Residential heaters. Even though they are designed for the residential market, these water heaters (see Figure 1) can be appropriate for many small commercial facilities—and even some large facilities. These units are available with tank sizes ranging from 20 to 120 gallons and are manufactured in large quantities. As a result, they are relatively inexpensive and widely available. The term “energy factor” is used to express the efficiency of residential heaters and ranges from zero to one. It represents the portion of the energy going into the water heater that gets turned into usable hot water under standard conditions and takes into account the efficiency of the electric heating elements as well as heat loss through the walls of the tank. The higher the energy factor a water heater has, the more efficient it is.
Because electric resistance elements are nearly 100 percent efficient, the efficiency of electric water heaters depends primarily on how well they are insulated. Residential heaters are available with energy factors that range from 0.81 to 0.95. To determine the energy factor for a particular water heater, either obtain it from the manufacturer’s literature or look it up in the Air-Conditioning, Heating, and Refrigeration Institute’s (AHRI’s) Directory of Certified Product Performance.
Figure 1: Electric-powered tank water heater
An electric-powered tank water heater, also called an electric resistance storage water heater, generally consists of an insulated, glass-lined steel tank with two electric resistance elements that heat the water. Thermostats cycle the elements on when the water temperature sags below a setpoint. Cold water enters through a dip tube that extends into the bottom of the tank. An anode rod in the tank reduces the likelihood of corrosion, and there’s a temperature and pressure relief valve for safety.
Commercial heaters. Water heaters designed for the commercial market are available in a nearly unlimited range of tank and electric resistance element sizes. Manufacturers provide tanks that range in size from 5 to 1,000 gallons and have electric elements with inputs ranging from a couple to over one hundred kilowatts. Commercial heaters are rated for thermal efficiency in addition to standby loss, which represents the portion of the stored energy that is lost through the walls of the tank in a given amount of time. Although manufacturers typically don’t publish standby losses, they are often willing to share this information when asked. Alternatively, the AHRI provides this data in its Directory of Certified Product Performance.
Off-peak heaters. If you pay large electric demand charges, or if you are charged much lower rates during off-peak hours, you may benefit from installing a water heater that operates primarily or entirely off peak. Water heaters that are designed to operate this way incorporate larger storage tanks and controls that only allow the electric resistance elements to operate during off-peak hours (users should be sure that controls are adequately reducing or eliminating on-peak usage). Although these heaters take advantage of lower off-peak rates, they incur much greater standby losses, as their storage tanks must be large enough to carry them through the entire on-peak period. Because of the increased standby losses and energy consumption, these systems are usually only cost-effective in locations where there’s a large premium associated with on-peak power.
Heat pump water heaters. Some water heaters employ heat pumps to transfer heat from indoor air or exhaust airstreams to hot water storage tanks, thereby offsetting the energy consumed by the water heater’s electric resistance element and yielding energy savings. Because heat pump water heaters produce cool, dry air as a by-product, the best applications are those that can take advantage of both outputs simultaneously. Heat pump water heaters are especially well suited for commercial-sector applications where the demand for hot water is relatively constant and the need for cooling or dehumidification is continuous.
The initial cost of a commercial heat pump water heater is much greater than that for an electric or gas-fired boiler, but in the right applications, the annual energy savings can be large enough that payback periods are often just a few years. Additionally, many manufacturers are now producing integrated heat pump water heaters for the residential market that may be applicable in small commercial settings as well. These units are slightly larger than comparable electric-resistance water heaters, and are easily configurable to best suit the application in which they are installed. To help consumers select energy-efficient models, Energy Star publishes specifications for residential heat pump water heaters on its Heat Pump Water Heaters page and its website enables users to Find and Compare Products. For more information on this technology, see the Heat Pump Water Heaters topic.