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Electric Water Heaters

Traditionally, the only type of electric water heater has been an electric resistance water heater, which is inefficient but inexpensive. Within the last several years, tankless water heaters and heat pump water heaters have joined electric resistance water heaters in the electric water heater class. Tankless water heaters still use electric resistance to heat water, but only when water is needed, eliminating the energy losses due to tank losses. A heat pump water heater primarily uses a heat pump to improve the efficiency of the water heating process, but also has electric resistance backup heaters for high demand situations. Both types of technology can be an improvement over standard electric resistance water heaters, if applied appropriately.

A heat pump water heater (left) and an electric tankless water heater (right).
Kate Hudon; Faibel, Wikimedia Commons

Electric Resistance Water Heaters

Electric resistance water heaters are a common, but inefficient type of water heater. Resistive heaters, like those found in a toaster, convert electricity directly to heat and some of that heat will be lost to the surroundings. However, standard electric water heaters are inexpensive so they continue to be used in places without natural gas. (If a home has a natural gas line, a natural gas water heater is typically installed.) Generally, there are two resistive heating elements with one sitting near the bottom of the tank and the other near the top of tank. Cold water fills from the bottom through the dip tube and hot water is taken from the top of the tank when hot water is asked for. Sometimes, standard electric water heaters will be paired with a more energy efficient means of heating water, like a desuperheater from a ground source heat pump or a solar water heater, and the resistance heating elements are used minimally as backup. Despite their prevalence, standard electric water heaters will seldom be found in a Building America home unless they are paired with a solar water heater or heat pump water heater. Alternatively, a before/after retrofit monitoring project could include an existing standard electric water heater that must be monitored for the sake of comparison.

Electric Tankless Water Heater

Also known as instant or on-demand water heaters, tankless water heaters heat water when hot water is asked for, rather than heating a large tank of water. One disadvantage to a storage tank type water heater is that a large portion of the energy used to heat the water is lost due to tank losses and tankless water heaters eliminate that problem. Without the large storage tank, tankless water heaters take up less space. Due to the high heating capacity, there is essentially limitless hot water available when using a tankless water heater. However, there is usually some delay in the delivery of hot water and if hot water use is intermittent (like when washing dishes), delivery of hot water can be inconsistent or non-existent if the flow rate is too low to trigger the heating element.

Heat Pump Water Heaters

Heat pump water heaters (HPWHs) are a type of electric water heater that extracts the heat in the ambient air to heat the water using a heat pump. Most heat pump water heaters can heat the water with either the heat pump or back up electric resistance heaters, or both. Heat pump water heaters can be up to three times more efficient than a standard electric resistance water heater, but in most climates are not competitive with gas water heaters.