There are two main kinds of hot-water supply system, storage and demand. In a storage system, the water is heated and then stored in a tank until needed. When the storage tank and heating unit are combined into a single assembly, with the heating unit located directly under the tank, the system is a direct-fired storage system. Most domestic hot-water supply systems are of this type (see Fig. 1). If the heating unit and storage tank are completely separate, the system is called an indirect-fired storage system.
In the demand type of hot-water storage system, the water is heated and supplied on demand—there is no storage tank at all. This system is also known as an instantaneous, or tankless, system. In a demand system a copper coil—usually called a tankless coil—is surrounded by hot water or steam obtained from a boiler. Cold water circulates through the coil and is heated by the surrounding hot water or steam. The domestic hot water then flows either directly into a distribution system for immediate delivery upon demand or into a storage tank, from which it's withdrawn upon demand. Demand systems are common in dwellings when a boiler has been installed as part of the heating system, except in parts of the country where the water is excessively “hard.” Tankless-coil installations (for those dwellings that already have hot-water or steam boilers installed) are quite practical and can save about 30 % on the cost of heating water during the heating season.
STORAGE TYPE HOT-WATER SYSTEMS
The storage tanks used in direct-fired systems are usually made of galvanized steel, although copper-lined tanks are also manufactured. The interior of galvanized-steel tanks may be coated with a layer of vitreous porcelain enamel, known as glass in the trade to further increase the life of the tank. Glass-lined tanks are usually guaranteed against corrosion for 7 to 10 years; unlined galvanized tanks are guaranteed for only 2 to 5 years, depending on the thickness of the zinc coating.
Galvanized-steel tanks will also have a magnesium rod inserted into the tank that runs the length of the tank and acts as the anode of a galvanic couple. That is, the rod is slowly consumed by an electrolytic process and , by being consumed, helps to protect the steel against corrosion. Since the magnesium rod does waste away in time, one of the important duties of a conscientious homeowner should be to remove the rod from time to time, inspect its condition, and replace it when necessary. Otherwise, once the rod has been destroyed, the tank will sooner or later corrode through.
In copper-lined tanks the usual thickness of the copper is 3 lb per sq ft. Copper-lined tanks are, of course, immune from the corrosion that attacks steel.
Tank sizes vary according to the method used to heat the water. Storage tanks used with gas- or oil-fired heating units usually have 30-, 40-, and 50-gallon capacities. Electric units heat more slowly, so they require tanks with a greater storage capacity, usually 40-, 52-, 66-, and 80-gallons.
All the tanks are surrounded by insulation at least 1 in. thick to maintain the temperature of the water stored within the tanks. Mineral wool and fiber glass are the usual insulating materials used. The entire tank is then covered by sheet metal having a baked-on enamel finish.
Tank Safety Precautions
The tanks used with direct-fired systems must be factory-tested at 300 lb per sq in. and be capable of withstanding a normal operating pressure of 1 50 lb per sq in. Any tank meeting these requirements, as well as the design and construction standards of the American Society of Mechanical Engineers, will have an ASME stamp located somewhere on the tank.
It is always possible—if not very probable—that a system malfunction of some kind may cause pressure to build up in the storage tank or the temperature of the water to reach the boiling point. In either case, the result is a potentially dangerous condition.
An excessive buildup in tank pressure (that isn't the result of an excessive temperature increase) isn't particularly dangerous, though it can result in water damage to the house. The result of excessive pressure is that the tank ruptures rather than explodes. The water bursts from the tank at the instant of rupture, but the amount of pressure will not have been exceptionally high in the first place, and , in the second place, the water pressure drops abruptly once the rupture has occurred. The result will be a flood of hot water on the floor and a ruined tank but no particular danger to the house or its occupants.
If, however, the pressure buildup is the consequence of a temperature buildup, the result might very well be an explosive release of energy that will completely demolish the house.
When water is heated, the molecules of which it consists move more and more rapidly. This increased molecular motion causes the molecules to knock against each other with greater force and frequency, which gives each molecule a little more elbow room, so to speak. One of the effects of this increased elbow room is a decrease in the density of the water, which is why hot water tends always to rise above cold water.
Another effect of the increased molecular motion in a closed hot-water storage tank is an increase in the pressure exerted by the hot water against the walls of the tank. As long as the temperature of the water remains below the boiling point, the pressure within the tank will be normal system pressure plus the weight of the water pressing against the walls of the tank. But once the temperature of the water increases above 212°F, it begins to exert an excessive pressure against the tank walls. At some point the pressure within the tank will exceed the strength of the metal, and the tank will rupture. As the super- heated water escapes into the atmosphere, it expands into steam with explosive force.
The table below shows how the pressure within a 30-gallon hot-water tank increases as the temperature increases. It should be understood that it's not the increase in pressure that's responsible for the explosion. The explosion is the result of the almost instantaneous expansion in the volume of the escaping superheated water as it flashes into steam. The total expansion of water into steam is about 1 700 times the original volume of the water. The energy contained in this expansion, assuming a water temperature of about 300°F, is equal to the force of 1 lb of nitroglycerine.
Pressure and Temperature Relief Valves
The need for installing protective devices on a hot-water tank should be evident. These devices usually take the form of pressure and temperature relief valves (see Fig. 2).
Separate pressure and temperature relief valves can be installed on a hot-water tank—if the tank has been tapped for two relief valves; but most direct-fired storage tanks contain a combination pressure-temperature relief valve.
The pressure setting on either an independent or a combination valve is usually 25 lb per sq in. above the normal water supply pressure. If, for example, the pressure of the incoming water is 40 lb per sq in., the valve setting would be 65 lb per sq in. But, whatever the incoming water pressure, the valve set ting should never exceed the tank’s working pressure, of course. Some pressure valves have a fusible plug installed in them. The plug melts at about 210°F. Thus, if the valve should stick in the closed position, the plug will melt and relieve any excess pressures that would otherwise result from a temperature increase.
The temperature setting on both an independent and a combination relief valve is also 210°F. The temperature relief valve operates through a sensing bulb immersed in the tank. The sensing bulb must always be located within 6 in. of the top of the tank, where the water is hottest.
All relief valves include a safety test lever. When this lever is pulled up, the relief valve will open and water will be discharged from the tank. Thus, the valve can always be checked for correct operation. When the lever is released, the valve must reseat itself firmly to prevent any further discharge of water.
The outlet of a relief valve should always be piped to a floor drain or into a laundry tub. Failure to do this may cause the valve to spit water onto the floor, or even flood the floor if it should ever stick open for any reason, or be set too low. This drain line should never be connected directly into a floor drain as it will then be impossible to check the operation of the valve visually.
A tank may have a high-temperature energy shutoff device installed instead of a temperature relief valve (see Fig. 3). The energy shutoff device is part of the thermostat circuit that controls the operation of the heating unit. If the temperature within the tank should ever exceed 210°F, the energy shutoff device will immediately shut off the fuel flowing to the heating unit.
Some manufacturers supply only an energy shutoff device on their tanks, with the pressure or pressure-temperature relief valve being an optional extra. A hot-water tank should never be installed with only an energy shutoff device as protection against excessive tank temperatures. If the device should become inoperative for any reason, there would not be any other way available of preventing an excessive temperature rise. Either a pressure relief valve or a combination pressure- temperature valve should, therefore, be installed at the same time as the energy shutoff device.
Gas- and Oil-Fired Heating Units
Both gas- and oil-fired heaters operate in much the same way. The fuel passes through piping into a burner, If the burner isn't already in operation, the fuel is ignited by a pilot flame (for gas) or a spark (for oil), and the hot combustion gases pass up a flue that extends through the center of the tank, thus heating the water. The gases emerge from the top of the tank into an external flue or vent that leads either to a chimney or directly to the outside of the house.
In both gas- and oil-fired systems, the water temperature is sensed by a thermostat that maintains the water temperature at the desired value by regulating the flow of fuel to the burner. The thermostat can be adjusted to provide hot water between 120°F and 180°F, with the usual setting being 140°F. If the pilot flame should go out for any reason, or if a spark fails to ignite vaporized fuel oil, the fuel flow is automatically shut off. The burner must then be restarted manually.
Gas-fired installations have the advantage over oil-fired installations in that a fuel storage tank isn't required with gas. Thus, anyone considering the purchase of an oil-fired heater must already have an oil tank installed, usually because he has an oil-burning boiler or furnace installed; otherwise, he must install an oil storage tank plus the necessary piping, which adds considerably to the cost of the installation.
An oil-fired burner will cost two to three times as much as a gas-fired burner, assuming the two burners have the same water-heating capacity. The basic reason is that oil requires a more complex burner mechanism than gas. Gas comes into the house under pressure and can be burned as-is. The oil must first be vaporized and blown into the combustion chamber before it's ignited. The complexity of the pump, fan, and burner that are required thus adds to the cost of the installation.
Anyone having a choice between gas and oil, therefore, should take into account the comparative costs of the fuel-burning equipment as well as the cost of the fuel.
Electric Hot-Water Heating Units
The great advantage of an electric hot-water system over gas or oil is the simplicity and cleanliness of the installation. Heat is supplied by two immersion heating elements installed within the tank (see Fig. 4). These heating elements operate either independently or together to produce the required amount of hot water. Hot-water tanks having a relatively small capacity, 40 gal, say, need only one or two 110-volt, 1500-watt heating elements installed in the tank. Larger tanks require 208- or 220-volt circuits and heating elements having an output of up to 6000 watts.
The great disadvantage of using electricity to heat water is the cost of the electricity itself. Assume, for example, that we have two hot-water heaters, one gas-fired and the other electrically operated. Both units have a 30-gal capacity and a recovery rate of 33.6 gal per hr. (The recovery rate of a hot-water system is the number of gallons of cold water per hour that the burner is capable of heating to the desired temperature.) In this example, a 33.6 gal per hr recovery rate means the burners can completely replenish the hot water supply in the tanks in one hour. To provide this recovery rate, the gas-fired burner must have a heat output of 34,000 Btu per hr, and the electric heater must have a heat output of 8200 watts per hr, which is equivalent to about 27,900 Btu per hr.
What these figures show is that even though fewer Btu per hr are required for the electrically operated heater because of the greater efficiency with which electrical energy is converted into heat energy, nevertheless, the heater must operate longer; and , because of the higher cost of electricity per Btu, the dollar cost of obtaining hot water from electricity is much higher.
The cost of the electricity can be reduced in one of two ways. One method is to reduce the size of the hot-water tank. The smaller the tank, the wattage input of the heating elements remaining the same, the faster the water will heat up. One can in this way avoid the waste involved in having to heat a large quantity of water and in keeping the water hot. In effect, what one is doing is converting a storage-tank system into a modified demand system, and experience shows that demand systems operate with about 30 % less fuel consumption than direct-fired storage-tank systems.
The other method of reducing electrical costs is to use a very large tank and heat the water in the tank only during certain off-peak periods when electrical rates are much lower. Many utilities offer lower off-peak rates to encourage the installation f electric hot-water heaters.
The utility will install a separate line that supplies the electricity going to the storage-tank heating elements. A clock installed in this circuit by the utility closes the circuit only during the off-peak hours, at which times the heating elements can be energized. To make the most efficient use of this cheaper power, the capacity of the tank must be large enough to meet all the hot-water requirements of the family during times when the hot-water electrical circuit is deenergized.
DEMAND HOT-WATER SYSTEMS
In a demand hot-water system (see Fig. 5), the incoming cold water passes through a copper coil, which may be located either inside the boiler or outside, If inside, the coil is sub merged within the boiler water. If outside, the coil is located inside a metal housing through which hot water or steam from the boiler circulates. This assembly of housing and coil is a simple version of what is better known as a shell-and-tube heat exchanger. The heated water flows from the coil directly into the distribution system whenever a faucet is opened.
There is no way in which the temperature of the water within the coil can be controlled directly in a demand system, since the hot-water temperature will depend on the temperature of the fluid within the boiler, the temperature of the cold water, and the rate at which the cold water flows through the coil. Manufacturers, therefore, rate the heating capacity of their coils on the basis of assumed temperature values. The usual assumptions are that the boiler water is either at 1 800 or 200°F, and that the coil has sufficient surface area so that x gallons of cold water flowing through the coil per minute will have its temperature raised 100°F. The larger the coil, the greater the heating capacity of the coil in gallons per minute. For domestic use, a tankless coil should have a minimum capacity of 2 ¾ to 3 ¾ gal of 140°F water per minute, depending on the size of the family and the number of bathrooms in the house (see Table 1).
Table 1. Guide to the Selection of Hot-Water Heaters (Based on the number of bathrooms and bedrooms in a dwelling). Storage and tankless type domestic water heaters.
When there is no demand for hot water, there is no hot- water flow. Whenever there is a demand, the hot water flows through a thermostatically controlled mixing valve and into the hot-water distribution system. Since the water may have been re-circulating through the coil for a considerable period of time, its temperature may be somewhere between 180 and 200°F.
The mixing valve is necessary, therefore, to cool, or temper, the hot water to 140°F. The valve may, of course, be set to a lower or higher temperature, if desired. The mixing valve must also be of the fail-safe type. That is, if it should develop a defect, it will go automatically to its full-cold setting, thus preventing the accidental scalding of anyone who might be using hot water at the time.
A pressure relief valve should be installed in the line between the coil and the mixing valve. The valve is usually set at the operating pressure of the boiler, that's , 15 lb per sq in. (gauge) for steam and 30 lb per sq in. (gauge) for water.
Storage Tanks Used with Tankless Coils
The minimal-sized tankless coils installed in most dwellings may be said to have a steady but small output of hot water. In most of these dwellings the coil can't be depended upon to supply sufficient hot water if the demand should ever be excessive; when, for example, in a two-bathroom house, both bathtubs are run at the same time. It is sensible, therefore, to install a storage tank that will increase the capacity of the system to meet any unusually heavy demand. The tank should have a minimum capacity of 30 gal, and it should be covered with at least 1 in. of insulation, which will reduce the fuel costs by about 30 % .
Such a tank is installed in the system as shown in Fig. 5. As may be seen, whenever there is no demand, the hot water recirculates between the coil and the tank, which keeps the tank filled with hot water. The system is a gravity system, in which the water recirculates because of the difference in its density between the two units. The greater the temperature differential of the water, the greater the difference in its density, and the faster the water will recirculate. In installations where the temperature difference isn’t all that great, the difference in height between the tank and the coil becomes important, since height emphasizes slight differences in density. For most efficient circulation, therefore, the tank must be as high as possible above the outlet of the coil. A difference of 5 ft should be considered a minimum. If this can't be managed because the ceiling is too low, it might be necessary to install a pump in the circuit to recirculate the water.
Sometimes a tankless coil is used to boost the temperature of the incoming cold water when a direct-fired storage tank is actually connected to the domestic hot-water system. This is called a modified tankless-coil installation. Combining the two systems in this way can reduce substantially the cost of supplying hot water to the fixtures during the winter months.
The storage tanks used in modified tankless-coil installations are made nowadays either of galvanized steel, monel metal, or stainless steel, but in an old installation the tank may be made of copper. Tanks made of steel should be glass-lined; if not, they should at least have a magnesium rod installed to prevent corrosion. All tanks should have either separate pressure and temperature relief valves installed on them or a combination pressure-temperature valve, for the reasons already described. The tanks should also be insulated to help maintain the temperature of the water within.
HOT-WATER DISTRIBUTION SYSTEM
The Cold-Water Supply Line
In a hot-water storage tank, the incoming cold water enters the top of the tank through a pipe that extends about 6 in. from the bottom of the tank. This extension prevents dilution of the hot water at the top of the tank with cold water; and the fact that the pipe ends about 6 in. from the bottom of the tank prevents water currents from stirring up any sediment that may have collected at the bottom.
A check valve is sometimes installed in the water-supply lines to prevent backflow from the tank. Backflow may occur because of an over-temperature condition in the tank. The increased pressure that's a consequent of this over-temperature forces the hot water out of the tank and into the cold-water line. And, in fact, this is a very convenient way of relieving a potentially dangerous condition. But if other equipment should be installed in the cold-water line—a water meter or water softening equipment, say—a check valve must be installed in the line to protect this equipment against any such backflow. And if a check valve is installed in the line for this reason, a relief valve must then also be installed between the tank and the check valve to relieve any overpressure that might develop in the line.
Noncirculating versus Circulating Distribution Systems
A noncirculating distribution system transmits hot water from the tank, via one or more risers, to the branch lines that supply hot water to the plumbing fixtures. This is a one-way system, and , though extremely common because it's the least expensive method of installing a hot-water distribution system, it has several drawbacks. Hot water will flow through the system only when a faucet is opened. Between times, the water cools in the lines. The next time a hot-water faucet is opened, the water must run for a while before hot water finally emerges from the faucet. When the faucet is turned off again, all the hot water now filling the line will cool off again. Apart from the slight inconvenience in having to wait for hot water, a noncirculating system is very wasteful of hot water and of the energy it takes to heat the water. (To which it might be added that insulating the lines with at least 1/4-in.-thick insulation will reduce the heat loss by 75 to 80 % .)
A circulating system, on the other hand, though more expensive to install because of the additional piping required to return hot water to the storage tank again, has the advantage that hot water is always immediately available at every faucet, since the hot water is circulating continuously between the plumbing fixtures and the storage tank. Circulating systems are, however, very uncommon in dwellings.
SIZING HOT-WATER HEATING EQUIPMENT
Storage Tank Capacity
When a storage tank is part of a hot-water installation, the demand for hot water is usually calculated on a gallon-per- hour basis. Most households use most of their hot water during two peak periods every day—in the morning and at night. Between times, hot-water usage is very light, except on clothes-washing days. To ensure there is always sufficient hot water available for the family’s needs, the capacity of the storage tank must be calculated on the basis of these periods of heaviest demand.
The capacity of the storage tank can be calculated in several different ways, all of which give different results. The simplest method is to consult a table, such as Table 1, which is published by the Federal Housing Administration. It shows the minimum tank and heater sizes considered satisfactory for most dwellings.
Another widely used method for determining the capacity of the hot-water storage tank is based on the cold-water consumption of the dwelling. Experience shows, for example, that in large private dwellings and in most apartment buildings the cold-water demand will average around 100 gal per person per day. Experience also shows that the hot-water demand averages about one-third the cold-water demand. Thus, for example, in a house having five occupants, the cold-water demand will be about 500 gal per day. The hot-water demand, therefore, will be one-third this figure, or 167 gal per day.
It has also been found that the maximum hourly demand for hot water will average about one-tenth the total daily demand, or, using the above figure, 167 ÷ 10 = 16.7 gal per hr.
Finally, since some of the water in the tank is bound to cool, only about three-quarters of a tank is considered to be filled with hot water. To take this cooled water into account, the maximum hourly demand is multiplied by a factor of 1.25. In our example, therefore, the maximum hourly demand—16.7 gal—is multiplied by 1.25 = 20.875 gal as the tank size. In practice, a 30- to 40-gal storage tank is universally recommended for a family of five. Whether in fact this tank capacity is excessive or not would be difficult to say, since the amount of hot water required will also depend on the pattern of hot- water usage and the recovery rate of the heater.
In fact, the size of the storage tank depends on a family’s pattern of water usage and on the recovery rate of the heater. As we mentioned above, if a family concentrates its demand for hot water in one or two periods of the day, then a large tank is necessary. If the hot-water demand is more uniform through out the day, a smaller tank will be satisfactory. On the other hand, the greater the recovery rate of the heating unit, the faster will additional hot water be produced and the smaller the tank capacity need be.
Another difficulty in selecting the correct size of hot-water equipment depends on how much the water must be heated. E.g., the temperature of the groundwater in the northernmost states may be as low as 40°F, but in the southernmost states the groundwater temperature may be as high as 75°F. The optimum temperature of domestic hot water is usually assumed to be 140°F. Water hotter than this increases the rate at which carbonate deposits build up in the hot-water tank, in those parts of the country where the water is hard. In addition, a steel tank corrodes at a faster rate as the water temperature increases.
Therefore, if the water is to be heated to 140°F, in the northern parts of the United States the incoming water must be heated 100°F (i.e., 40°F + 100°F = 140°F), while in the southern parts of the United States the cold water need by heated only 65°F (i.e., 75°F + 65°F = 140°F). This, of course, affects the selection of the hot-water heating equipment. In a southern state, the burner need not have the Btu output that's necessary in a northern state. Table 2 can be used as a guide in the selection of a suitable hot-water system. The table shows the input in both Btu and kilowatts for storage tanks of different capacities. Tables 3 and 4 show the amount of heat input required to heat cold water to achieve different recovery rates. Since the size of most hot-water equipment is calculated on a per-hour basis, the recovery rate per gallon can be used as a guide in selecting a tank size; that's , tank capacity per gallon will equal the recovery rate in gallons per hour.
The size of a tankless coil is calculated on the basis of maxi mum hot-water consumption per minute. A hot-water consumption table should be used as a guide in making up an estimate. E.g., the typical shower will consume about 30 gal of hot water in about 5 mm, or 6 gal per mm. A clothes washer will complete a wash cycle in about 10 mm, which means its per-minute consumption of hot water will be about 7.5 gal, and so on. By adding up the maximum probable demands in this way, based on the habits of the particular family, one can arrive at a reasonable estimate of the required hot-water output of a tankless coil.
Table 2. Calculations for Determining Hot-Water Heater Size
Table 3. Gas Water Heater—Recovery Capacity (gal per hr) for Different Btu Inputs and Water Temperature Rises
Table 4. Electric Water Heater—Recovery Capacity (gal per hr) for Different Btu Outputs and Water Temperature Rises