If they haven’t already, more and more plumbing engineers will be hearing about the electrification of water heaters. However, gas-fired water heaters have been working fine for well over a century. So why consider a change?

Like other components of the built environment, the notion of electrifying water heaters has to do with the larger movement to reduce or eliminate the carbon emissions from the construction and operation of buildings and infrastructure. 

As we know, burning fossil fuels — to power an entire factory or produce a building’s hot water — releases carbon into the atmosphere, which exacerbates global warming and climate change. Decarbonization is the process of eliminating or reducing carbon emissions, especially carbon dioxide (CO₂), from being released into the atmosphere. From an operations standpoint, therefore, a low-carbon building minimizes the systems running on fossil fuels and uses building energy sources that produce zero or low levels of greenhouse gas emissions.

Switching from heating water with natural gas to electricity is an example of decarbonization by electrification — the conversion of a machine or system to the use of electrical power exclusively. When this occurs, the building no longer needs to be connected to a piped natural gas utility system.

To be clear, at this time, electrification at the building level, in most instances, does not decarbonize the building or its systems. Most U.S. utilities’ electrical grids are generated using fossil fuels, such as coal and natural gas. 

However, building-system level electrification is a step in the right direction by consolidating the fossil fuel burned by millions of small appliances to a relatively few large power plants. By eliminating the need for fossil fuel at the site level, we can focus on eliminating source carbon emissions to reach decarbonization at scale. 

As building designers, we typically have little control over the source of emissions. However, we can control site carbon emissions by eliminating fossil fuel combustion. This is important for owners who want a zero-carbon building as soon as their utility electrical grid is no longer burning fossil fuels.

Water Heater Electrification

Two options exist for electrifying water heaters: electric resistance and heat pumps.

Electric resistance water heaters are very simple — connecting electricity to an electric resistance element creates heat and heats the water. The benefits of an electric resistance water heater are that it is simple with virtually no limitations to water setpoint temperatures. 

Electric resistance water heaters have a coefficient of performance (COP) of 1.0. Simply put, COP is a measure of the work put in compared to the output: COP = Energy Out/Energy In. If you put 1 Btuh or 1 W in and you get 1 Btuh or 1 W out, you get a COP of 1.0, meaning an electric resistance water heater is effectively 100% efficient. A good gas-fired water heater might be +/- 95% efficient (meaning some heat is lost up the flue). 

Heat pump water heaters use a refrigerant circuit to heat the water. A small air-source heat pump water heater contains a refrigerant circuit about the same size as the one in a home refrigerator. However, the system operates opposite to a refrigerator. Instead of making the inside cold and heating the surrounding air, a heat pump water heater makes the inside hot while it cools the surrounding air. It can have a COP of 2.0, 3.0 or even higher — making it more efficient than a 100% efficient electric resistance water heater. 

A heat pump can be more efficient than 100% because it is not creating energy; as its name implies, it pumps or moves energy (i.e., heat). A COP of 3.0, therefore, means it moves three times more energy than it uses by absorbing heat from the surrounding air and pumping it into the water. 

Like refrigerators, heat pumps contain motors that drive compressors, raising the refrigerant inside the unit to a higher pressure, adding heat to the water. This is known as the “heat of compression.”

Heat Pump Considerations

Superior efficiency notwithstanding, a heat pump water heater can present some important caveats to consider when comparing it to a resistance system. 

One characteristic is complexity. A heat pump system includes several more components and moving parts than a resistance system. This adds cost and potentially reduces the useful life of the water heater. 

Location is another important consideration. The simplest heat pump water heater is an ambient air-source unit, which removes heat from surrounding air, providing the additional benefit of space cooling. Heat pump water heaters should be installed in locations that can remain in the 40 F to 90 F range year-round. Since they always emit cool air, it is best to install them in a location that will benefit from year-round cooling. 

If a building has no space available where indoor air heat transfer is possible, two alternatives exist:

1. Outdoor air-source heat pumps. In some cases, the entire water heater is installed outdoors. In other cases, the system decouples the water heating tank side from the “cooling” components, locating the tank indoors and the cooling components outdoors to collect heat from the atmosphere.

2. Water-source heat pumps. Instead of pulling heat from the surrounding air, these units transfer heat from a water-based source. This water could be open-source geothermal, closed-loop geothermal, cooling tower condenser water or a campus-wide chilled water system. (This can be a great benefit to a campus if the other buildings’ heating systems can be designed around a similar technology.)

An additional caveat to consider for a heat pump water heater is the heating temperature setpoint, which can be limited by the refrigerant’s thermodynamic properties. For example, R-410a refrigerant has a peak temperature in the 140 F range. This refrigerant works well for applications requiring the water heater to be set at or less than 140 F. 

However, a different refrigerant may be required for applications needing elevated water storage temperatures (for example, to alleviate concerns for Legionella or other waterborne pathogens).

Some current models use R-134A, providing temperatures of 160 F or higher. However, R-134A and R-410A are HFCs (hydrofluorocarbons), which have high global-warming potential and are due to be phased out in the not-so-distant future. Some of the new, environmentally friendlier refrigerants can also approach or exceed 160 F. Some manufacturers are using R-744, also known as CO₂. Such CO₂ heat pumps can heat water up to 185 F.

In lieu of a water heater designed only using resistive or heat pump technology, available options can combine the two technologies. If the water heater can use the higher efficiency of the heat pump system during the majority of its operating time but also has the resistive heating element available for periods of high demand, it is possible to reduce the size and cost of the heat pump equipment. 

Under such conditions, this can be a good choice, bridging the gap between the technologies and combining their individual strengths. This also can provide better redundancy for clients with critical applications; if either the heat pump or resistive element fails, the water heater can still provide partial capacity until the unit can be repaired.

Heat pump water heaters provide an option to efficiently use electricity to provide hot water for buildings. As technology advances, additional options and configurations will allow designers to consider the use of heat pump water heaters for an even wider array of project types.

While gas-fired water heaters have been reliable and remain viable, electrification of the utility grid structure is happening and will continue. By being aware of heat pump water heater options, plumbing engineers can help building owners reach decarbonization goals and be prepared for the eventual decarbonization of building energy sources.

Dave Bodenschatz, PE, LEED AP, is the director of mechanical engineering for IMEG Corp.’s Technical Operations Team, which provides the firm’s designers with the latest industry knowledge and tools. 

Eric Busch, PE, LEED AP, is a mechanical engineer at IMEG Corp. He designs mechanical systems for new construction and renovations and has significant experience designing plumbing systems.