Steam heating systems
For the most part, steam heating systems are no longer used in new residential construction, but you will find them in older homes. In fact, in some of these homes, the boiler and its associated controls may be relatively new. The boiler used in this system is basically the same as that used in a hot-water system. It will be made of either cast iron or steel. (See the discussion on boilers on pages 192–193.) You can tell whether a boiler is being used for a steam system or a hot-water system by the type of controls and gauges used. A quick indication is a water-level gauge. (See FIG. 14-15.) If you see such a gauge, the boiler is being used to generate steam.
Unlike the hot-water system that is completely filled with water, the steam boiler is only partially (about three-quarters) filled with water, depending on the size and make. The remaining portion of the boiler, the distribution piping, and the radiators will be filled with air. After the system is fired, the water heats up until it boils, as in a tea kettle. The steam thus formed rises in the pipes of its own accord, without the aid of a fan or pump, pushing the air ahead of it as it moves along. The air from the pipes and radiators is then dissipated into the rooms through air-vent valves that, depending on the piping configuration, are located on the radiator or near the end of the steam main.
When the steam comes into contact with the cool radiator surface, it condenses back into water and in the process gives up its heat. The resulting water then flows back to the boiler for reheating. If because of a blockage in an air vent (often the result of painting the vent) the air cannot be evacuated from a radiator, a pressure will be built up within the radiator that will prevent the steam from entering. In this case, the radiator will be ineffective because it will not heat up. This condition can be easily corrected by replacing the air vent.
There are two basic distribution piping configurations used with steam heating systems: one pipe and two pipe. You can easily tell which configuration is being used by the number of pipes connected to the radiator. In a one-pipe configuration, there is only one pipe connected to the radiator; in a two-pipe configuration, there are two.
In a one-pipe arrangement, the steam is distributed to the various radiators through the same pipe that carries the condensate back to the boiler. (See FIG. 14-13.) The radiators used in this system must be pitched so that the condensate flows back through the supply valve. Otherwise, the condensate can accumulate and block the steam flow. Each radiator has a manually operated supply valve and an air vent. Some radiators are equipped with an air-vent valve that has an adjustable opening. This opening can be increased or decreased in size, thus allowing the air in the radiators to be evacuated at a faster or slower rate. Adjustable air vents are often found in larger homes and are used as a means of providing a uniform steam supply to all of the radiators. Radiators closest to the boiler will receive steam before those that are farther away. In some cases, depending on the distance apart and the size of the radiator, those closest to the boiler can be fully heated before those farthest away receive any steam. By decreasing the vent opening on those radiators closest to the boiler and increasing the opening on those farthest away, it is possible for all of the radiators to receive steam at about the same time.
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 one-and 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 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.