A heat pump water heater is a hot water heater that transfers heat from the surrounding indoor air (or the outdoor air, if the HPWH is connected to the outdoors by vents) into a hot water tank, which is typically located in an open, unfinished basement or garage. This heated water is then piped throughout the home to showers, dishwashers, sinks, and washing machines. Because heat is transferred instead of generated (i.e. by burning fossil fuels or using an electric resistance tank), heat pump water heaters can be three times more efficient than conventional water heaters.

The Technology

A heat pump water heater has two primary components: a heat pump, which absorbs heat from the surrounding air, and an insulated storage tank, which stores heated water for later use.

Heat Pump Water Heater Technology Diagram

To heat water, (1) a fan draws air from the surrounding space across a coil filled with cold, low-pressure refrigerant. The refrigerant absorbs heat from the air and is compressed (2) into a hot vapor. Coils containing the heated refrigerant wrap around the hot water tank, transferring heat into the stored water (3). Once the refrigerant loses heat and has condensed back into a liquid, the process repeats. If the heat pump can’t meet the hot water heating needs of the home (due to high usage or the space becoming too cold for sufficient heat absorption), backup electric resistance elements (4) can boost the temperature of the water as needed. By pulling heat from the surrounding air, heat pump water heaters also provide dehumidification to the space in which they are located.

Just how much dehumidification the system can offer depends on how much hot water a household uses: the more the system is used, the more heat and humidity it will pull from its surroundings. For this reason, most Massachusetts homeowners prefer to install heat pump water heaters in unfinished basements, saving energy due to more efficient hot water heating and reduced dehumidifier usage—though a heat pump water heater won’t entirely replace your dehumidifier.

Just like traditional electric resistance water heaters, heat pump water heaters come in a variety of sizes to meet a home’s annual domestic hot water needs—typically 50, 65, and 80 gallons. Some heat pump water heaters require a few more inches of vertical clearance than a traditional tanked hot water heater. If you currently have an electric resistance water heater, heat pump water heaters can use the same hookups and electrical connections and you will be able to transition to a heat pump water heater without an electrical upgrade. If you are using a different fuel source to heat your hot water now and have a 100 amp electric panel, talk to your installer to see if an electrical upgrade is required. The heat pump water heater will also likely need access to a floor drain or pump for managing condensate buildup.

Home improvement scenarios that work with heat pump water heaters:

  • Existing home, replacing hot water system
  • Existing home, replacing heating system that also heats hot water
  • Existing home undergoing major renovations
  • New home construction

Adding Ducts to a Heat Pump Water Heater

Heat pump water heaters use the heat in the air around them to heat water, cooling and dehumidifying the space they are in. While this can be beneficial in the summer months, it can make spaces like basements less comfortable in the winter. With many models, homeowners have the option to have ducting kits installed, which will allow the heat pump water heater to vent cold exhaust air to the outdoors (or a different room in the home) and/or bring in air from outdoors. Ducting can be a valuable option for some homeowners, for example in cases where the space your water heater is installed in is too small for sufficient air circulation—or if it is beneficial to bring in air from a warmer room/blow cold exhaust air into another area. Some homeowners may want to blow cold air into a pantry to keep it cool or into an attic or crawl space to keep the basement from getting too cold.

However, in Massachusetts, ducting with the outdoor air entails seasonal considerations. Bringing in outdoor air will only work during the summer where warmer air can potentially improve the performance of the system; during colder months, the intake air will be too cold to allow the heat pump water heater to operate. Ducting cooled air outdoors and not ducting air back in will depressurize your home, increasing air infiltration from the outdoors, which can reduce comfort.

Split Heat Pump Water Heaters

While most HPWHs available in the US today have the heat pump co-located with the hot water tank, split systems are also becoming available. Split systems include an indoor hot water tank and an outdoor compressor unit connected by refrigerant lines. The outdoor compressor heats the refrigerant by pulling heat from the outdoor air. Some outdoor units use an exterior metal panel that can absorb solar radiation to assist in heating the refrigerant; these types of systems are called solar-assisted heat pumps. The heated refrigerant then passes indoors and heats the water in the hot water tank before returning to the outdoor system. Split- and solar assisted heat pump water heater systems do not impact indoor air temperatures nor require condensation draining but are subject to weather-based variability in performance based on outdoor air temperatures or daylight. These two kinds of split water heating systems are typically more expensive to install but are well-suited to the unique requirements of some homes. These systems are beginning to be available in Massachusetts, but are still only offered by certain installers.

Helping to Choose Your Heat Pump Water Heater

The Northwest Energy Efficiency Alliance maintains an Advanced Water Heating Specification that tests HPWHs for their efficiency in cooler climates like the Northeast. Tested products are categorized by tiers based on efficiency and additional features, including holding a 10-year warranty, reduced noise levels, and reduced usage of less efficient electric resistance backup. Check out the Qualified Products List for more information on what products might best meet your needs.

Measuring Efficiency

One of the primary metrics of a water heater’s performance is its Uniform Energy Factor (or UEF). The UEF is a measure of how much water a system of a certain size can heat in a given time period relative to how much energy it uses. The higher the UEF, the more efficient the water heater. Heat Pump Water Heaters generally have a UEF between 2.2 and 3.5+, compared to UEFs of 0.6-0.95 for conventional water heaters.

Older water heaters may just have Energy Factor (EF) listed on labels. UEF is a new metric developed in 2015 that more accurately reflects water heating efficiency by incorporating standby losses.

Solar-Assisted Heat Pump Water Heater Outdoor Unit

Solar-Assisted Heat Pump Water Heater – Outdoor Unit

Split Heat Pump Water Heater Indoor Unit

Split Heat Pump Water Heater – Indoor Unit

Heat Pump compressor installed on house foundation wall.

Split Heat Pump Water Heater – Outdoor Unit

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