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Supply valve
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Hartford loop Wet return
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Fig. 14-13. One-pipe distribution configuration for steam heating system.
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In a two-pipe configuration, the steam is supplied to the radiator by one pipe and the condensate returned to the boiler through another. The radiators in this system are not equipped with individual air-vent valves. They have a steam trap on the condensate return pipe. A steam trap allows the air bound in the radiator and the condensate to flow in the return pipe but closes on steam contact and does not allow the passage of steam. The air in the return line is then vented by a main vent. A two-pipe steam system can be converted to a forced-hot-water system. This cannot be done with a one-pipe steam system. In both oneand two-pipe systems, when the condensate is returned to the boiler, if the return line in the boiler room is above the boiler-water level, it is called a dry return. If the return line is below the boiler water level, it is called a wet return. When the system has a wet return, there should be a special piping arrangement at the boiler, a Hartford loop. (See FIG. 14-14.) The purpose of the Hartford loop is to prevent water from draining out of the boiler in the event of 200 Heating systems I
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a leak in the wet-return piping. If a leak occurs in the return line, boiler water will drain down only until it reaches the top of the Hartford loop. There will still be sufficient water to prevent damage to the boiler if it continues to fire. If the heating system has a wet return, look for a Hartford loop. If you do not see one, you should consider its installation. Controls In addition to the thermostat, every steam system should have a high-pressure limit switch, a low-water cutoff, and an automatic pressure-relief valve. Also, to determine whether the boiler is operating properly, there should be a water-level gauge and a pressure gauge. (See FIG. 14-15.) The high-pressure limit switch is connected electrically to the burner control. When the steam pressure exceeds a predetermined setting, the limit switch will shut down the burner, thereby preventing the pressure from building up further. The limit control should be physically connected to the
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Boiler water line Hartford loop Boiler Wet return
Fig. 14-14. Hartford loop in the condensate return line of a steam heating system. The Hartford loop prevents water from draining out of the boiler in the event of a leak in the wet-return piping. boiler with a pipe that has a curl that looks like a pigtail. The pigtail has water in the bottom of the loop, which prevents the corrosive action of the steam from affecting the control. The low-water cutoff is a control that shuts down the burner when the level of the water in the boiler drops below the design level. There are two types of low-water controls one mounted inside of the boiler and one externally mounted. The latter is preferred because it provides a convenient means for testing its operation. This unit has a blowoff valve that when opened drops the water level in the control, causing it to operate. The manufacturer of this unit recommends that it be opened and blown down once each month to prevent a sludge accumulation that can affect its operation. All too often, homeowners neglect to perform this simple operation. I have checked many a unit that apparently had not been flushed in years; when I opened the blow-off valve, either nothing flowed out or there was a thick dark-brown sludge oozing
Fig. 14-15. Oil-fired steam boiler, showing the level gauge, low-water cutoff, high-pressure limit switch, and pressure gauge. out. The units that are mounted in the boiler are self-cleaning and do not require any action on the part of the homeowner. The water-level gauge provides a convenient means for determining the level of the water in the boiler. It is usually mounted on the side of the boiler; when there is an exteriormounted low-water cutoff, it is often part of that assembly. The water level should be at the midpoint or two-thirds up the glass gauge. The exact position is not important. What is important is that you are able to see the water level. If the entire level gauge is filled with water, there is too much water in the system. In fact, the system can be flooded. If too much water is introduced into the boiler, the water level will rise until it fills the entire distribution system and radiators. If any of the radiator valves or fittings Steam heating systems 201
are not watertight, and sometimes they are not, the water will leak out all over the room. When there is no water visible in the gauge, water must be introduced into the boiler. This can be done by manually opening the fill valve in the water-supply line. Some systems have an automatic boiler-water feeder that introduces water to the required level as needed. This is a desirable feature, but occasionally such units malfunction. Often level gauges are coated with sediment so that the water level is not visible. In this case, the glass gauge must be cleaned and all the accumulated sediment removed. The relief valve is a safety valve that automatically discharges when the operating pressure exceeds the design pressure. For a residential steam system, the relief valve is set to discharge at 15 psi, although the normal operating pressure is considerably less, usually about 1 to 2 psi. Some large systems may even operate at a negative pressure (vacuum). Domestic water heater As with hot-water boilers, steam boilers can also be equipped for generating domestic hot water. When it produces domestic hot water, the boiler must be fired all year long. When heat is not required, the boiler-water temperature is controlled by an aquastat. The aquastat activates the burner when the boiler-water temperature drops below a preset figure and shuts off the burner when the boiler-water temperature rises to about 200 F. When steam heat is required, the burner is activated by the thermostat. In this case, the thermostat overrides the aquastat control so that steam is produced. In a forced-hot-water heating system that also produces domestic hot water, a flow-control valve is needed to prevent the boiler water from circulating as a gravity system during those months when heat is not required. In a comparable steam system, no flow-control 202 Heating systems I
valve or equivalent is necessary because the boiler water is not heated sufficiently to produce steam. Advantages and disadvantages The advantages of a steam heating system can be appreciated more in a large building than in a residential structure. It is a relatively simple system that does not require a pump or fan for circulation of the steam. Since there is no water in the pipes when the system is not operating, there is no problem of the pipes freezing and bursting. If a repair or replacement is needed to a section of pipe or a fitting, it is not necessary to drain the system. Also, a steam leak would result in very little water accumulation. On the other hand, a steam system is slow in responding to an initial rapid change in heat demand because the boiler-water temperature must be brought up to 212 F before steam circulation begins. Unless the condensate is returned to the boiler by means of a pump, the boiler must be located below the radiator in the lowest rooms.
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