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Water Heating

In commercial buildings, where large amounts of hot water are used, the energy tied up in water heating can be a significant component of the building's total energy consumption. For example, in the lodging industry, 42 percent of energy use goes for water heating. Other commercial buildings with heavy hot water demand include restaurants, commercial laundries, buildings with industrial processes, and other types of buildings such as dormitories or other high-density housing facilities.

There are a number of technologies available to heat water efficiently. However, before implementing these technologies, it is important to first reduce hot water use with water-saving fixtures and appliances. Conserving water and heating it efficiently should be addressed during the whole building design process.

Water heating technologies covered in this section include:

See also the Water Heating Technology Fact Sheet (PDF 836 KB) Download Adobe Reader for additional information on water heating options.

DOE conducts research and development on water heating. Learn more.

Conventional Water Heating

Most commercial water heating is done with storage water heaters that use gas, oil, or electricity. Ranging in size from 20 to hundreds of gallons (75.7 liters and larger), storage water heaters remain the most popular type for water heating in the United States. A storage heater operates by releasing hot water from the top of the tank when the hot water tap is turned on. To replace that hot water, cold water enters the bottom of the tank, ensuring that the tank is always full.

Because the water is constantly heated in the tank, energy can be wasted even when no faucet is on. This is called standby heat loss. Newer, more energy-efficient storage models can significantly reduce the amount of standby heat loss, making them much less expensive to operate. To determine the most energy-efficient model, consult the Water Heater Efficiency Certification Program sponsored by the Gas Appliance Manufacturers Association, Inc. (GAMA), now part of the Air-Conditioning, Heating, and Refrigeration Institute (AHRI). A Procedural Guide (PDF 496 KB) Download Adobe Reader to the Program can be downloaded at the AHRI Web site.

The efficiency of most gas water heaters currently in use is about 65 percent and standby losses are about 6.5 percent of stored capacity per hour. An accepted measure of the energy performance of water heaters is the energy factor (EF), which takes into account thermal losses from the tank. The National Appliance Energy Conservation Act (NAECA) has established minimum energy factors for water heating equipment used in residential applications, and since many commercial applications use the same type of water heating equipment, water heaters in commercial applications tend to fall under the NAECA minimum performance standards.

Criteria for Selecting a Water Heater

As with any purchase, balance the pros and cons of the different water heaters in light of your particular needs. There are numerous factors to consider when choosing a new water heater: fuel type, capacity, efficiency, and cost.

Determining Capacity

Although some consumers base their purchases on the size of the storage tank, the peak hour demand capacity, referred to as the first-hour rating (FHR), is actually the more important figure. The FHR is a measure of how much hot water the heater will deliver during a busy hour. Therefore, before you shop, estimate the peak hour demand and look for a unit with an FHR in that range.

Gas water heaters have higher FHRs than electric water heaters of the same storage capacity. Therefore, it may be possible to meet your water heating needs with a gas unit that has a smaller storage tank than an electric unit with the same FHR. More efficient gas water heaters use outside air for combustion air intake and exhaust.

Rating Efficiency

Once you have decided what type of water heater best suits your needs, determine which water heater in that category is the most fuel efficient. The best indicator of a heater's efficiency is its Energy Factor (EF), which is based on recovery efficiency (i.e., how efficiently the heat from the energy source is transferred to the water), standby losses (i.e., the percentage of heat lost per hour from the stored water compared to the heat content of the water), and cycling losses.

The higher the EF, the more efficient the water heater. Electric resistance water heaters have an EF between 0.7 and 0.95; gas heaters have an EF between 0.5 and 0.65, with some high-efficiency gas condensing models ranging from 0.9 to 0.95; oil heaters range from 0.7 to 0.85; and heat pump water heaters range from 1.5 to 2.0. Product literature from manufacturers usually gives the appliance's EF rating.

Comparing Costs

When choosing among different models, it is wise to analyze the life-cycle cost—the total of all costs and benefits associated with a purchase during its estimated lifetime.

Units with longer warranties usually have higher price tags, but these models tend to be more durable, especially in areas with hard or mildly corrosive water. Often, the least expensive water heater to purchase is the most expensive to operate.

Drainwater Heat Recovery

Any hot water that goes down the drain carries away energy with it. Drainwater heat recovery systems save energy by using the heat in drains to preheat water coming into the water heater.

Estimates based on the DOE test procedure for water heaters indicate that the equivalent of 350 billion kWh worth of hot water is discarded annually through drains, and a large portion of this energy is, in fact, recoverable. To capture and put to use heat from wastewater produced by all sources in a dwelling would require a regenerator-type, double-walled heat exchanger, one that can capture heat from wastewater generated by one fixture or appliance (e.g., a clothes washer) and apply this heat to assist another hot water demand that may occur at a later time. If wastewater generation is concurrent with the need for hot water (e.g., a shower), a nonregenerative, straightforward heat exchanger can be used. Heat exchanger systems of each type are available for use in buildings. The nonregenerative type is most successfully used in laundries, dishwashing, and process applications.

Heat Pump Water Heaters

Heat pump water heaters can provide up to 60 percent energy savings over conventional water heaters.

Heat pumps are a well-established technology for space heating. The same principle of transferring heat is at work in heat pump water heaters (HPWHs) except that they extract heat from air (indoor, exhaust, or outdoor air) and deliver it to water. Some models come as a complete package, while others work as an adjunct to a conventional water heater. Because it extracts heat from air, the HPWH delivers about twice the heat for the same electricity cost as a conventional electric resistance water heater.

The simplest HPWH is the ambient air-source unit, which removes heat from surrounding air, providing the additional benefit of space cooling and dehumidification. Exhaust air units extract heat from a continuously exhausted air stream and work better in heating dominated climates because they do not cool ambient air. Some units can even be converted between the two modes of operation for optimum operation in either summer or winter. In mild climates you can locate units in unheated but protected spaces such as garages, essentially using outdoor air as a heat source.

A variation of the stand-alone HPWH is the desuperheater feature available on some central air conditioners. It provides economical supplemental water heating as a byproduct of air conditioning. Desuperheater water heating can be part of an integrated package with a heat pump or air conditioner system. In most such systems, the heat pump water heating only occurs during normal demand for space conditioning, with resistance electric coils providing water heating the rest of the time. During the cooling season, the desuperheater actually improves the efficiency of the air conditioning system while heating water at no direct cost. In an average climate, a desuperheater might meet 20 to 40 percent of annual water heating demand.

Applications

Properly applied, HPWHs save energy in almost every situation. Initial investment is recouped fastest if electric rates and hot water usage are high and there is a steady need for the cool air generated as a byproduct. HPWHs have difficulty being cost effective when low-cost natural gas is available. Best energy savings are accomplished when temperatures are mild or warm. Because HPWH efficiency and capacity drop as temperature drops, avoid applications where the ambient air is cold.

The high water heating efficiency combined with the cooling benefits tends to favor applications where there are large hot water demands—which occur for much of the day—and where there is a simultaneous need for spot cooling. Laundries, restaurants, and some dormitories are representative of good applications.

There are several commercial building types that are typically good candidates for HPWH. They include:

  • Laundries (coin-operated and commercial), where there is a large daily hot water requirement and where space cooling would be useful

  • Restaurants, particularly in the kitchen where large hot water demands for dishwashing coincides with a need for cooling the kitchen and its occupants; locate the HPWH evaporator to take advantage of the heat from the dishwasher

  • Hotels and motels are large users of hot water; locate the evaporator of the HPWH near ice machines to improve their performance

  • Health clubs for spa heating and service water heating

  • Schools, particularly in the kitchen where hot water is used for food preparation and clean-up

  • Multifamily housing and apartments where a single system provides hot water to all residents

  • Finally, in these and other buildings, where the cost of energy for conventional water heating is high and where a more efficient water heating option would be attractive.

Pool and Spa Water Heating

HPWHs are also useful and efficient for pool and spa heating. The water heating principle is identical to that previously described for a HPWH, which uses air as the heat source. In the case of pool heaters, the cooled air can be exhausted outdoors, or can be used to cool and dehumidify the air above the pool. Dehumidification is a particularly attractive feature for indoor pool or spa applications. Systems which provide pool water heating and operate over a number of ventilation air modes are available from a number of vendors in sizes ranging to more than 500,000 Btu/h of water heating.

Costs

Most of the heat delivered to the water comes from the evaporator of the unit, not through the electrical input to the machine. Consequently, the efficiency of the HPWH is much higher than for direct-fired gas or electric storage water heaters. The installed cost of commercial HPWH systems is typically several times that of gas or electric water heaters; yet the low operating costs can often offset the higher total installated cost, making the HPWH the economic choice for water heating. The HPWH becomes increasingly attractive in building applications where energy costs are high, and where there is a steady demand for hot water. This attractiveness is less a function of building type than it is of water demand and utility cost.

Demand (Tankless or Instantaneous) Water Systems

Demand water heaters are more commonly seen in Japan and Europe, but are also used in the United States. Unlike conventional tank water heaters, tankless water heaters heat water only as it is used, or on demand. A tankless unit has a heating device that is activated by the flow of water when a hot water valve is opened. Once activated, the heater delivers a constant supply of hot water. The output of the heater, however, limits the rate of the heated water flow.

Gas and Electric Demand Water Heaters

Demand water heaters are available in propane (LP), natural gas, or electric models. They come in a variety of sizes for different applications, such as a whole building water heater, a hot water source for a remote bathroom or hot tub, or as a boiler to provide hot water for a heating system. They can also be used as a booster for dishwashers, washing machines, and a solar or wood-fired hot water system.

Applications

You may install a demand water heater centrally or at the point of use, depending on the amount of hot water required. For example, you can use a small electric unit as a booster for a remote bathroom or laundry. These are usually installed in a closet or underneath a sink. The largest gas units, which may provide all the hot water needs of a small commercial building, are installed centrally. Gas-fired models have a higher hot water output than electric models.

Solar Water Heating

An estimated one million residential and 200,000 commercial solar water-heating systems have been installed in the United States. Although there are a large number of different types of solar water-heating systems, the basic technology is very simple. Sunlight strikes and heats an "absorber" surface within a "solar collector" or an actual storage tank. Either a heat-transfer fluid or the actual potable water to be used flows through tubes attached to the absorber and picks up the heat from it. (Systems with a separate heat-transfer-fluid loop include a heat exchanger that then heats the potable water.) The heated water is stored in a separate preheat tank or a conventional water heater tank until needed. If additional heat is needed, it is provided by electricity or fossil-fuel energy by the conventional water-heating system. By reducing the amount of heat that must be provided by conventional water-heating, solar water-heating systems directly substitute renewable energy for conventional energy, reducing the use of electricity or fossil fuels by as much as 80 percent.

Today's solar water-heating systems are well proven and reliable when correctly matched to climate and load. The current market consists of a relatively small number of manufacturers and installers that provide reliable equipment and quality system design. A quality assurance and performance-rating program for solar water-heating systems, instituted by a voluntary association of the solar industry and various consumer groups, makes it easier to select reliable equipment with confidence. Building owners should investigate installing solar hot water-heating systems to reduce energy use. Before sizing a solar system, water-use reduction strategies should be put into practice.

Types of Solar Hot Water Systems

There are five types of solar hot water systems:

  • Thermosiphon systems
  • Direct-circulation systems
  • Drain-down systems
  • Indirect water-heating systems
  • Air systems

The Department of Energy's Office of Energy Efficiency and Renewable Energy has more information on solar energy technologies and their applications at the Solar Energy Technologies Program.

Thermosiphon systems

These systems heat water or an antifreeze fluid, such as glycol. The fluid rises by natural convection from collectors to the storage tank, which is placed at a higher level. No pumps are required. In thermosiphon systems fluid movement, and therefore heat transfer, increases with temperature, so these systems are most efficient in areas with high levels of solar radiation.

Direct-circulation systems

These systems pump water from storage to collectors during sunny hours. Freeze protection is obtained by recirculating hot water from the storage tank, or by flushing the collectors (drain-down). Since the recirculation system increases energy use while flushing reduces the hours of operation, direct-circulation systems are used only in areas where freezing temperatures are infrequent.

Drain-down systems

These systems are generally indirect water-heating systems. Treated or untreated water is circulated through a closed loop, and heat is transferred to potable water through a heat exchanger. When no solar heat is available, the collector fluid is drained by gravity to avoid freezing.

Indirect water-heating systems

In these systems, freeze-protected fluid is circulated through a closed loop and its heat is transferred to potable water through a heat exchanger with 80 to 90 percent efficiency. The most commonly used fluids for freeze protection are water-ethylene glycol solutions and water-propylene glycol solutions.

Air systems

In this indirect system the collectors heat the air, which is moved by a fan through an air-to-water heat exchanger. The water is then used for domestic or service needs. The efficiency of the heat exchanger is in the 50 percent range. Direct-circulation, thermosiphon, or pump-activated systems, require higher maintenance in freezing climates. For most of the United States, indirect air and water systems are the most appropriate. Air solar systems, while not as efficient as water sytems, should be considered if maintenance is a primary concern since they do not leak or burst.

Types of Collectors

There are three basic types of collectors: flat-plate, evacuated-tube, and concentrating.

A flat-plate collector—the most common type—is an insulated, weatherproofed box containing a dark absorber plate under one or more transparent or translucent covers.

Evacuated-tube collectors are made up of rows of parallel, transparent glass tubes. Each tube consists of a glass outer tube and an inner tube, or absorber, covered with a selective coating that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn (evacuated) from the space between the tubes to form a vacuum, which eliminates conductive and convective heat loss. The vacuum also helps them achieve extremely high temperatures (170-350° F); so they are appropriate for commercial and industrial uses.

Concentrating collectors are usually parabolic troughs that use mirrored surfaces to concentrate the sun's energy on an absorber tube (called a receiver) containing a heat-transfer fluid. They provide hot water and steam, usually for industrial and commercial applications.

Parabolic-trough collectors use curved mirrors to focus the sunlight on a receiver tube (sometimes encased in an evacuated tube) running through the focal point of the mirrors, and can heat their transfer fluid to as much as 570°F (299°C). Because they use only direct-beam sunlight, parabolic-trough systems require tracking systems to keep them focused toward the sun and are best suited to areas with high direct solar radiation. And because they are particularly susceptible to transmitting structural stress from wind loading and require large areas for installation, parabolic-trough collectors are usually ground mounted. For electrical generation or industrial uses that require very high temperatures (greater than 392°F [200°C]), a heat-transfer fluid such as oil is used, but depending on the danger of freezing, antifreeze or water is used in the heat-transfer loop for domestic water-heating systems. Parabolic-trough collectors generally require greater maintenance and supervision and particularly benefit from economies of scale, so are generally used for larger systems.

Low-, Mid-, and High-Temperature Collectors

The collectors can be low-temperature, mid-temperature, or high-temperature. The glazed, flat-plate collectors most commonly used for commercial or residential domestic hot water are classified as "mid-temperature" collectors, generally increasing water temperature to as much as 160°F (71°C). Flat-plate collectors consist of an insulated, weather-tight housing or box, a clear glass or plastic cover glazing, a black absorber plate, and a system of passages for the heat-transfer fluid to pass through the collector. Special coatings on the absorber maximize absorption of sunlight and minimize re-radiation of heat. Gaskets and seals at the connections between the piping and the collector and around the glazing ensure a watertight system.

Low-temperature collectors, which generally increase water temperature to as much as 90°F (32°C), are less expensive because they consist simply of an absorber with flow passages and have no covering glass (glazing), insulation, or expensive materials such as aluminum or copper. These collectors are less efficient in retaining solar energy when outdoor temperatures are low, but are quite efficient when outside air temperatures are close to the temperature to which the water is being heated. They are highly suitable for swimming pool heating and other uses that require only a moderate increase in temperature and are most commonly used in warmer areas. For the last several years, they have been the most frequently installed collectors. In warm climates, low-temperature collectors are sometimes used in hybrid systems that heat a pool in the winter and supplement domestic water heating in the summer, when pool heating is not needed.

Large facilities or ones with quasi-industrial operations such as laundries may be able to efficiently use more sophisticated high-temperature collectors. Although they are also used in mid-temperature systems, evacuated-tube collectors can be designed to increase water/steam temperatures to as much as 350°F (177°C). They may use a variety of configurations, but generally encase both the absorber surface and the tubes of working fluid in a tubular glass vacuum for highly efficient insulation. Evacuated-tube collectors are the most efficient collector type for cold climates with low-level diffuse sunlight. They can be mounted either on a roof or on the ground, but they need to be protected from vandalism and can be damaged by hail or hurricanes.

Solar Equipment Certification

The Solar Rating and Certification Corporation (SRCC) is an independent, nonprofit trade organization that creates and implements solar equipment certification programs and rating standards. SRCC certifies solar thermal equipment that meets minimum standards jointly set by private and public sectors. The compiled information is published in the Directory of SRCC Certified Solar Collector and Systems Ratings. The guidebook rates the performance, durability, and safety of solar thermal collectors and systems. It also lists certified products and consumer tips for suitable solar product selection. This and other publications are available for downloading from the SRCC Web site.