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Consumption The U.S. Department of Energy reports that the average U.S. household consumes about 60 gal (228 L) of hot water per day, about 15 gal/person (57 L/person) per day. An average European uses daily 36 L (10 gal) of 60°C hot water per day. Energy consumption for heating domestic water is the second largest energy user (after space heating) in the residential sector in industrialized countries, accounting for about 14% of the energy used in a typical household. Typical hot water usage in a U.S. household by percentage is listed in Tbl.14. Img.28 A residential storage tank water heater. Img.29 A gas connection to a residential storage tank water heater. Note the drain valve on the right side of the tank. Img.30 Plumbing connections to a residential storage tank water heater. Dielectric unions are used to minimize a galvanic reaction between the copper pipes and the water heater. Img.31 A short storage tank water heater. Img.32 A commercial, gas-fired storage tank water heater. Img.33 A commercial, electric storage tank water heater. Note the presence of an electrical connection and lack of presence of a vent. Img.34 Two ultra-efficient, gas-fired, power-vented, storage tank water heaters connected in series. Note the drain pans at the base of each unit that are designed to drain water into a floor drain. Img.35 A gas-fired instantaneous water heater. Img.36 A water heating boiler that provides hot water to a commercial building. Types of Water Heaters A water heater is an appliance or system that heats water for domestic or building service hot water use. There are three types of water heaters: storage tank water heaters, instantaneous on-demand (tankless) water heaters, and circulating water heaters. Different types of water heaters are shown in Imgs **28 through **36. Fig.11 A gas-fired storage tank water heater. Fig.12 An electric storage tank water heater. Storage Tank Water Heaters A storage tank water heater consists of a storage tank and a heating medium. Typically, storage tank sizes include 30, 40, 50, 60, 65, 75, 80, 100, and 120-gal (120- to 400-L) capacity. They are available in natural gas, propane (LP), fuel oil, and electric models. Storage tank water heaters remain the most popular type for residential heating needs in the United States. An average-size, three-bedroom residence uses a 50-gal (200-L) water heater. Electric water heaters have coil-like elements that extend into the tank, which heat water as electric current passes through the elements. See Fig.11. Gas and fuel oil fired water heaters have burners located on the bottom of the tank and a vent that passes up through the center of the tank. The vent tube extracts residual heat from the combustion products before the gases are exhausted to the top of the tank into a vent that carries them outdoors. See Fig.12. A storage tank water heater stores water for use on demand. Street pressure drives water flow. As hot water is drawn from the tank, it’s replaced by water from the water service. This cools the water contained in the tank. A thermostat that extends into the tank serves as a control of the heating medium: it switches the heating medium on when the temperature of the water in the tank has cooled below a set-point temperature and switches the heater off when the water in the tank reaches the preset temperature setting. The storage tank is insulated to reduce standby losses. All storage tank water heaters are required to have T/P relief valves. Separate valves may be used, or a combination T/P relief valve may be installed. Both temperature relief valves and the combination T/P relief valves must be installed so that the temperature-sensing element is located in the top 6 in of the tank. Types of storage tank water heaters include the following: • Residential storage tank water heaters are designed for the residential market, but can be appropriate for many small commercial facilities. These water heaters, which are available with tank sizes up to 120 gal and gas inputs up to 75 000 Btu/hr, are manufactured in large quantities. As a result, they are relatively inexpensive and widely available. • Commercial storage tank water heaters are similar to residential models except that they are available with much higher gas input ratings (1 000 000 Btu/hr or more) and larger storage tanks (up to 250 gal). They also feature larger pipe connections, more rugged controls, and a few features that are only rarely found in residential heaters, such as flue dampers and electronic ignition. • Ultra-efficient water heaters use power burners and enhanced heat exchangers to force hot combustion gasses into chambers and tubes that are submerged in the stored water. Ultra-efficient water heaters are vented with plastic pipes that go directly through an outside wall. Because they draw combustion air directly from outside, through one of those pipes, their combustion processes are sealed off from the occupied space. Storage tank water heaters can also be classified on their method of venting: non-venting (electric), naturally aspirated, power vented, and sealed combustion. Naturally aspirated water heaters have a flue that runs vertically through the center of the tank. Products of combustion are vented by relying on the natural buoyancy of the flue gases. Care should always be taken to ensure that naturally aspirated water heaters cannot backdraft because of strong winds at the chimney or because of high exhaust airflows in the building. Power-vented water heaters use a fan to exhaust flue gases. Sealed-combustion water heaters have supply air and exhaust air connections to the outdoors. These units don’t require any indoor air to operate. There are two types of sealed-combustion water heaters. Some units have a flue up the center of the tank (similar to the power-vented water heaters). The second type of unit has a heat exchanger that wraps around the tank. Instantaneous (Tankless) Water Heaters Instantaneous water heaters, sometimes called tankless water heaters or demand water heaters, supply hot water on demand. They don’t rely on a standby storage in a tank to artificially boost their capacity. Instead, they have 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. They are more efficient because they don’t have any tank standby losses. (See Fig.**) Instantaneous water heaters are available in natural gas, LP, and electric models. They come in a variety of sizes for different applications, such as a whole-house water heater, a hot water source for a remote bathroom or hot tub, or as a boiler to provide hot water for a home heating system. They can also be used as a booster heater for dishwashers and washing machines. The largest gas-fired units can provide all the hot water needs of a household, and are installed centrally. Gas-fired models have a higher hot water output than electric models. As with many storage tank water heaters, even the largest whole house tankless gas models may not supply enough hot water for simultaneous, multiple uses of hot water (i.e., showers and laundry). Large users of hot water, such as the clothes washer and dishwasher, need to be operated separately. Some types of instantaneous water heaters are thermo statically controlled. They can modulate (vary) their output temperature according to the water flow rate and the inlet water temperature. This is useful when using a solar water heater for preheating the inlet water. Instantaneous point-of-use water heaters fit compactly under a fixture (e.g., sink or lavatory) and heat water at the location where it’s used. This device ensures that hot water is available without delay. Because only a cold water line needs to be brought to the fixture, it also saves on piping installation costs. This type of heater typically is electrically powered and is available in a 1 kW, 120 V, single-phase through 32 kW, three-phase power design. Instantaneous water heaters are rated in gpm of hot water output at a specified temperature rise (temperature increase of incoming water). Residential units are available up to 5 gpm capacities. An average-size residence will typically demand 2 to 4 gpm (7.6 to 15.2 L/min). Smaller, 1/4 to 2 gpm (1 to 8 L/min) point-of-demand units can be installed near demand points, such as under a sink or lavatory. Fig.13 An instantaneous (tankless) water heater. Circulating Water Heaters Circulating water heaters consist of a separate storage tank that stores water heated by a heat exchanger. The heat exchanger may be a separate unit that is heated by stream or hot water from a boiler or may be contained in a boiler. The boiler may be designed to exclusively heat water for DHW or BSHW use or may be used for other purposes. Fig.14 An indirect water heating system. Fig.15 An indirect water heating system for a large building. Tankless Coil and Indirect Water Heaters A building's space heating boiler can also be used to heat water. Two types of water heaters that use this system are tankless coil and indirect. No separate storage tank is needed in the tankless coil water heater because water is heated directly inside the boiler in a hydronic (hot water) heating system. The water flows through a heat exchanger in the boiler whenever a hot water faucet is opened. During colder months, the tankless coil works well because the heating system is used regularly. How ever, the system is less efficient during warmer months and in warmer climates when the boiler is used less frequently or not at all; that is, the boiler must operate when hot water is needed. A separate storage tank is required with an indirect water heater. Like the tankless coil, the indirect water heater circulates water through a heat exchanger in the boiler. But this heated water then flows to an insulated storage tank. Because the boiler does not need to operate as frequently, this system is more efficient than the tankless coil. In fact, when an indirect water heater is used with a highly efficient boiler, the combination may provide one of the least expensive methods of water heating. See Figs. **14 and **15. Heat Pump Water Heaters Heat pump water heaters extract energy from outdoor air and use it to produce hot water very efficiently. Heat pump water heaters use an electric motor to run a compressor. The compressor draws a gaseous refrigerant through an evaporator, raising its pressure until it liquefies in the condenser. This heat pumping process heats the condenser and cools the evaporator. In removing heat from air, the heat pump both cools and dehumidifies the air, thus helping to also meet cooling needs. The heat pump water heater makes economic sense in hot humid regions where natural gas in unavailable (e.g., tropical islands like Hawaii), electric energy cost is high, and the need for dehumidification is virtually constant. In most cases, the extra costs of heat pump water heaters over standard electric water heaters are paid back in a few years. Fig.16 A solar water heating system. Solar Water Heaters A solar water heater typically includes collectors mounted on the roof or in a clear area of the yard, a separate storage tank near the conventional heater in the home, connecting pipes, and an electronic controller. Throughout the year, the solar system preheats the water before it reaches the conventional water heater. Solar water heating systems can reduce the annual fuel cost of supplying hot water to a residence by more than half. There is, however, the added cost of the solar system. A schematic of a solar water heating system is shown in Fig.16. Solar water heating systems are discussed in Section 11. Tbl. **15 CHARACTERISTICS OF SELECTED GAS-FIRED TANK-TYPE AND INSTANTANEOUS (TANKLESS) WATER HEATERS COMPILED FROM INDUSTRY SOURCES. Water Heater Type: Water Heater Features Storage Tank Type Instantaneous (Tankless) Water Heater Features Tank capacity First hour rating Btu input Thermal efficiency Energy factor Dimensions Wall hanging Life expectancy Desuperheaters A desuperheater is an attachment to an air conditioner or heat pump that allows waste heat from that device to assist in heating domestic water. In hot climates, a desuperheater can provide most of a home's hot water needs. Energy Efficiency The energy factor (EF) is a standardized measure used to ex press the efficiency of residential heaters. It was developed by the U.S. Department of Energy. It represents the amount of heat required to warm up hot water for a typical home in a year, divided by the amount of energy input into the heater to warm that water. A water heater with an EF of 0.60 transfers 60% of the energy it consumes to the hot water; the remaining energy is vented outdoors as waste heat. The higher the EF, the more energy efficient the water heater and the less energy consumed. By varying the tank insulation, burner design, and a few other features, manufacturers make residential heaters available in energy factors ranging from 0.43 to greater than 0.95. Naturally aspirated water heaters units have the lowest EF, typically below 0.60. Many units also use a standing pilot light for ignition that further degrades seasonal performance. Power-vented water heaters have lower standby losses and therefore a higher EFs that range from 0.58 to 0.63. Sealed-combustion water heaters with the flue in the center of the tank will have an EF as high as 0.65. Those with a heat exchanger that wraps around the tank can have an EF over 0.80. Electric resistance water heaters have an EF ranging from 0.7 and 0.95; gas water heaters from 0.5 to 0.6, with a few high efficiency models ranging around 0.8; ultra-efficient water heaters have thermal efficiencies of 94%, an EF of 0.94; oil water heaters from 0.7 and 0.85; and heat pump water heaters from 1.5 to 2.0. An instantaneous (tankless) water heater with an electronic ignition will have an EF above 0.90. Instantaneous water heaters will always have a higher EF in comparison to storage tank water heaters because they don’t have tank standby losses. Characteristics of selected gas-fired tank-type and instantaneous (tankless) water heaters are provided in Tbl.15. Unlike residential heaters, commercial heaters are not rated for overall efficiency. Instead, they are rated for thermal efficiency, which represents the portion of input gas energy that goes toward heating the water drawn from the tank. With few exceptions, commercial heaters are available with thermal efficiencies ranging from 78 to 80%. These heaters also must meet standards for standby loss, which represents the portion of the stored energy that is lost when the burners are not operating. Standby losses are typically not published but can be obtained from the manufacturer. The instantaneous energy efficiency of a heat pump water heater system depends on incoming water temperature, intake air temperature, the heat transfer characteristics of the heat pump, and various conductive and convective losses throughout the system. Additionally, although in most circumstances the hot water output is useful throughout the year, the cold air out put may not be. Accordingly, there is no simple index that ac counts for both outputs and describes overall heat pump water heater efficiency. As a result, the heat pump water heater industry relies on two indexes of energy efficiency: coefficient of performance (COP), which is favored by manufacturers of larger heat pump water heater systems. The EF of a typical heat pump water heater will be about 2.2. The COP is a measure of the instantaneous energy output of a system in comparison with its instantaneous energy input. Standby losses and the interaction of changing water and air temperatures are not reflected in measurements of COP. Accordingly, the COP of a standard hot water system is close to 1, and the COP of a typical heat pump water heater may be 3. Buyers of commercial systems should be aware that COP quoted by manufacturers sometimes reflects the combination of the production of cold air and hot water in relation to energy input. This is helpful if full use is made of the cold air, but not otherwise. Fig.18 A hot water recirculating system continuously circulates hot water through the hot water supply piping, making hot water available at the fixtures without evacuating stagnant water. Fig.17 A noncirculating hot water line conveys hot water to the individual fixtures. For hot water to be received at the most remote fixture, stagnant water in the hot water supply line must first be evacuated. Img.37 Properly marked domestic cold water, hot water, and hot water recirculating lines. (--) Hot Water Recirculating Systems In many medium and large commercial buildings and even some residences, a hot water recirculating system continuously circulates hot water from the water-heating unit through the hot water supply piping and back to the water heater through hot water recirculating piping. This ensures that hot water is always available at the taps, thus avoiding the need to run water for a long time to obtain water at the desired temperature. See Img **37 and Figs. **17 and **18. There are three types of hot water recirculating strategies as follows. Continuous Recirculating In continuous recirculating, water is constantly recirculated from the water heater through the piping. This type of system is best suited for buildings having round-the-clock occupancy. Hospitals and multiunit residential buildings typically require continuous hot water recirculating. Timed Recirculating Timed recirculating involves use of an electronic or electro mechanical timer to shut off circulation of hot water when the building is not occupied. This shutdown would apply to commercial, institutional, and industrial buildings that are closed at night and/or on weekends. Nighttime housekeeping and maintenance operations should not be neglected when considering shutdown periods. Thermostatically Controlled Recirculating Thermostatically controlled recirculating relies on a sensor located at a remote location in the recirculating line, which senses water temperature and activates the recirculating pump when water temperature drops below a pre-determined setting. Small- to medium-size buildings typically have a single recirculating loop, a recirculating pump, controls, and a water heating system with storage tank. In large buildings, several re circulating loops may be necessary. Each loop will typically have its own piping and controls. One recirculating pump will usually serve all loops with balancing valves controlling flow to a specific loop. The pump requires a minimal horsepower drive in the range of 1 /12 or 1/16 hp (just enough to keep the water moving). To reduce heat loss, the hot water recirculating piping must be insulated. Prev: WATER TEMPERATURES |