Steam Heating Systems

Steam is a very effective heating medium. Until recently, this property of steam has resulted in its being the most commonly used method of heating residential, commercial, and industrial buildings. Over the past 40 years or so, steam heating has been largely replaced in res­idences and small buildings by other heating systems that have often proven to be less expensive to install and operate or that operate at similar or greater levels of efficiency in small structures.

The basic operating principles of steam heating are relatively simple. A boiler is used to heat water until it turns to steam. When the steam forms, it rises through the pipes in the heating system to the heat-emitting units (radiators, convectors, etc.) located in the various rooms and spaces in the structure. The metal heat-emitting units, being cooler, cause the steam to condense and return to the boiler in the form of water (condensate, also called condensation) for reheating.

Classifying Steam Heating Systems

There are a number of different methods of classifying steam heat­ing systems, but the most commonly used methods include one or more of the following features:

1. Pressure or vacuum conditions

2. Method of condensate flow to the boiler

3. Piping arrangement

4. Type of piping circuit

5. Location of condensate returns

Steam heating systems can be divided into low-pressure and high-pressure types, depending on the operating pressure of the steam used in the system. A low-pressure system commonly oper­ates at a pressure of 0 to 15 psig, whereas a high-pressure system uses operating pressures in excess of 15 psig.

Both vapor and vacuum steam heating systems operate at low pressures (0 to 15 psig) and under vacuum conditions. The latter system uses a vacuum pump to maintain the vacuum; the vapor system does not, relying instead on the condensation of the steam to form the vacuum.

The condensate from the heat-emitting units is returned to the boiler by either gravity or some mechanical means. When the former method is used, the system is referred to as a gravity return system. If mechanical means of returning the condensate are employed, the system is referred to as a mechanical return system. The three types of mechanical devices used to return the condensation in mechanical return systems are: (1) the vacuum return pump, (2) the condensate return pump, and (3) the alternating return trap. (Each of these devices is described in the appropriate sections of this chapter.)

Using the piping arrangement as a basis for classification, a steam heating system will be either a one-pipe or a two-pipe system. A one — pipe system is designed with a single main that carries the steam to the heat-emitting units and the condensate back to the boiler. In other words, it functions as both a supply and a return main. In a two-pipe system, there is both a supply main and a return main.

The piping circuit may best be described as the path taken by the steam to the riser (or risers). In a divided-circuit installation, two or more risers are provided for the steam supply. A one-pipe-circuit installation, on the other hand, employs a single riser from the boiler to carry the steam supply to the heat-emitting units. A loop — circuit installation is used when it is necessary to operate heat — emitting units at locations below the water level of the boiler.

A steam heating system may also be classified according to the direction of steam flow in the risers (i. e., supply mains). An upfeed system is designed so that the risers are below the heat-emitting units. In other words, the steam supply moves from the boiler up to the heat-emitting units in the rooms and spaces within the struc­ture. An upfeed system is also sometimes referred to as an upflow, or underfeed, system. A downfeed system is one in which the steam supply flows down to the heat-emitting units. In this system, the supply main is located above the heat-emitting units.

Sometimes the location of the condensate return is used as a basis for classifying a steam heating system. If the condensate return is located below the water level in the boiler, it is referred to as a wet return. A dry return is a condensate return located above the water-level line.

Gravity Steam Heating Systems

Gravity systems are generally limited to residences and small build­ings where the heat-emitting units can be located at least 24 inches above the water-level line of the steam boiler.

A gravity steam heating system is characterized by the fact that the condensate is returned to the boiler from the heat-emitting units

By means of gravity rather than mechanical means. Both one — and two-pipe installations are used, with the former being the oldest and most commonly used for a number of years (Figures 8-1 and 8-2). These one-pipe steam systems were designed to move the steam from the boiler to the heat-emitting units at no greater than 4 oz of steam pressure.

Steam Heating Systems

Steam Heating Systems

Gravity steam heating systems are the cheapest and easiest to install because they are adaptable to most types of structures, but they do have a number of inherent disadvantages. The principal disadvantages of gravity steam heating systems are:

1. The return lines in one-pipe gravity systems must be large enough to overcome the resistance offered by the steam flow­ing up from the boiler in the opposite direction.

2. There is the possibility of water hammer developing in one — pipe systems because the steam and condensate must flow in opposite directions in the same pipe.

3. Air valves (which are required) sometimes malfunction by either spurting water or failing to open. If the latter situation is the case, excess heat will build up in the system.

4. Automatic control of the steam flow from the boiler results in room-temperature fluctuations.

5. Comfortable room temperatures are possible by manually regulating the valves on individual heat-emitting units, but this results in some inconvenience.

6. In two-pipe gravity systems, the condensate return from each heat-emitting unit must be separately connected to a wet return or water sealed. This is expensive.

The principal reason for the development of the two-pipe gravity system is to create a means of overcoming the resistance offered by the steam flow to the condensate returning to the boiler.

One-Pipe, Reverse-Flow System

The one-pipe, reverse-flow system is the simplest and cheapest of this type to install. This system is easily distinguished by the absence of any wet or dry returns to the boiler. Supply mains from the boiler are inclined upward and connect with the room heat — emitting units, there being no other piping. This system is called reverse flow because the condensate flows back through the mains in a reverse direction or opposite to that of the steam flow.

The operation of a typical one-pipe, reverse-flow system is shown in Figure 8-3. Steam from the boiler flows into the main or mains (inclined upward) through the risers to the heat-emitting units at the bottom. The steam pushes the air out of the mains, ris­ers, and heat-emitting units and escapes through air valves placed at the other end of the heat-emitting units, as shown. The conden­sate forming in the units flows back to the boiler through the same piping, but in the opposite or reverse direction to the steam flow.

For satisfactory operation, every precaution should be used to install the correct pipe sizes, especially for the mains. If the piping is too small, it will be necessary to carry excess pressure in the boiler to ensure proper operation. Without the excess pressure, operation of the remote heat-emitting units will be unsatisfactory. Satisfactory operation of these remote units can be achieved without excess boiler pressure if the pipes used in the system are the correct size.

Two Radiator Load Three Radiator Load

Steam Heating Systems

The smaller the main, the greater the speed of the steam flow and the greater the resistance to the flow of condensate. It is very difficult for the condensate to flow back to the boiler against the onrushing steam in a long main that is in an almost horizontal posi­tion. The main should be inclined as much as conditions in the basement permit.

Leaky radiator valves can be a problem. When the valve does not close tightly, steam will work its way into the radiator and stop the condensate from coming out. The result is that the radiator soon fills with water, and when turned on again, there is difficulty getting the condensate out. This produces gurgling, hissing, and the more violent effect known as water hammer.

Upfeed One-Pipe System

One-pipe steam heating systems can also be of the upfeed type. In a standard upfeed one-pipe steam heating system, both the steam and the condensate travel through the same pipes, and the heat-emitting

Units are located above the supply mains (hence the name upfeed system).

An attempt to reduce the amount of condensate return in the pipes carrying the steam has resulted in a modification of the upfeed one — pipe system. In the modified system, the condensate is dripped at each radiator (and therefore from the main itself) into a wet return.

The upfeed one-pipe system shown in Figures 8-4 and 8-5 con­sists of a main or mains branching off from the boiler steam outlet and inclined downward, instead of upward as in the previously described one-pipe system. This arrangement causes the condensate in the mains to flow in the same direction as the steam.

The mains connect with return pipes, which carry all the con­densate back to the boiler. It is only in the risers that reverse flow of the condensate takes place, and accordingly they should be large enough to take care of this reverse flow without undue turbulence.

Steam from the boiler flows through the main or mains, accruing condensate with it; the condensate returns (hence the name) to the boiler at a much lower level, as shown in Figures 8-4 and 8-5. After traversing the mains, steam flows through the risers and into the radiators. Condensation takes place as the steam warms the radia­tors, and the condensate drains through the risers and return pipes to the boiler.

Steam Heating Systems


Steam Heating Systems

A distinction is made between a wet, or sealed, return and a dry return. A wet return is below the water level in the boiler, whereas a dry return is above the water level. The advantage of a wet return is that it seals and prevents steam at a slightly higher pressure from entering the return. Under most circumstances, a wet return is preferable to a dry return. The latter may be necessary to clear doorways or other openings.

There are no disadvantages to a wet return when the system is properly installed and the valves maintained in tight condition. In most installations, these requirements are often lacking, resulting in many noninherent disadvantages. Among the installation prob­lems that should be avoided are (1) pipes that are too small for the job, (2) sharp turns, (3) air pockets, (4) not enough pitch to the mains, (5) not enough air valves, and (6) air valves that are too small or cheap.

Upfeed One-Pipe Relief System

Figure 8-6 shows an upfeed, one-pipe, relief system applied to an eight-radiator installation. Connected to the main outlet A are two or more branch mains AB and AC, which supply the various risers.

Steam Heating Systems

Figure 8-6 One-pipe, under — or upfeed, relief system showing locations of dry and wet returns.

Steam is supplied to each riser, and the condensate drains in the riser in reverse direction to the steam flow.

The condensate returns from the risers to the boiler by gravity through drip pipes (or drop pipes), which are virtually continua­tions of the return pipe, so that the condensate will flow back into the boiler.

Steam (usually at 1 to 5 psig) passes from the boiler to mains AB and AC. These branches being slightly inclined, any conden­sate will drain into the drip pipes. The steam passes through the risers to the radiators, where its heat is radiated to warming the rooms, thus causing condensation. Because the risers are suffi­ciently large, the condensate is carried by gravity in a direction reverse (that is, opposite) to the direction of the steam flow, drains down the drip pipes, and returns to the boiler through the low — level (wet) return pipe.

The condensate is forced back into the boiler by the pressure resulting from the greater head of the column of condensate in the drip pipes, as compared with the head of water in the boiler. Moreover, the water in the drip pipes, being at a lower temperature than the water in the boiler, is heavier, which upsets the equilibrium of the two columns.

This system is characterized by a slight difference in pressure (pressure differential) in the various parts of the system caused by frictional resistance offered by the pipe to the flow of the steam.

The steam flow is variable in different parts of the system due to variable condensation and badly proportioned pipe sizes. This can be explained by Figure 8-6. If the water level in the boiler is at D, then in operation with wet returns, the pressure difference will be balanced by the condensate standing at different levels in the differ­ent drip pipes, as at E and F, these levels being such that the differ­ence in head and density will restore equilibrium. The effect of a wet return can be obtained with the dry return (shown at the left in Figure 8-6) by attaching a siphon to the bottom of the drip pipe. Water from the drip falls into the loop formed by the siphon; and after it is filled, it overflows into the dry return.

The water will rise to different heights, G and H, in the legs of the siphon to balance the difference in pressure at points P and P’. If the siphon were omitted and the drip pipe connected directly to the dry return, there would be a tendency for the condensate in the dry return to back up instead of draining into the boiler because the pressure in the drip pipe at P is greater than the pressure in the dry return. In general, the pressure varies because there is a gradual reduction in pressure as the steam flows from the boiler to the remote parts of the system. This is caused by frictional resistance offered by the pipe and fittings (Figure 8-7).

The steam flows into the pipes only when condensation is taking place. The variation in pressure exists, on the other hand, only when the steam is flowing in the pipes. The effects of pressure variation can be explained with the aid of Figure 8-8. With the plant in oper­ation and condensation taking place in the radiators and condensate draining into the drip pipes (suppose the pressure in the boiler is 5 psig—4 psig in drip 1 and 3 psig in drip 2), then to balance these pressure differences the water will rise in drip 1 to L 2.3 ft above the water level in the boiler, because there is a pressure difference of 1 psig (5 — 4) and the weight of a column of water 2.3 ft high is 1 lb for each square-inch of cross-section. Similarly, for drip 2, the pres­sure difference is 2 psig; hence, the water will rise in this column to an elevation above the water level in the boiler equal to 2.3 X 2, or 4.6 ft, to balance the 2-psig pressure difference. Strictly speaking, these figures are correct only when the temperature of the water in the two drip pipes is the same as the temperature of the water in the boiler. For simplicity, the difference in weight or density of the cold water in the drips and hot water in the boiler were not considered. Under these circumstances, the heavy cold water in the drips would rise to lower elevations than those shown at L and F in Figure 8-8.

Sometimes there are problems encountered on long lines with radiators located at the end of the line and near the level of the

Steam Heating Systems

Water in the boiler. On long lines where there is considerable reduc­tion of pressure, the water sometimes backs up into the radiator, as in Figure 8-9, interfering with its operation.

Steam Heating Systems

Figure 8-8 Effects of pressure variation in the different parts of a system.

Steam Heating Systems

Radiators located at elevations below the water level in the boiler may be operated by means of a steam loop (Figure 8-10). In the steam loop, the condenser element may consist of a pipe radiator placed on the floor above the boiler. The ample condens­ing surface thus provided will render the loop very active in removing the condensate, and at the same time the heat radiated from the condenser is utilized in heating. The drop leg is pro­vided with a drain cock D, and the connection to the boiler is provided with a check valve. To start the system, turn on the steam at the boiler and open D until the steam appears. The con­densation of steam in the condenser (upper radiator) will cause a rapid circulation in the riser, carrying with it the condensate from the lower radiator, which, in passing over the goose neck, cannot return but must gravitate through the upper radiator and drop leg past the check valve and into the boiler. The pipe at the bot­tom of the main riser, which acts as a receiver for the condensatie from the lower radiator, should be one or two sizes larger than the pipe in the main riser.

Downfeed One-Pipe System

The downfeed one-pipe system (also referred to as the one-pipe oversystem or simply the downfeed system) is characterized by hav­ing the heat-emitting units located below the supply mains. The condensate drips through the risers, thereby keeping the supply main relatively free of condensate.

Steam Heating Systems

Water level in boiler.

The downfeed one-pipe system is well suited for buildings 2 to 5 stories high because, if the heat-emitting units were fed from below, the risers would have to be excessively large. Instead of a steam main encircling the basement, the main is carried to the attic, forming a central riser for all the heat-emitting units. The branches in the attic correspond to the mains in the systems already described but do not carry any condensate from the heat-emitting units. These branches connect with the drops or downflow pipes that feed the heat-emitting units and drain the condensate.

As shown in Figure 8-11, steam flows from the boiler up the cen­tral upflow pipe through the branches and down through the drops or downflow pipes. Because the branches are inclined, any water or condensate in the steam drains into the upflow riser. The steam passes to the heat-emitting units through the connecting pipes. Condensate forming in the units drains through these connecting pipes into the downflow pipes. Air valves or vents are provided to rid the system of air. It should be noted that there is no reverse flow

Figure 8-11 One-pipe overhead feed system installed in a tall building.

Steam Heating SystemsOf condensate in the drop pipes. As a result, pipe sizes can be made smaller, because with parallel flow, steam velocities can be higher than in the one-pipe system.

One-Pipe Circuit System

In the one-pipe circuit system the steam main is carried entirely around the basement, taken off from the boiler by an elbow at the high point, as in Figure 8-12.

Steam Heating Systems

Note that the main must incline all the way from the high point to the low point. To allow for this inclination requires twice as much riser between high and low points as with the divided-circuit system, that is, where there are two mains taken off from a tee connection.

The one-pipe system is adapted for a rectangular building of low or moderate size. The size of the main (since all the steam flows through it) must be larger than in the divided-circuit system. However, especially in large installations, savings in piping may be made by installing a tapered main. A tapered main is one that is reduced in size along its length by connecting lengths of different­sized pipes with reducers. Eccentric reducers should be used to avoid water pockets, which would interfere with the proper drainage of the condensate. The risers are connected by being tapped from the main at various points to serve the heat-emitting units.

In a one-pipe system, the condensate drains into the main, flows in the same direction as the steam flow, and is carried to the drip pipe and then into the boiler. Since there is no return pipe as with the relief system, the circuit arrangement is less expensive to install.

Proportioning a tapered main is very important. The amount of condensate increases from the beginning to the end of the main and is considerable near the end, depending upon the number of heat-emitting units. Allowance should be made for this, and too much tapering should be avoided.

One-Pipe, Divided-Circuit Nonrelief System

The one-pipe, divided-circuit nonrelief system differs from the one just described in that there are two mains at the high point taken off by a tee as in Figure 8-13. These mains terminate in a U-shaped drip connection (LF) at the low point. Each should be vented with a quick vent as shown. Evidently each main takes care of only half the total condensate.

This steam heating system is suited to long buildings with a boiler located near the center. The end of each main is connected to a separate drip pipe connected with a common return, giving sepa­rate seals for each end.

For proper operation, these ends should not be at a lower eleva­tion than 14 in above the boiler water line. The individual seals make the two halves of the divided circuit independent, which is desirable for unequal loads. Thus, there may be considerable differ­ence between the pressure at L and F, each being what is necessary to balance the load.

One-Pipe Circuit System with Loop

The one-pipe circuit system with loop is adapted to L-shaped build­ings, a circuit being used for the main building and a loop (tapped from the circuit) servicing the wing (Figure 8-14). The mains are

Steam Heating Systems

Figure 8-13 One-pipe, under — or upfeed, nonrelief, divided-circuit system.

MAIN ——————————————————————— WING

Steam Heating Systems

Figure 8-14 One-pipe, under — or upfeed, nonrelief, circuit system with loop for L-shaped building.

Installed for the proper drainage of the condensation by providing two high points—one at the beginning of the circuit and the other at the beginning of the loop—thus giving ample margin above the boiler water line for adequate pitch in both the circuit and the loop. This is clearly shown in Figure 8-14.

The low point of the loop is higher than the low point of the circuit because the pressure at the end of the loop is less than the pressure at the end of the circuit. This requires a longer vertical drip pipe since the liquid column rises higher to balance the lower pressure.

Two-Pipe Steam Heating Systems

In two-pipe systems, separate pipes are provided for the steam and the condensate; hence they may be of smaller size than in one-pipe systems, where a single pipe must take care of both steam and con­densate. Various piping arrangements are used in two-pipe systems (e. g., circuit, divided-circuit, and loop) to best meet the require­ments of the building. Steam is supplied to the heat-emitting units through risers, and the condensate is returned through downflow or drip pipes.

Two-Pipe, Divided-Circuit System

A two-pipe, divided-circuit system is shown in Figure 8-15. In this two-pipe system, steam passes from the boiler at the high point to the mains, along which risers bring the steam to the heat-emitting

Steam Heating Systems

Units. From the opposite side of each unit is connected a drop or drip pipe. Figure 8-15 shows a wet return on the left side and a dry return on the right side.

In the wet return, the condensate is returned to the boiler by means of individual drips provided at each connection. There is a dif­ferent arrangement for the dry return. The drip pipe from each heat — emitting unit terminates in a loop or siphon, which is tapped to the dry return. In operation, the condensate gradually fills the siphons and flows over into the dry return, passing into the boiler drip pipe.

A two-pipe system should also be provided with a check valve so placed as to allow water to pass into the boiler but prevent undue outflow. Under certain conditions, this prevents the water in the boiler from being driven out into the return system by the boiler pressure. The disadvantage of a check valve is that it sometimes gets stuck, a problem that can interfere with the operation of the system. An equalizing pipe with a Hartford connection loop may be used in place of a check valve to avoid this situation (see Hartford Return Connection in this chapter).

Vapor Steam Heating Systems

A vapor steam heating system is one that commonly uses steam at approximately atmospheric pressure or slightly more, and that oper­ates under a vacuum condition without the aid of a vacuum pump (see Vacuum Steam Heating Systems and Figures 8-16 through 8-18).

The steam pressure at the boiler necessary to operate a vapor system is generally very low (often less than 1 lb), being no more

Steam Heating Systems





Figure 8-16 A vapor steam heating system. (Courtesy Dunham-Bush, Inc.)

Than is required to overcome the frictional resistance of the piping system. Under most operating conditions, the pressure at the vent will be zero or atmospheric.

Vapor steam heating systems may consist of various combinations of closed or open, upfeed or downfeed, and one-pipe or two-pipe arrangements, depending on the requirements of the installation.

Steam Heating Systems

Figure 8-17 Piping of a vapor system to a boiler with only one supply

Tapping. (Courtesy Dunham-Bush, Inc.)

Any of these combinations will have certain advantages and disadvantages.

Open (Atmospheric) Vapor Systems

A vapor system with a return line open to the atmosphere without a check, trap, or other device to prevent the return of air is some­times referred to as an open, or atmospheric, system.

An open vapor system is frequently used when the steam is deliv­ered from its source under high pressure. When this is the case, pressure-reducing valves should be installed in the system to reduce the pressure of the steam to a suitable operating level. An open vapor system is also used when there is no need to return the con­densate to the boiler (i. e., when it is wasted within the system). A condensate-return pump should be used when the system design requires the return of the condensate to the boiler.

In an open vapor system, the pressure at the boiler is 1 to 5 oz, or enough to overcome the frictional resistance of the piping

Steam Heating Systems

Figure 8-18 Piping of a vapor system to a boiler with two or more supply tappings. (Courtesy Dunham-Bush, Inc.)

System. The pressure at vent is zero gauge, or atmospheric. In oper­ation, steam is maintained at about 5 oz pressure in the boiler by the action of the automatic damper regulator. The amount of heat desired at the radiators is regulated by the degree of opening of the supply valve. Steam enters at the top of the radiator and pushes out the air through the outlet connection, which is open to the atmosphere. The condensate returns to the boiler by gravity. This system has the advantage of heat adjustment at the radiator, but the devitalizing effect in the air is somewhat greater than in the vacuum systems because the steam entering the radiators is at a higher tem­perature than the steam of lower pressure in the vacuum system. It is, however, simple.

Figure 8-19 illustrates a very simplified, mechanically controlled, vapor steam heating system. Though it does not present the latest practice, it does present control principles very plainly.

The success of a vapor steam heating system depends upon the proper working of the automatic damper regulator in keeping the boiler pressure within proper limits. To accomplish pressure regulation, the dampers are controlled by a float working in a float

Steam Heating Systems











Figure 8-19 Open atmospheric vapor system.


Chamber in communication with the water space in the boiler, as shown in Figure 8-19.

When the pressure in the boiler is the same as that of the atmo­sphere (0 psig), the water level in the float chamber (Figure 8-19) is the same as that in the boiler, and the index hand points to zero.

As steam generates, the steam pressure increases and the water level in the boiler is forced downward. The latter action causes the level in the float chamber to rise until the pressure due to the differ­ence AB (Figure 8-19) of water level balances that in the boiler.

The float, in rising, connected as it is by pulleys and chains to the dampers, closes the ashpit damper, thus checking the draft and pre­venting a further increase of steam pressure.

In this system, the steam feed is connected to the top of the radia­tors and the air and condensation is taken from the bottom because steam is lighter than either air or condensation. Accordingly, when steam is admitted, it floats on top of the air, thus driving the air out through the lower connection.

The chief feature of a vapor system is that the amount of heat given off by each radiator may be regulated by the steam valve. Thus, in Figure 8-19, the valve of radiator C is opened just a little, which will admit only just enough steam to heat a larger portion of the radiator; with the valve wide open on E, the entire radiator is heated.

Steam Heating Systems

Figure 8-20 Steam entering top of radiator and pushing air out the bottom.

The kind of radiator used is the downflow type in which steam enters at one end at the top and the air and condensation pass out at the other end at the bottom.

As steam enters a cold radiator, it forces the cool air in the radiator out through the trap into the return piping. The operation of a typical downflow radiator is shown in Figures 8-20 through 8-22. Figure 8-20 shows the steam entering and air passing out through thermostatic retainer valve. Figure 8-21 shows more steam entering and conden­sate and the balance of the air passing out through the trap, the action progressing until (as in Figure 8-22) the radiator is full of steam.

As the radiator warms up, the steam gives off heat and con­denses. The condensate, being heavier than steam, falls to the bot­tom of the radiator and flows to the trap through which it passes into the return piping. After the air is forced out, the steam fills the radiator and follows the condensate to the trap. The trap closes when the steam enters it because the steam is hotter than the water. This excess heat expands the valve control element, closing and holding the valve against its seat with a positive pressure, thus pre­venting the steam from flowing into the return piping (Figure 8-23).

The trap closes once the radiator is completely filled with steam, and heat is given off as the steam condenses. The condensate thus formed, which is cooler than the steam, flows in a steady stream to

Steam Heating Systems

Figure 8-21 Condensate and air leaving the radiator.

Steam Heating Systems Steam Heating SystemsThe trap, which it slightly chills, causing it to open and allowing the condensate to pass out into the return piping (Figure 8-24).

When properly working, the trap adjusts itself to a position corresponding to the temperature of the condensate, just as a

Steam Heating Systems

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Figure 8-23 Progressive action of thermostatic check or trap from beginning of air entrance to closing of valve by expansion of actuating element.

Thermometer does to the room temperature, and permits a continu­ous flow of condensate from the heat-emitting units (Figure 8-25).

Closed Vapor Systems

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Figure 8-24 Progressive action of thermostatic check or trap closed position to open position due to contraction of actuation element when chilled by relatively cool condensate.

подпись: figure 8-24 progressive action of thermostatic check or trap closed position to open position due to contraction of actuation element when chilled by relatively cool condensate.A vapor steam with a return line closed to the atmosphere is some­times referred to as a closed vapor system. The condensate returns by gravity flow to a receiving device (an alternating receiver or

Steam Heating Systems

Figure 8-25 Detail of trap showing the valve in intermediate position for ideal continuation flow of condensate.

Boiler-return trap), where it is discharged into the boiler. The con­densate from the alternating receiver is discharged against the boiler pressure.

Because air cannot enter a closed vapor system, a moderate vac­uum is created by the condensing steam. As a result, steam is pro­duced at lower temperatures, and the system will continue to provide heat after the boiler fire has died down.

Figure 8-26 shows the arrangement of a typical upfeed, two-pipe vapor system with an automatic return trap. The heat-emitting units discharge their condensate through thermostatic traps to the dry return pipe. These systems operate at a few ounces of pressure and above, but those with mechanical condensation return devices may operate at pressures upward of 10 psi. The simplest method of venting the system consists of a 3/4-in pipe with a check valve open­ing outward. Most systems employ various forms of vent valves, which allow air to pass and prevent its return. A dry return is pro­vided so that the air will easily go out the vent pipe.

Vacuum Steam Heating Systems

A vacuum heating system is one that operates with steam at pres­sures less than that of the atmosphere. The object of such systems is to take advantage of low working temperatures of the steam at
these low pressures, giving a mild form of heat such as is obtained with hot-water heating systems. Vacuum systems such as these, which operate at all times at pressures less than atmospheric, should not be confused with the combined atmospheric and vac­uum systems described in the next section. There are distinct design differences between the two systems.

A distinction should be made between a vacuum system and a subatmospheric system. The latter differs from an ordinary vacuum system in that it maintains a controlled partial vacuum on both the supply and return sides of the system instead of only on the return side. In the vacuum system, steam pressure above that of the atmo­sphere exists in the supply mains and heat-emitting units practically at all times. The subatmospheric system is characterized by atmo­spheric pressure or higher existing in the steam supply piping and heat-emitting units only during severe weather (Figures 8-27 and 8-28).

There are a number of different methods of classifying vacuum systems (e. g., one-pipe or two-pipe and vacuum pressure or subat — mospheric). For the purposes of this chapter they will be classified according to the type of vacuum: natural vacuum systems and mechanical vacuum systems.












DEVICE________ __











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HARTFORD RETURN CONNECTION Figure 8-26 Two-pipe, upfeed vapor system with automatic return trap.


подпись: 212

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Figure 8-27 A typical variable-vacuum steam heating system installation. (Courtesy Dunham-Bush, Inc.)


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Figure 8-28 A subatmospheric steam-heating system. (Courtesy Nash Engineering Co.)

Natural Vacuum Systems

Any standard one — or two-pipe steam system may be converted into a natural vacuum system by replacing the ordinary air valve with a mercury seal or connecting thermostatic valves to the radiator return outlet on radiators and providing a damper regulator on coal-burning boilers adapted to vacuum working. The mercury-seal system is shown in Figure 8-29.

A mercury seal is virtually a barometer, consisting, as shown in Figure 8-30, of a tube (A) that dips just below the surface of the mercury in a cup (B). When the steam is raised in the boiler to pressures above atmospheric, it drives all air out of the system, the air leaving by bubbling through the mercury in cup B.

If the fire is allowed to go out, the steam will condense and pro­duce a vacuum, provided all pipe fitting has been carefully done and the valve stuffing boxes are tightly packed.

In Figure 8-29 the loop at C prevents water from being carried over into the seal pipe when purging the system of air. If air should again enter the system, it can be expelled by raising the steam pres­sure above atmospheric. In very cold weather, the system can be operated at pressures above atmospheric by closing valve D. When fires are banked for the night, valve D may be opened and the system worked as a vacuum system. The flexibility of vacuum sys­tems is in sharp contrast with low-pressure systems, where steam

Steam Heating Systems








подпись: air -10 Li. o 00 LU

5 o

I lJILuj

J l…………… I *•> I

Figure 8-30 Operation of mercury seal.

Disappears from the radiators as the temperature drops below 212°F. According to weather demands, the radiators may be kept at any temperature from, say, 150 to 220°F.

Another method of maintaining a natural vacuum is by using thermostatic valves instead of a mercury seal. A thermostatic valve has an expansion element that operates to close the valve when heated by hot steam and to open the valve when chilled by the rela­tively cold condensate.

The two kinds of thermostatic valves used are the single-unit (or retainer) valves and two-unit, or combined retainer and air-check, valves sometimes called master thermostatic valves.

Figure 8-31 shows the details of a master thermostatic valve, consisting of a thermostatic unit and an air check. The thermostatic unit has an expanding element, the air check, consisting of a group seat poppet check valve that is practically airtight and therefore will retain the vacuum within the system for a considerable length of time. The air check operates when excess pressure is generated in the boiler to purge the system of air, the check at other times remaining closed.

The thermostatic valve remains open while the system is being purged of air and condensation but closes when steam enters the valve chamber—it retains vacuum in the air line.

Figure 8-32 shows the operation of the natural vacuum system with retainer and master thermostatic valves. Individual thermo­static retainer valves A, B, and C are placed in the outlet of each radiator, which pass air or water but close to steam. At the end of the air line is a master thermostatic valve (D), which operates when the system is purged of air by excess pressure.

Steam Heating Systems

Figure 8-31 A natural vacuum system with retainer and master thermostatic valve showing sectional views of the valves.

The drip should be proportioned to prevent water entering the air line in case of high vacuum in such a manner that the vertical distance M between the water level in the boiler and the lowest point of the air line is not less than 2 ft for each inch of vacuum to be carried in the system.

The successful operation of natural vacuum systems depends largely on efficient damper regulators on coal-burning boilers (i. e., efficient draft control), for unless the fire is held in proper check, the pressure will rise and break the vacuum. This can waste fuel, for there may be sufficient heat in the boiler to supply steam to the system with a 5- or even 10-in vacuum and hold that heat in the system for hours.

Automatic damper regulators are designed to act by pressure, temperature, or a combination of these two. Figure 8-33 shows a regulator that acts on the pressure principle. It consists of a diaphragm connected at B to a lever having its fulcrum at A and having a weight W free to slide along a slot between the stops.

In starting, the weight is placed on the left side of the lever as shown (Figure 8-33). This tilts the lever (position LF) and opens the

Steam Heating Systems

Figure 8-32 Natural vacuum system with retainer and master thermostatic valves.

Damper. The weight is adjusted by the stop so that sufficient pres­sure is produced to clear the system of air before the regulator tips to position L’F (shown in dotted lines) and closes the damper.

The regulator is gradually closing as the pressure comes on. When the regulator is entirely closed, the weight slides to the right and remains in this position until the vacuum in the system becomes strong enough to gradually open the damper—just enough to maintain a vacuum.

In the morning, the regulator may be set to the open position from the floor above by the pull chain M. This generates pressure and purges the system of any air that might have accumulated; then the regulator weight automatically goes to the vacuum side of the regu­lator and maintains the vacuum heat until more fuel is required or

Steam Heating Systems

Figure 8-33 Damper regulator for natural vacuum system operating on the pressure principle.

Further regulation is necessary. Temperature controls or damper regulators that depend on temperature changes for their operation may also be used (Figure 8-34). Since the temperature of steam increases with the pressure, evidently the expansion and contrac­tion of a rod exposed to the steam can be made to operate the damper.

Figure 8-34 shows the construction of a typical thermostatic reg­ulator. The expansion element or rod is fastened at A in a closed cylindrical chamber through which steam from the boiler passes to the main. The end B is free to move, passing out of the chamber through a stuffing box. The motion of the rod is considerably mag­nified by the bell crank lever, which is connected to the damper by a chain attached at C.

In operation, as the pressure of the steam rises, so does its tem­perature; the rod (which is made of a metal having a higher coeffi­cient of expansion than that of the cylindrical chamber) expands, and its free end, B, moves to the right, thus causing end C of the lever to descend, closing the damper.

When the pressure falls, the rod contracts, and the spring that keeps the bell end in contact with the rod causes end C of the lever to rise and open the damper.

The lever will assume some intermediate position in actual oper­ation, thus holding the steam at some predetermined pressure, which may be varied by means of the screw adjustment (D).

Steam Heating Systems

Figure 8-34 Damper regulator for natural vacuum system operating on the temperature principle.

The major objection to regulation by temperature is that there is no provision for securing excess pressure to purge the system of air in starting. This must be done by hand control of the damper. This objection can be overcome by the method of combined pressure and temperature regulation, which employs pressure for starting and temperature for running. In starting, the thermostatic portion of the regulator is closed off from the systems during which pressure is gen­erated sufficient (about 1 pound) to purge the air from the system. After purging, the regulator automatically opens a valve to the ther­mostatic position, which then maintains the temperature desired, its range embracing both vacuum and low-pressure operation.

Mechanical Vacuum System

A mechanical vacuum system is one in which an ejector or pump is used to maintain the vacuum. The hookup of the ejector system is shown in Figure 8-35. The ejector, which may be operated either by steam or water, is started before steam is turned on in the sys­tem. Thus, after the air is removed, steam quickly fills the heat — emitting units, because the air is automatically removed as fast as it accumulates.

The system commonly used in exhaust heating employs an alleged or so-called vacuum pump, which ejects the air and condensate from


Steam Heating Systems

The system. In operation, this device pumps out most of the air (or other gas) from the condenser, maintaining a partial vacuum. The pump cannot obtain a perfect vacuum because each stroke of the pump piston or plunger removes only a certain fraction of the air, depending on the percentage of clearance in the pump cylinder, resis­tance of valves, and so forth. Hence, theoretically an infinite number of strokes would be necessary to obtain a perfect vacuum (not considering resistance of the valves, clearance, etc.).

Condensation of steam creates the vacuum, and the pump that removes the air maintains a vacuum. A wet pump (that is, one that removes both air and condensate) is the type generally used. A dry pump removes only the air.

The essential features of a mechanical vacuum pump system are shown in Figure 8-36. This system is of the fractional valve distribution type. In operation, air, being heavier than steam, passes off through thermostatic retainer valves to the pump. When the steam reaches the retainer valves, they close automatically to


Steam Heating Systems

Figure 8-36 Mechanical vacuum air-pump system as applied to fractional valve distribution.

Prevent the steam passing into the dry return line to the pump and breaking the vacuum. The condensate is pumped from the receiver back into the boiler by a feed pump and passes on its way through a feed water heater, where it is heated by the exhaust steam from the air and feed pumps.

Combined Atmospheric Pressure and Vacuum Systems

A combined atmospheric and vacuum system works at pressures in the boiler from 1 to 5 oz of gauge pressure (that is, above atmo­spheric). This pressure is needed on coal-burning installations to operate the damper regulator.

The desired vacuum in the heat-emitting units is obtained by throt­tling the supply steam with the unit feed valves. The working principle of this system is shown in the elementary sketch found in Figure 8-37. In operation, when steam is raised in the boiler, it passes through the steam main risers and supply valves to the heat-emitting units.

Steam Heating Systems

The proper working of this system is obtained by an automatic device or trap that closes against the pressures of either steam or condensation and allows air, but not the steam, to pass out. The trap (Figure 8-37) consists of three elements: (1) a diaphragm valve (point L), (2) a float valve (point A), and (3) a thermostatic valve (point F). Connection R, in Figure 8-37, connects the steam outlet of the boiler to the diaphragm. When there is no pressure in the boiler, diaphragm valve L is held closed by the spring.

When the fire in the boiler is started and the air in the boiler expands, the diaphragm is inflated and moves the valve spring to the right (against the action of the spring), which opens valve L. This makes a direct opening through float valve A and thermostatic valve F, thus opening the system to the atmosphere. Valve L remains open as long as there is a fraction of an ounce of pressure on the boiler. When steam forms and passes through the system, it drives all the air out of the system through the three open valves (L, A, and F).

The heat of the steam causes the expansion element of valve F to expand and close the valve; thus the system is filled only with steam.

The vacuum is obtained on the principle that the steam admitted to the radiators condenses while giving off heat through the radia­tor walls. This causes a tremendous reduction in volume of the steam remaining in the radiators, resulting in a pressure that is less than atmospheric in radiators; i. e., a vacuum is formed. The steam condenses because of a reduction in temperature below that corre­sponding to the pressure of the steam.

When the radiator gives off heat in heating the room, the tempera­ture of the steam in the radiator is lowered. This reduction in temper­
ature causes some of the steam to condense in a sufficient amount to restore equilibrium between temperature and pressure of the steam.

The pressure falls because the temperature falls. If a closed flask containing steam and water is allowed to stand for a length of time, the atmosphere being at a lower temperature than that inside, the flask will abstract heat from the steam and water, but the heat will leave the steam more quickly than it leaves the water. The result is a continuous condensation of the steam and reevaporation of the water, during which process the temperature of the whole mass and the boiling point are gradually lowered until the temperature inside the flask is the same as that outside. This process is accomplished by a gradual decrease in pressure. Figures 8-38 through 8-40 illustrate the effect of pressure on the boiling point.

A one-pipe system may be converted into a combined system by replacing the air vent on the heat-emitting units and making the piping absolutely tight. In this conversion, a compound gauge recording both pounds of steam and vacuum inches is required (Figure 8-41).

Exhaust-Steam Heating

The term exhaust-steam heating relates to the source of the steam rather than to its distribution. In fact, after the exhaust steam enters the heating system, its action is no different from that of live steam


подпись: equilibrium between pressure
Figure 8-38 Equilibrium between the steam and the water. Equalized temperature.

Figure 8-39 Reducing the pressure by applying cooling water to the closed vessel. Reduction of pressure causes the water to boil.



подпись: cooling water
re-establish equilibrium
Taken from a heating boiler, because it is adapted to both low — pressure and vacuum systems.

The chief difference between exhaust systems and those already described are the provisions for delivering steam from the engine to the heating system free from oil and at a constant pressure and for

Steam Heating SystemsFigure 8-40 Equilibrium reestablished between pressure and temperature.

5- lb Gauge 51/2 in Vacuum

Steam Heating Systems

Figure 8-41 Compound gauge used on combined atmospheric — vacuum systems.

Returning the condensate to the high-pressure boiler (Figure 8-42). Figure 8-43 shows the essential features of an exhaust-steam heat­ing system having fractional control vacuum distribution.

The necessary devices between the engine and the inlet to the heating system are: (1) the oil separator, (2) the trap, (3) the back pressure valve, and (4) the pressure-regulating valve. In addition, for mechanically producing the vacuum and returning the conden­sation to the high-pressure boiler, an air pump, receiver with vent, and feed pump are required. A feed water heater should also be provided both for economy and to permit returning the condensa­tion and makeup feed water to the boiler at the proper temperature.

In operation, exhaust steam from the engine first passes through the heater and then through the oil separator, which frees it from the lubrication oil, the latter passing off into the oil trap. The steam now enters the heating system at A, its pressure being prevented from ris­ing above a predetermined limit by the back pressure valve (regu­lated by weight B) and maintained at a predetermined constant pressure by the pressure-regulating valve (adjusted by weight C).

The pressure-regulating valve is, in fact, an automatic steam “make-up” valve, which admits live steam to the heating system when the exhaust is not adequate to supply the demand, thus “mak­ing up” for this deficiency and maintaining the pressure constant.

The condensate and air are removed from the system at D by a wet pump (as distinguished from a dry pump, which removes only



подпись: oil separat or
Steam Heating SystemsBackpressure VALVE









To a

подпись: to a

Distribution showing application

подпись: distribution showing application

With vacuum

подпись: with vacuumSETTLING CHAMBER steaM TRAP

Figure 8-42 Typical exhaust-steam heating system power plant containing an open feed water heater.


Steam Heating Systems

Figure 8-43 Exhaust-steam heating system with fractional valve control and automatic makeup.

The air). The condensate and air are discharged into a receiver, from which the air passes off through a vent and the condensate is pumped by a feed pump back into the boiler after passing through a feed water heater.

There is a continued loss of water through various leaks; the feed pump inlet (alleged suction) is connected at E, with the supply from the street main or other source, the amount entering the system being controlled by the make-up valve.

The construction of a typical regulating valve and its connec­tions are shown in Figure 8-44. The valve is controlled by means of governing pipe A (Figure 8-44), connecting the diaphragm chamber to the accumulator, the latter being connected to the heating main at the point from which the pressure regulator is to be governed.

The accumulator is always half full of water, and its elevation must be such that the water line in the accumulator is level with the diaphragm so that there will not be an unbalanced column of water to exert pressure on the diaphragm.

The water is provided to protect the diaphragm from the steam, the pressure of the water being transmitted from the surface of the water in the accumulator to the diaphragm.

The working principle of the regulating valve and its connected devices is relatively simple. In operation, when the exhaust side F is at the predetermined pressure, this brings sufficient force against the under, or water, side of the diaphragm to overcome the downward thrust due to the adjustable weighted level and close the valve.

If the engine slows down or there is heavy demand for heat, so that the exhaust steam is not adequate to supply the demand, the pressure in the exhaust side F will fall, and the downward thrust of the adjustment weight will overcome the opposing pressure of the water on the diaphragm and open the valve, admitting live steam from the boiler side L into the exhaust side F in sufficient quantity to restore the pressure.

The inertia of the water in the accumulator acts as a damper to prevent oversensitiveness of the valve or hunting (i. e., the behavior of any mechanism that runs unsteadily, oscillating either too far or too little in an attempt to adjust itself to momentary fluctuations of pressure or other conditions that cause this action).

The spring under the diaphragm acts to balance the downward thrust of the lever and hold the valve in closed position when the pressure is the same on both sides of the diaphragm.

The back pressure valve is used to prevent exhaust pressure exceeding a predetermined limit. This is virtually a lever safety valve designed to work at very low pressure. Some back pressure valves are so light that they will open or close with a variation of only 2 oz. The position of the weight on the lever, whose fulcrum is at F (Figure 8-45), determines the exhaust pressure at which the valve will open.

Proprietary Systems

Over the years, a number of automatic heating systems have been designed and patented by manufacturers of steam heating equip­ment. Because these heating systems are protected by patent, they are referred to by the name of the manufacturer. Three of the most popular of these proprietary steam heating systems are:

1. The Trane vapor system

2. The Dunham differential system

3. Webster moderator systems

The Trane vapor system, illustrated in Figure 8-46, is a combined atmospheric and natural vacuum system installed for residential heating. In this system, the air and water return runs in


Steam Heating Systems

Figure 8-44 Pressure-regulating valve and accumulator for maintaining a constant pressure in steam heating main of an exhaust-steam heating system.


подпись: 229

Steam Heating Systems

The same direction and is practically the same length as the supply main. When properly worked out, this feature gives the same effect as though each convector radiator were placed at an equal distance from the boiler. This tends to synchronize the heating effect of all the heat emitting units in the system.

When the fire is started in the boiler of the Trane system, the water becomes heated and steam is formed, which flows through the supply main and enters the radiators, displacing the air, which is heavier than the steam. The air and condensate drain from the radi­ators through radiator traps and return piping to a point near the boiler where the air is exhausted through the quick vents and float vents at the end of the steam and return mains. The water is returned to the boiler by the direct-return trap.

As the rooms become warm, less steam is condensed and the pres­sure in the boiler begins to rise. The rising pressure causes the damper regulator to operate, closing and opening the drafts and maintaining just the amount of pressure necessary for proper heating.

When the fire in the boiler becomes lower, condensate forms, air is prevented from entering the system, and a vacuum is created. As a result, the operation changes from atmospheric to natural vac­uum, hence the name combined atmospheric and natural vacuum system. The reduced pressure of the vacuum allows the water to boil and furnish steam to the radiators at a lower temperature, a decided economy when the fire is low.

The boiler connections for the Trane system are shown in Figure 8-47. It is very important that the steam connection to the return


Steam Heating Systems

Figure 8-46 The Trane vapor heating system, a combined low-pressure and natural vacuum steam heating system.


подпись: 231


подпись: 232

Steam Heating Systems

Figure 8-47 Recommended boiler connections for the Trane vapor system.


Trap be taken from the steam space of the boiler and not from the supply piping of the header. The top of the direct-return trap must be placed at least 22 in above the water line of the boiler. In no case should the top of the trap be less than 4 in below the air and water return main. The dimensions from the water line to the end of the mains and the top of the return trap are the minimum allowable. Greater clearance above the water line should be employed where possible. When the ends of the steam and return mains occur in remote parts of the building away from the boiler, use the connec­tions shown in Figure 8-48. The ends of such mains must always be vented before dropping to the wet return. The vent pipe must be installed from the piping below the trap up to the return main when a wet return is used. If a dry return is used, the vent pipe may be omitted.

Steam Heating Systems

Figure 8-48 Pipe connections for use where steam and return mains drop to wet return in remote part of building.

Steam Heating Systems

Figure 8-49 Connections for dripping steam main where it rises to a higher level.

If desired, a Hartford connection may be used between the inlet to the boiler and the return connections from the steam and return mains (see Hartford Return Connection in this chapter).

The recommended size for vertical piping immediately below the float vents and the quick vents is lVi-in pipe at least 6 in long. This affords a separate chamber for air and water.

The connections for a dripping steam main that rises to a higher level are illustrated in Figure 8-49. The trap and strainer may be omitted, provided the lower main is at least 18 in above the water line and the return is connected directly into the return header of the boiler.

When the motor-operated steam valves are used on the mains, the boiler connections should be arranged as shown in Figure 8-50. An equalizer line is required to equalize between the steam main and the return main when a vacuum forms in the former after the motor-operated valve closes. A swing check valve prevents the flow of steam into the return main (Figure 8-51). Figure 8-52 shows the arrangement of boiler connections when air is eliminated from the return main through the No. 9 vent trap. Sometimes reversed circu­lation resulting from rapid condensation of steam will tend to cre­ate greater vacuum in the steam main than in the return main. This can be prevented by using the boiler connections illustrated in Figure 8-49.

Steam Heating Systems

Figure 8-50 Pipe connections when motor-operated steam valve is used on mains.

Steam Heating Systems


Steam Heating Systems




Figure 8-52 Connection when air is eliminated from the return main through the vent trap No. 9.

A typical convector radiator used in the Trane system is shown in Figure 8-53. It is equipped with an angle valve and an angle trap having horizontal laterals below the floor. Connections for Trane convector radiators are illustrated in Figure 8-54. The upper unit shows a vertical trap and a straightway valve concealed within the convector radiator. The lower unit is equipped with an angle valve and trap with a downfeed riser dripped through the angle trap.

The Dunham differential vacuum heating system is a simple two-pipe, power, vacuum (air-pump) return system working nor­mally at pressures below atmospheric (subatmospheric) in the sys­tem and employing orifice supply valves on the radiators. The principal advantage of the Dunham system is that it continuously distributes heat at a variable rate equal to heat losses from the structure. This is accomplished by reducing the capacity of the sys­tem by decreasing the volume temperature of the steam.

Steam Heating Systems



Figure 8-53 Trane convector radiator with angle valve and angle trap having horizontal laterals below floor.

Standard radiators, pipe fittings, and boiler connections are used in the Dunham system. The principal features of the Dunham sys­tem that distinguish it from other heating systems are:

1. The traps and valves

2. The condensation pump

3. The controller

Each of these components is differentially controlled; that is, each is actuated by a pressure differential (the difference in pressure in the supply piping and the pressure return piping). The Dunham system distributes the steam proportionately to all radiators, the pressure range being from 2 psig to pressures considerably lower than atmospheric. Because of the relatively constant differential in pressures between the steam and return lines, the radiators are filled with steam.

A condensate pump is connected to the differential controller and to the supply and return piping. The controller starts the pump when the pressure difference between the supply and return piping tends to fall or disappear, and stops it when the pressure differential is restored. The condensate pump is a wet air pump that handles both air and condensate.

Steam Heating Systems

Figure 8-54 Typical connections for Trane convector radiators.

The thermostatic radiator traps are actuated by temperature changes within the radiator. Drip traps are installed at drip points to which large volumes of condensate flow. These are combined thermostatic and float traps. A control valve regulates the admis­sion of a continuous flow of steam into the heating main.

Webster moderator systems are special control systems used pri­marily with two-pipe vapor, vacuum, and vented return systems or in modified form with some one-pipe steam heating systems. These

Webster moderator systems are controlled by the weather with an automatic outdoor thermostat. One or more hand-operated variators can be used to adjust the outdoor thermostat. The two moderator control systems produced by Webster are based on the following operating principles:

1. Continuous steam flow

2. Pulsating steam flow

The continuous steam flow arrangement is called the Webster electronic moderator system (Figure 8-55) and is suitable for medium to large buildings requiring one or more control valves. This system consists of the following basic components:

1. Outdoor thermostat

2. Main steam control valve

3. Variator

The steam supply may be taken from a high — or low-pressure boiler or any other source. The initial pressure should not be over 15 psig, using a reducing valve if necessary. The return piping may be either open or closed.

The moderator control regulates the pressure difference and will function equally well regardless of whether the pressure in the return piping is atmospheric or below. The main steam-control valve is adjusted automatically by the moderator control, which acts to reverse the direction of the motor, causing it to move the valve in the closing direction when less steam is required and in the opening direc­tion when more steam is required. The outdoor thermostat automat­ically varies the steam flow in accordance with changes in outdoor temperature. Depending upon the outdoor temperature, the ther­mostat automatically selects the position of the main steam-control valve. Its position may be advanced or reduced by the variator to give more or less steam than is called for by the outdoor temperature.

Changes in pressure difference in the heating system are auto­matically compensated for by a pressure-actuated mercury tube in the control cabinet. One end of the tube is connected to the steam supply mains and the other end to the return main. If the supply pressure is unduly increased, mercury rises in the tube to unbalance resistances contained therein and the main steam valve begins to close in amount sufficient to balance resistances. A reverse action takes place when the pressure difference falls below that called for by the control equipment.


Steam Heating Systems

Figure 8-55 Typical arrangement of Webster E-5 electronic moderator system.

The pulsating steam flow arrangement is called the Webster mod­erator system. This system comprises a central heat control of the pul­sating flow type for new or existing steam or hot-water systems. It is designed for small- and medium-size buildings and for zoning of large buildings. It directly controls the operation of burner, stoker, blower, or draft damper motors. The basic components of this system are:

1. Outdoor thermostat

2. Pressure difference controller

3. Control cabinet

4. Capillary tubing

These four components work together to open and close a valve in the steam main or to start and stop the automatic firing device at the boiler, generally through a relay. Figure 8-56 shows the general arrangement of a Webster EH-10 moderator system for a building served by a central station or street steam. A Webster EH-10 mod­erator system used for controlling a burner on a steam boiler is illustrated in Figure 8-57.

The pressure difference controller maintains the correct pressure difference between supply and return piping. In combination with metering orifices, this device gives an even distribution of the steam to all radiators in the system, thereby preventing over — or underheating.

Control is accomplished by varying the length of intervals during which steam is delivered to the radiators. These intervals are longest in cold weather and shortest in mild weather. The timing is such that the longest off interval is comparatively short so that heat output from radiators is practically continuous.

The timing mechanism inside the control cabinet is powered by a synchronous motor, which turns a cam. Timing gears between motor and cam set the length of the operating cycle. Rising on the cam is a roller connected to the arm of a switch. When the roller is on the high part of the cam, the switch is in the position for open­ing the control valve or starting the firing equipment. When the roller is on the low part of the cam, the switch is on the position for closing the control valve or stopping the firing equipment. The length of the on interval is changed automatically by the outdoor thermostat or by adjusting the variator by hand. The average length of the cycle is 30 minutes. Other gears can be furnished for cycle lengths from 12 to 60 minutes.

A variator is included in the control cabinet for manual adjust­ment of the rate of heat delivery to the building. The variator



goose neck





Locate in accessible position preferably on north wall of building.

Do not place near warm currents of air.






SERVICE 110 V 60 C ‘


Steam Heating Systems


All wiring and complete installation to be made in accordance with local electrical codes and electrical specifications.

All wiring to carry 24 volts unless otherwise noted.

Electric contractor to furnish necessary cutouts, service switches, and, where required, box for transformer.








Steam Heating Systems





Steam Heating Systems Steam Heating Systems Steam Heating Systems





Figure 8-56 Typical arrangement of Webster EH-10 moderator system in a building served by a central station or street steam.

Changes the relationship of the switch lever and roller to the cam by moving the cam itself. This is accomplished by mounting the motor, cam, and gear train on a movable carriage.

High-Pressure Steam Heating Systems

High-pressure steam heating systems operate at pressures above 15 psig (generally in the 25- to 150-psig range) and are usually found in large industrial buildings in which space heating or steam process




SERVICE 110 V 60 Hz

Steam Heating Systems







Steam Heating Systems



□ □

OUTDOOR THERMOSTAT Locate in accessible position, preferably on north wall of building Do not place near warm currents of air.

All wiring and complete installation to be made in accordance with local electrical codes and electrical specifications. All wiring to carry 24 v. unless otherwise noted. Electrical contractor to furnish necessary cutouts, service switches, and, where required, box for transformer

EH-10 /





1. y









X^iTEST / R-14A

/ / 1 CON — 4—)X







SERVICE 110 V 60 Hz 20 WATTS



NOTE: Dotted lines indicate wiring in accordance with burner manufacturer’s requirements.


Steam Heating Systems Steam Heating Systems


Figure 8-57 General arrangement of the Webster EH-10 moderator system controlling a burner of a steam boiler.

Equipment (e. g., water heaters and dryers) is used. An example of a typical two-pipe, high-pressure steam heating system is shown in Figure 8-58. This system is also referred to as a medium-pressure steam heating system when the steam pressures are in the lower ranges.

Steam Heating Systems

Figure 8-58 A typical two-pipe, high-pressure, steam heating system.

(Courtesy Dunham-Bush, Inc.)

One advantage of this system is that the high pressure of the steam permits the use of smaller pipe sizes. The high steam pressure also makes possible the elevation of the condensate return lines above the heating units, because the return water can be lifted into the return mains. High-pressure condensate pumps and thermo­static traps are commonly used in these systems to handle the con­densate and return it to the boiler.

High-pressure steam heating systems are more expensive to operate and maintain than low-pressure systems. In most cases, a licensed stationary engineer must be hired for the installation—a factor that tends to increase the operating cost.

Steam Boilers

The boiler is the source of heat for a steam heating system, and it will operate on a number of different fuels; however, regardless of the fuel used, the operating principle will be the same. Water is

Heated until it boils and changes to steam. The steam is then dis­tributed to the heat-emitting units throughout the structure either by natural or mechanical means.

Low-pressure steam heating boilers are used in residences and small buildings. The design and construction of these boilers are very similar to the boilers used in hot-water heating systems in the same size. Both boilers are described in considerable detail in Chapter 15 (“Boilers and Boiler Fittings”).

To size a steam boiler, first measure the height in inches of one cast-iron radiator in the system. Then, count the number of sections and the number of tubes or columns (Figure 8-59). The sections are the divisions or separations of a cast-iron radiator as seen when standing directly in front of it. When you look at the radiator from its narrow end, you can see that each section consists of one or more vertical pipes.

Steam Heating Systems

Every cast-iron radiator is made up of individual sections, which when joined together, form a single radiator module.

Table 8-1 Abbreviated Column-Type Radiator EDR Chart (Number of Columns and Height)







1 Column

2 Columns

3 Columns

4 Columns














These vertical pipes are called columns in the traditional cast — iron radiators and are 21i2 inches wide. In newer radiators, they are called tubes and are only 1V2 inches wide.

Find the square foot Equivalent Direct Radiation (EDR) of one section of the radiator from Table 8-1. Multiply that figure by the number of sections in the radiator to arrive at the square foot EDR rating of that radiator. Multiply the square foot EDR rating by 240 Btu per hour to obtain the heating capacity of that one radiator. Calculate the design heating capacity for each of the remaining radiators in the heating system. The sum of the design heating capacities of all the radiators is the total radiation heating demand on the boiler.

Control Components

The controls used to regulate steam boilers and ensure their safe and efficient operation are similar in most respects to those used with hot-water boilers. A list of the principal controls used with steam boilers includes:

1. ASME safety valve

2. Steam pressure gauge

3. Low-water cutoff

4. Boiler water feeder

5. High-limit pressure control

6. Water gauge glass

7. Water cocks

8. Primary control (burner mounted)

9. Operating control (with tankless heater)

These and other boiler controls are described in considerable detail in Chapter 15 (“Boilers and Boiler Fittings”), Chapter 4 of Volume 2 (“Thermostats and Humidistats”), and Chapter 9 of Volume 2 (“Valves and Valve Installation”).

Hartford Return Connection

A well-designed steam heating system will have a Hartford return connection (Figure 8-60) in the condensate return line. The pur­pose of the Hartford return connection (or Hartford loop, as it is sometimes called) is to prevent excessive loss of water for the boiler when a breakdown (such as a water leak) occurs in the return line.

As shown in Figure 8-60, an equalizer connects the lower outlet to the steam outlet. The Hartford connection is taken off from the equalizer an inch or two below the normal water level. Evidently low pressure can only draw water out of the boiler connection. This gives a low water level, and it cannot recede further because steam will flow into the connection leg as the water recedes in the leg due to pressure difference.

More details about different boiler connections are found in Chapter 15, “Steam and Hot Water Space Heating Boilers,” and in Chapter 9, “Valves and Valve Installations” in Volume 2.


Steam Heating Systems

Pipes and Piping Details

A steam heating system requires careful planning of the piping to ensure both an efficient and a safe operation. For example, the pipe material used (e. g., wrought iron or Schedule 40 black steel) is important because the capacity of a particular weight pipe will depend on both its size and the material from which it is constructed.

The expansion of pipes when they become heated is another fac­tor that must be considered when designing a steam heating system. Sufficient flexibility in the piping can be provided for by correctly designed offsets, slip joints or bellows, radiators and riser runouts, U-bends, or other expansion loops. The pitch of connections from risers must be sufficient to prevent the formation of water pockets when pipe expansion occurs.

Pipe materials, pipe sizes (and pipe-sizing methods), pipe ex­pansion rates, pipe fittings, and piping details, such as wet and dry returns, drips, and connections to heat-emitting units, are covered in Chapter 8 of Volume 2 (“Pipes, Pipe Fittings, and Piping Details”).

Steam Traps

A steam trap is an automatic device installed in a steam line to con­trol the flow of steam, air, and condensate. In operation, it opens to expel air and condensation and closes to prevent the escape of steam. All steam traps operate on the principle that the pressure within the trap at the time of discharge will be slightly in excess of the pressure against which the trap must discharge.

The principal functions of steam traps in steam heating sys­tems include (1) draining condensate from the piping system, radiators, and steam processing equipment; (2) returning conden­sate to the boiler; (3) lifting condensate to a higher elevation in the heating system; and (4) handling condensate from one pres­sure to another.

Steam traps may be classified on the basis of their operating principles as follows:

1. Float traps

2. Bucket traps

3. Thermostatic traps

4. Float and thermostatic traps

5. Flash traps

6. Impulse traps

7. Lifting traps

8. Boiler return traps


Pumps are used in steam heating systems to dispose of condensate or return it to the boiler and to discharge excess air and noncon — densable gases to the atmosphere. The specific function (or func­tions) of a pump depends on the type of steam heating system in which it is used. The two basic steam heating pumps are:

1. Condensate return pumps

2. Vacuum heating pumps

Heat-Emitting Units

A heat-emitting unit is a device that transmits heat to the interior of a room or space. The two heat-emitting units used in steam heating systems are radiators and convectors. Simply defined, a radiator is a heat-emitting unit that transmits heat from a direct heating surface principally by means of radiation. A convector may be defined as a heat-emitting unit that transmits heat from a heating surface principally by means of convection. The heating surface of a convector is usually of the extended finned tube construction.

A detailed description of the heat-emitting units used in steam heating systems is contained in Chapter 2 of Volume 3, “Radiators, Convectors, and Unit Heaters.”

Air Supply and Venting

Two types of venting occur in heating systems. One deals with passing smoke and gases resulting from the burning of com­bustible fuels (e. g., coal, oil, and gas) to the outdoors and is described in the various chapters on furnaces and boilers. Another type of venting deals with the relief of pressure in steam heating systems by allowing a certain amount of air to escape (i. e., be vented) from the radiators. Venting radiators is described in Chapter 2 of Volume 3, “Radiators, Convectors, and Unit Heaters.”

Unit Heaters

A unit heater is essentially a forced draft convector. A centrifugal fan or propeller is used to force the air over the heating surface and into the room or space through deflector vanes. The principal operating components of a steam unit heater are shown in Figure 8-61. Unit heaters find their widest application in industrial and commercial buildings, gymnasiums, field houses, auditoriums, and other types of large buildings.

All unit heaters are described in considerable detail in Chapter 2 in Volume 3, “Radiators, Convectors, and Unit Heaters.”

Air Conditioning

Central air conditioning can be added to a steam heated structure by installing a water chiller or a separate forced-air cooling system (i. e., a split system).

Steam Heating Systems

The water chiller and steam boiler may be installed as separate units, or a complete package containing both units may be used. Water chiller installations are very rarely used in residential air con­ditioning. Split-system air conditioning dominates this field (i. e., with respect to steam heated residences).

Troubleshooting Steam Heating Systems

Steam heating systems are easy to operate but require much higher levels of maintenance than other types. It is especially important to keep a watchful eye on the control system. If the controls malfunc­tion, excessive steam pressure and overheating could cause the boiler to rupture, resulting in serious injury to the occupants of the house and expensive damage to the structure.

If the steam heating system is in good condition, it can last for years without any problems. When a problem does occur, how­ever, the homeowner may experience some difficulty in finding local help qualified in servicing and repairing boilers and steam heating systems. This is especially true in rural areas and small towns.

The first step the homeowner must take is to determine the cause of the problem in the steam heating system. Once that is determined, the homeowner then can decide whether to make the necessary repairs, replace the boiler with a new one, or replace the entire system with another type of space heating system. An example of a problem and possible remedies are listed in Table 8-2.

One of the single best sources of information for troubleshooting steam heating systems is Dan Holohan’s Pocketful of Steam Problems (with Solutions!). It contains easy to understand descriptions of hun­dreds of steam heating problems, their possible causes, and suggested remedies. It covers in depth all types of steam heating systems. The book can be ordered from

Dan Holohan Associates, Inc.

63 North Oakdale Ave.

Bethpage, NY 11714


Mailroom@heatinghelp. com

Symptom and Possible Cause

Water hammer

(a) Condensed water trapped in a section of horizontal steam piping

(b) Lift installed in return line after the trap

(c) Steam bubbles trapped and imploding in water-filled wet return lines or pump discharge piping

Possible Remedy

(a) Provide correct pitch of piping in steam lines down and away from boiler to drip traps; install drip traps ahead of risers, at end of steam main, and every 300 to 400 feet along steam piping; mount screen and dirt pocket of Y — type strainers horizontally; provide gravity condensate drainage from steam traps if there is a modulating steam regulator in the steam supply line.

(b) Install a flash tank on the drip trap discharge, and direct the cooled condensate into a common return line.

(c) Use properly sized gravity return lines to allow sufficient space in the piping to allow condensate (water) to flow in the bottom and steam to flow in the top without mixing.

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers