- Quick recovery electric resistance storage water heater - The most common type of electric water heater used in the United States. There are normally two 4500-watt elements, one upper and one lower in a tank holding 50 to 66 gallons. The tank is usually made of steel lined with glass for corrosion protection.
- Base-loaded electric resistance storage water heater - A storage water heater with one or two 500- to 3000-watt elements and an 80- to 120-gallon tank. The greater storage volume and lower wattage elements levels the electrical load, shifting demand from utility peak load hours to off-peak hours.
- Off-peak electric resistance storage water heater - A storage water heater unit in which the elements are controlled by a timer or remote control device such as a radio or power line carrier signal that allows operation only during utility off-peak hours. There are usually has one or two elements with power levels from 500 to 4500 watts and tank volume is usually 80 to 120 gallons. The elements are typically prevented from operating for 2 to 12 hours. Multiple tanks may be used.
- Interruptible electric resistance storage water heater - A storage water heater in which operation of the elements is remotely controlled, typically by radio or power line carrier signal, to disable element operation during utility peak loads. Typical units use 4500-watt elements and a 50- to 80-gallon tank. Interruption periods are typically less than 4 hours.
- Point-of-use electric resistance water heater - A storage water heater installed near the point where hot water is used. The location reduces the time required to obtain hot water and avoids the use of a pumped recirculation loop and the associated high heat loss. Typical residential units use a 1- to 20-gallon tank while commercial applications usually call for larger units.
- Instantaneous electric resistance water heater - Instantaneous water heaters have little or no storage capacity and heat water as it is needed with heating elements activated by a flow switch. Residential installations commonly have 6000- to 9000-watt heating elements capable of heating about one gallon per minute from 60 to 110° F. Commercial installations are available with much higher power ratings and are often custom built for the application.
- Electric resistance pipe wrap - Pipe wrap heaters features a self-temperature-regulating wire which is wrapped around pipes to offset heat loss so hot water is immediately available at the point of use. Electric pipe wrap is also used for to provide freeze protection for piping.
- The plastic tank electric resistance water heater is similar to other types of electric resistance units except the tanks are made of nonmetallic materials. Advantages include less susceptibility to corrosion and lighter weight, which makes shipping and installation easier.
- The tanks of concrete-lined electric resistance water heaters are made of steel lined with concrete. These reportedly are less susceptible to corrosion than are glass-lined steel tanks, but they are substantially heavier.
- A newcomer to the U.S. market, the plastic-lined electric resistance unit has a plastic-lined steel tank, which weighs approximately the same as a glass-lined tank but is reported to be less susceptible to corrosion.
- Tanks for unpressurized electric resistance water heaters differ from others described here in that water is stored at atmospheric pressure instead of city water pressure. A special pump utilizes city cold water pressure to pump hot water out of the tank and send it to the point of use. Advantages include availability of lower cost tanks in sizes above 120 gallons, light weight, ruggedness for shipping and handling, and ease of installation. This type of tank is now under development and should be available in the United States soon.
- A booster heater is installed somewhere between one point of use and the main water heater and is used to raise water temperature to the final desired level. Boosters allow the main building water heater thermostat to be set at the lowest temperature appropriate for the majority of uses, rather than at the highest temperature needed for single point of use. Tank or tankless (instantaneous) units can be installed, or the boosters can be built into end-use devices such as dishwashers.
Waste Heat Recovery
- A waste heat recovery (desuperheater) water heater is installed in a vapor compression system between the compressor and the condenser. It utilizes hot refrigerant vapor exiting the compressor to heat water, but does nor remove enough heat from the refrigerant to cause it to condense. This makes it possible to retrofit existing vapor compression systems with desuperheaters without redesigning the refrigerant circuit. This electrotechnology is attractive because water heating energy is essentially provided free. heated water is circulated into a conventional water heater tank for storage. A backup water heating energy source is necessary to provide hot water when the vapor compression device (e.g., air conditioner, heat pump, or refrigerator) is not in use.
- The waste heat recovery (full-condensing) multifunction technology is similar to that of desuperheater water heaters, but the system is designed to fully condense refrigerant, allowing the capture of more waste heat for water heating. More importantly, it allows stand alone water heating capability. For example, when incorporated in an air conditioner or heat pump, the unit has multiple operating modes: space heating only, space cooling only, space cooling plus water heating, and water heating only. On an annual basis, because of their stand-alone water heating capability, such systems provide considerably more hot water than do desuperheaters.
Heat Pump Water Heaters (HPWHs)
- A heat pump water heater (HPWH) without a tank is an air-source vapor compression heat pump that is specifically designed to heat domestic water and to be connected to a conventional water heater tank. Such units can be retrofit to existing water heaters. They are highly efficient--annual coefficients of performance range from 2.0 to 3.0, which means they consume one-half to one-third as much electricity as electric resistance units.
- A HPWH with a water storage tank is somewhat more efficient than a tankless unit. It must either replace or be connected in series with an existing water heater. Annual coefficients of performance range from 3.0 to 4.0.
- A HPWH with a single ducting for airflow manipulation (tank or tankless) includes a ducting connection and a fan capable of forcing air through the ducting. Such units are useful for moving cool, dehumidified air from the heat pump into areas where cooling is desired or away from areas where cooling is not desired. Alternatively, the ducting can supply the heat pump with higher-temperature air to achieve better water heating performance or remove heat from unconditioned areas such as attics. If ducting is routed to the outdoors, the units can assist in building ventilation.
- A HPWH with double ducting for airflow is similar to a single-ducted unit but has two sets of ducting connections and two fans for forcing air through ducting. It can simultaneously heat water and achieve air-to-air heat exchange for ventilation in tightly constructed buildings.
- A swimming pool HPWH is tankless except that a swimming pool serves as a tank. Unlike units designed to heat potable water, these water heaters are constructed of materials capable of withstanding exposure to typical swimming pool chemicals, since most swimming pools are heated to less than 90°, the equipment is optimized for better low-temperature performance.
- A spa HPWH is similar to a swimming pool HPWH but is designed to heat water to higher temperatures, typically 110 to 120°F.
- A water-to-water HPWH is a vapor compression water heating device that utilizes water instead of air as the heat source. This type of system is found mostly in commercial applications for utilizing waste heat in cooling-tower water circuits. Variations exist for use with other types of water loops, such as ground-coupled heat exchangers. Both residential and commercial equipment is available