Heat Pump Water Heaters

How Heat Pump Water Heaters Work

A heat pump water heater transfers heat from the surrounding indoor air (or the outdoor air, if the water heater is connected to the outdoors by vents) into a hot water tank 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 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.

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. 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.

Heat pump water heaters generally have a UEF of up to 4 as compared to UEFs of 0.6-0.95 for conventional water heaters. Mass Save provides rebates for heat pump water heaters with 3.2 UEF or greater.

Helping to choose your heat pump water heater

If you’re within Mass Save’s territory and wish to receive a rebate and/or qualify for the HEAT loan, select a heat pump water heater that meets Mass Save’s requirements that is ENERGY STAR certified.

Whether you’re eligible for Mass Save programs or not, another helpful resource is the Northwest Energy Efficiency Alliance (NEEA)’s Advanced Water Heating Specification. The organization tests heat pump water heaters for their efficiency in cooler climates like the Northeast. Tested products are categorized by tiers (with highest being best) 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.