the downsizing of the system, similar to how storage-type water heaters are selected. Water storage acts like a thermal battery, allowing the system to maintain a more consistent temperature while offering flexibility to operations. 

The now smaller heat pumps also use less connected power and operate for longer periods, increasing their longevity. 

Plumbing Industry’s Acceptance of Heat Pumps

As with any applied systems, heat pumps are not a one-size-fits-all application. Due to power limitations, retrofit applications may find it difficult to move from natural gas-based heating to electric or heat pumps. Replacement of instantaneous systems with heat pumps requires some degree of water storage with a larger footprint. In all cases, some study is needed to inform the ultimate system solution.

In residential or light-commercial applications, the industry offers several water heater types to choose from. One heat pump option for residential includes a hybrid unit combining an air-source heat pump DHW system with an electric heating element. For energy efficiency, the system prioritizes the use of the heat pumps. With increased demand or if it is too cold outside for the heat pumps to be effective, the electric heating element can operate like a traditional electric water heater.

What about other innovative features or options available to be able to use either natural gas or electric water heaters? The options available for traditional gas and electric water heaters can be applied to heat pump systems regardless of the type. This includes leak detection, where the water heater has a sensor alert used in the event of a leak. This feature can even automatically close the shut-off valve in the event of a leak, reducing property damage, and is available for heat pump DHW systems.

How about the associated electrical cost of converting gas to a hybrid heat pump? One big challenge in replacing a gas-type water heater in residential or light commercial applications is the cost associated with upgrading the electrical service. However, there are now hybrid heat pump options that come standard with 120V plugs designed for a direct replacement of existing gas water heaters. 

Though more efficient and designed for direct replacement of gas-fueled water heaters, these hybrid heat pumps do not necessarily heat up the water faster. In these applications, it is recommended to double the storage tank size to better accommodate the high peak load. 

This article highlights the application and considerations associated with heat pump DHW systems. If we place the heat pump hybrid “heating performance” as, at a minimum, equal to the performance of the gas and electric storage-type counterparts, then the heat pump water heater has earned its place as a viable option to consider when selecting a water heater.

If we add the additional benefits of better efficiencies, cost of operation, local utilities’ tax credits or rebates, and federal tax credits that may be applied when installing a heat pump water heater, then perhaps heat pump DHW systems will become strong contenders. 

Lowell Manalo is the plumbing discipline leader for the western region at SmithGroup. He is a member of the American Society of Plumbing Engineers and has more than 20 years of experience designing plumbing systems for a variety of building types.