Of the various systems of heating dwellings that by hot-water is considered by many to be the most satisfactory. On account of its high specific heat, water at a temperature much below the boiling point furnishes the heat necessary to keep the temperature of the house at the desired degree. The temperature of the radiators is generally much lower than those heated by steam but the amount of radiating surface is greater than for steam heating plants of the same capacity. It is because of the relatively low temperature at which the water is used, that the greater amount of heating surface is required.

One objection to the use of hot water as a means of heating is, that once the heat of the house is much reduced, the furnace is a long time in raising the temperature to normal. This is due to the fact that the temperature of the water of the entire system must be uniformly raised, because of its continuous passage through the heater. On the other hand, this uniformity of the temperature of the water prevents sudden changes in the temperature of the house. Water-heating plants work with perfect quiet and may be so regulated to suit the outside temperature that the heat of the water will just supply the amount to suit the prevailing conditions.

The care required in the management of the boiler is less than that required in the steam plant because of the fewer appliances necessary for its safe operation. Another advantage in the use of the hot-water plant is its adaptability to the temperature conditions during the chilly weather of early fall and late spring, when a very small amount of heat is required. At such times the temperature of the radiators is but a few degrees warmer than the outside air. The amount of attention necessary for maintaining the proper furnace fire under such conditions is less then for any other form of heating. The increasing use of the hot-water plant for heating the average-sized dwelling attests to its excellence in service.

The Low-pressure Hot-water System.

—A hot-water system consists of a heater, in which the water receives its supply of heat, the circulating pipes for conducting the heated water to and from the radiators that supply heat to the rooms, and the expansion tank that receives the excess of water caused when the temperature is raised from normal to the working degree. In addition to the parts named there are a number of appliances to be described later, that are required to make the system complete.

A hot-water plant of the simplest form is shown in Fig. 32. The illustration presents each of the features mentioned above, as in a working plant. The different parts are shown cut across through the middle, the black portion representing water. Not only does the water fill the entire system but appears in the expansion tank when the plant is cold.

Hot-water heaters are quite generally in the form of internally fired boilers. The fire-box occupies a place inside the boiler and is surrounded, except at the bottom, by the water space. Commonly, these boilers are made of cast iron and are constructed in sections, the same as the steam boiler shown in Fig. 16. Manufacturers sell a single style for either steam or hot-water heating. The boiler in Fig. 32 is cylindrical in form. It is made of wrought iron and contains a large number of vertical tubes through which the heat from the furnace must pass on its way to the chimney.

As the water is heated it expands and rises to the top of the boiler because of its decreased weight. Since the water in the radiator is really a part of the same body of water, the heated portion rises through the supply pipe to the top of the radiator. As the hot water rises in the radiator, it displaces an equal amount of cold water, which enters the boiler at the bottom. This displacement is constant and produces a circulation that begins as soon as the fire is started and varies with the difference in temperature between the hot water leaving the boiler at the top and the cold water entering at the bottom.

As the water in the system is heated and expands, there must be some provision made to receive the enlarging volume. In this arrangement a pipe connects the bottom of the boiler with the expansion tank located at a point above the radiator. Under the conditions represented in the drawing the water does not circulate through the tank and as a consequence the water it contains is always cold.

In raising its temperature, water absorbs more heat than any other fluid and on cooling it gives up an equal amount. As a consequence it furnishes an excellent vehicle for transmitting the heat of the furnace to the rooms to be heated. Water, however, is a poor conductor and receives its heat by coming directly into contact with the hot surfaces of the furnace, and gives it up by direct contact with the radiator walls. To transmit heat rapidly and maintain a high radiator temperature, the circulation of the water in the system must be the best possible. The connecting pipes between the boiler and the radiators must be as direct as circumstances will permit and the amount of radiating surface in each room must be sufficient to easily give up an ample supply of heat. Even though the furnace is able to furnish a plentiful supply of heat to warm the house, it cannot be transmitted to the rooms unless there is sufficient radiating surface. A plant might prove unsatisfactory either because of a furnace too small to furnish the necessary heat or from an insufficient amount of radiating surface. Yet another factor in the design of a plant is that of the conducting pipes. Both the boiler and the radiators might be in the right proportion to produce a good plant, but if the distributing pipes are too small to carry the water required, or the circulation is retarded by many turns and long runs, the plant may fail to give satisfaction.

Fig. 33 shows a complete hot-water plant adapted to a dwelling. It is just such a plant as is commonly installed in the average-sized house but without any of the appliances used for automatic control of temperature. The regulation of the temperature is made entirely by hand, in so governing the fire as to provide the required amount of heat. In the drawing the supply and return pipes may be traced to the radiators as in the case of the simple plant. The supply pipe from the top of the boiler branches into two circuits to provide the water for the two groups of radiators at the right and left side of the house. To provide any radiator with hot water, a pipe is taken from the main supply pipe and passing through the radiator it is brought back and connected with the return pipe which conducts the water back to the boiler.

The expansion tank is located in the bathroom near the ceiling. It is connected with the circulating system by a single pipe which joins the supply pipe as it enters the radiator located in the kitchen. Like the expansion tank in Fig. 31 the water it contains is always cold. It is provided with a gage-glass which shows the level of the water in the tank and an overflow pipe which discharges into the bathtub, in case of an overflow. An overflow pipe must always be provided to take care of the surplus when the water in the system becomes overheated. This does not often occur but the provision must be made for the emergency. The overflow pipe is frequently connected directly with the sewer or discharged at some convenient place in the basement.

The High-pressure Hot-water System.

—In the hot-water plant described the expansion tank is open to the air and the water in the system is subjected to the pressure of the atmosphere alone. The heat of the furnace may be sufficiently great to bring the entire volume of water of the system to the boiling point and cause it to overflow but the temperature of the water cannot rise much above the boiling point due to the pressure of the atmosphere.

If the expansion tank is closed, the pressure generated by the expanding water and the formation of steam will permit the water to reach a much higher temperature. In the table of temperatures and pressures of water on page 3, it will be seen that should the pressure rise to 10 pounds, that is, 10 pounds above the pressure of the atmosphere, the temperature of the water would be very nearly 240°F. (239.4°F.). The difference in heating effect in hot-water heating plants under the two conditions is very marked. In the low-pressure system the temperature of the radiators cannot be above 212° but the high-pressure system set for 10 pounds pressure will heat the radiators to 240°, and a still higher pressure would give a correspondingly higher temperature. The amount of heat radiated by a hot body is in proportion to the difference in temperature between the body and the surrounding air. If we consider the surrounding air at 60° the difference in amount of heat-radiation capacity of the two radiators would be as 180 is to 132. The advantage of the high-pressure system lies in its ability to heat a given space with less radiating surface than the low-pressure system.

In Fig. 34 is illustrated an application of a simple and efficient valve arrangement that converts a low-pressure hot-water system into a high-pressure system without changing in any way the piping or radiators. The drawing shows the boiler and two radiators connected as for a low-pressure system, but attached to the end of the pipe as it enters the expansion tank is a safety valve B and a check valve A, as indicated in the enlarged figure of the valve. The safety valve is intended to allow the water to escape into the expansion tank when the pressure in the system reaches a certain point for which the valve is set. The check valve A permits the water to reËnter the system from the tank whenever the pressure is restored to its normal amount.

Suppose that such a system is working as a low-pressure plant. The hot water from the top of the boiler is flowing to the radiators through the supply pipe and the displaced cooler water is returning to the bottom of the boiler through the return pipe as in the other plants described. It is now found that the radiators are not sufficiently large to heat the rooms to the desired degree except when the furnace is fired very heavily. It is always poor economy to keep a very hot fire in any kind of a heater, because a hot fire sends most of its heat up the chimney. If the radiators could be safely raised in temperature, they would, of course, give out more heat and as a result the rooms would be more quickly heated and kept at the required temperature with less effort by the furnace. The difficulty in this case lies solely in there being insufficient radiator surface to supply heat as fast as required.

The increase in radiator temperature is accomplished by the pressure regulating valve B, attached to the end of the pipe as it enters the expansion tank. The expansion tank with the regulating valve is shown enlarged at the left of the figure. The valve B is kept closed by a weight marked W, that is intended to hold back a pressure of say 10 pounds to the square inch. A pressure of 10 pounds will require a temperature of practically 240°F. (see table on page 3). The check valve A is kept closed by the pressure from the inside of the system. When the pressure of the water goes above 10 pounds—or the amount of the weight is intended to hold back—the valve is lifted and an amount of water escapes through the valve B into the tank, sufficient to relieve the pressure. Should enough water be forced out of the system to fill the tank to the top of the overflow pipe, the overflow water is discharged through this pipe into the sink in the basement.

When the house has become thoroughly warmed, the demand for a high radiator temperature is reduced, the furnace drafts are closed, the water in the system cools and as it shrinks the system will not be completely filled. It is then necessary to take back from the tank the water that has been forced out by excess pressure. It is here that the check valve comes into use. So long as there is pressure on the pipes, this valve is held shut and no water can escape, but as the inside pressure is released by the cooling there will come a point where the water in the tank will flow back through the valve A and fill the system.

This is the type of valve used by the Andrews Heating Co. and designated a regurgitating valve. In practice it gives excellent service. The only danger of excessive pressure in the use of this device is the possibility of the valve becoming stuck to the seat through disuse. Any possible danger from such an occurrence may be eliminated by the occasional lifting of the valve by hand.

Heating-plant Design.

—A heating plant should be designed by a person of experience. No set of rules has yet been devised that will meet every condition. Carpenter’s rules given on page 25 serve for hot water as well as for steam as a means of determining the radiating surface required for an ordinary building, but the rules do not take into account the method of construction of the house and the consequent extra radiation demanded for poorly constructed buildings. In many cases the designer must rely on experience as a guide where the rules will not apply. In the case usually encountered, however, the rules given will meet the conditions.

What was said regarding the size of steam boilers required for definite amounts of heating surfaces, applies with equal force to hot-water boilers, because house-heating boilers are commonly used for either steam or hot-water heating. There are no established rules for determining the heating capacities of house-heating boilers. Manufacturers’ ratings are usually low. There are some manufacturers who make honest ratings for their boilers but they are in the minority. When the heating capacity of a boiler is not known from experience, the only safeguard against installing a boiler too small for the radiators to be heated, is to require a guarantee that the plant will give satisfaction when in operation and when considered necessary a certain percentage of the contract price should be withheld until the plant proves itself by actual trial.

Overhead System of Hot-water Heating.

—In Fig. 35 is illustrated another system of high-pressure hot-water heating that corresponds to the overhead system of steam heating. It differs from the high-pressure system already described in the method of distribution and in the radiator connections.

The flow pipe is taken to the attic and there joined to the expansion tank as a point of distribution. On the expansion tank is a safety valve set at 10 or more pounds pressure. The flow of the water is all downward toward the radiators. The circulation through the radiators is also different from the other plants described. The supply pipe joins directly to the return pipe and the connections to the radiators are made at the top and bottom of the same end. The circulation through the radiators in this case is due to the difference in gravitational effect between the hot and colder water at the top and bottom of the radiator. The system requires no air vents on the radiators as all air that might collect in the system goes up to the expansion tank. The safety valve on the expansion tank in this case is the common lever type. The overflow should empty into the sewer and be pitched to prevent any water being retained in the discharge pipe. If water should be retained in this pipe and should freeze, the system would become dangerous, because of the possibility of high pressures from a hot fire.

Expansion Tanks.

—Fig. 36 is a form of expansion tank in common use. It may be used for either the high-or low-pressure system. The body of the tank is made of galvanized iron and is made to stand a considerable amount of pressure. The gage-glass is attached at B, and the overflow at O. The pipe E connects the tank with the circulating system and D connects with the cold-water supply as a convenience for filling the system with water. The object in placing the stop-cock D near the expansion tank is to avoid overflowing the system in filling. The overflow pipe, as stated above, is most conveniently connected with the sewer, into which the water will run in case of an overflow, but the other methods shown are commonly used. There should be no valve in this pipe nor in the pipe E.

The expansion tank must be so located that there will be no danger of freezing. Should it be necessary to place the tank in the attic or where freezing is possible, the tank must be so connected as to become a part of the circulating system. Such an arrangement is shown in Fig. 37. The expansion tank is connected with a supply and return pipe as a radiator. This arrangement is sometimes used but it is not desirable. It is wasteful of heat and there is always a possibility of freezing in case the fire in the furnace is extinguished a sufficient time to allow the water to grow cold.

Any possibility of danger from excessive pressures in either the low-pressure or the high-pressure system must originate in the expansion tank. It is, therefore, desired to again mention the possible causes of danger. Any closed-tank system is liable to become overheated. The expansive force of water is irresistible and unless some means is taken to prevent excessive pressure some part of the apparatus is apt to burst. No closed-tank system should be used without a safety valve.

The low-pressure or open-tank system requires no safety appliances. So long as there is open communication between the tank and the boiler the pressure cannot rise but slightly above that of the atmosphere. There is only one cause that will lead to high pressure in such a system. If the pipe connecting the expansion tank is stopped an excessive pressure might generate. There is little or no danger of this happening.

In the closed-tank system the expansion tank should be of greater capacity than for the open-tank system. Its size is commonly about one-ninth of the volume of water used. The larger tank is necessary to prevent too rapid rise of pressure as the temperature of the water rises. The air in the tank acts as a cushion against which the pressure of the expanding water is exerted.

The extended use of hot-water heating has led to the invention of many appliances for the improvement of the circulation and heating effects. Pulsation valves are used for retaining the water in the boiler until a definite pressure has been attained that will lift the valve long enough to dissipate the pressure. Many of these systems possess merit and some of them are great improvements over the simple plant.

Radiator Connection.

—The method of connecting the radiators to the distributing pipes depends entirely on local conditions. In a well-balanced system any of the methods shown in Figs. 38, 39 or 40 might be used with good heating effects. The method of attaching the supply pipe to the radiator is, however, an important factor in case of accumulation of air. In Fig. 41 is shown the form of connection most commonly used. The drawing is intended to represent a cast-iron radiator with the valve at D, and the air vent at B. Should air collect in the radiator it will rise to the top and displace the water. The water will continue to circulate and heat as much of the radiator as is in contact with the water, but that part not in contact will receive no heat from the water and will, therefore, fail to fulfill its function. As soon as the air vent is opened the air will escape and allow the water to entirely fill the space.

In Fig. 42 a much different condition exists, when air accumulates. In this mode of connection the water enters through the valve V, and escapes at the bottom of the opposite end. When air fills the radiator to the line L, the circulation is stopped and the radiator will grow cold.

The position of the valve on these radiators is of little consequence. The valve is intended merely to interrupt the flow of the water and may occupy a place on either end of the radiator with the same result.

Hot-water Radiators.

—Radiators for hot-water heating are most commonly of cast iron and in appearance are the same as those used for steam heating. The only difference in the two forms is in the openings between the sections. Those intended for steam have an opening at the bottom joining the sections; while those for hot water have openings at both top and bottom to permit circulation of the water.

Automatic Hot-water Air Vents.

—It is sometimes desired to use automatic air vents on hot-water radiators. For such work a vent is used that remains closed as long as water is present and will open when the water is displaced by the accumulating air, but will again close when the air is discharged. In such vents the valve is controlled by a float, the buoyancy of the float when surrounded by water serving to keep the valve closed. These vents are not so positive in their action as automatic air vents for steam. The change in temperature which controls the steam vent does not take place with hot water. The automatic hot-water vents are not perfectly reliable. They may work with entire satisfaction for a long time and then fail from very slight cause. The failure of a hot-water vent is generally discovered by finding a pool of water on the floor or a wet spot on the ceiling or wall of the floor below.

One type of the automatic hot-water vent that has proven quite successful is shown in Fig. 44. The threaded lug is screwed into the radiator at the proper point. As the water enters the radiator the air is discharged through the vent, escaping at the opening C. When the water has risen to a sufficient height it enters the openings G and H until enough is present to raise the float A. The pointed stem attached closed the hole C with sufficient force to make an air-tight joint. The float A is a very light copper cylinder. Its buoyancy supplies the force to close the vent and its weight opens the vent when the water is displaced by air. It will be readily seen that very slight cause might prevent the performance of its duty.