Distribution Systems

In steam heating systems, steam that is produced in a boiler is circulated under pressure through insulated pipes, and then condensed in cast-iron radiators. In the radiator, the latent heat given off when the steam cools and becomes water is released to the air of the room. The condensed water then returns to the boiler through a network of return pipes. The system is reasonably efficient but difficult to control precisely, as the steam gives off its heat rapidly.

Steam pipes are larger than those for water, but smaller than air ducts. The steam moves by its own power, and the rate can't be controlled as water can. It is also harder to control temperature. The condensation of steam in pipes is noisy, and hot water systems (Fig. 24-4) are now more common than steam. In order for the condensed water to collect and drain in the steam pipes, the drainpipes must be sloped, taking up more space in construction.

The old style of cast-iron radiator, located near outside walls and under windows in every room, is not installed anymore, but are still in service in older buildings. Old-fashioned radiators are made of from 2 to 50 cast-iron sections, each with four or six tubes. Some models stand on the floor, others hang on wall brackets. Such radiators have pipes that are exposed to the room, and thus leaks in these radiators are easily repaired.

Radiators lose much heat to the adjacent exterior i( walls. Foil-covered cardboard radiator reflectors are available at building supply stores, and can be placed between the wall and the radiator. Keeping a radiator clean will also maximize heat reflection out into the room.

Fin-tube radiation devices, fan-coil units (FCUs), unit heaters, and IR heaters are currently used with steam. Fin-tube radiators (Fig. 24-5) are longer and smaller than the old-fashioned cast-iron radiators.

Figure 24-5 Baseboard fin tube convectors.

Radiators (Fig. 24-6) are also used in hot water systems. Hot water (or hydronic) systems are much easier to control. Only the sensible heat of the water is transmitted to the air, not the latent heat of vaporization as with steam. A very even, controlled release of heat to the air is achieved by regulating the temperature and rate of circulation of the water. Hydronic systems are silent when properly installed and adjusted, and produce comfortable heat.

Hot water baseboard and radiator systems are laid out in four different arrangements. In a series loop sys-

Contemporary wall radiator

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Contemporary wall radiator

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Figure 24-6 Radiators.

tem, water flows through each baseboard or fin tube radiator at the building's perimeter in turn. The water becomes somewhat cooler by the time it gets to the end of the loop. A damper at each baseboard adjusts heat by reducing the amount of air that passes over the fins. Because all elements are turned on or off together, a series loop should only serve spaces within a single zone that share the same heating requirements.

One-pipe systems are very common. A single pipe supplies hot water from the boiler to each radiator or convector in sequence. Special fittings divert part of the flow into each baseboard. A valve at each baseboard allows for the heat to be reduced or shut off. Because it uses a little more piping than the series loop system, it is not as economical to install. Temperatures are still lower at the end of the loop, as with a series loop system.

Two-pipe systems use one pipe to supply water from the boiler to radiators or convectors, and a second pipe to return water to the boiler. In a two-pipe reverse return system, the lengths of supply and return pipes for each radiator or convector are nearly equal. The water is supplied to each baseboard or radiator at the same temperature, as the supply pipe doesn't pass through the baseboard or accept cooled return water. This system uses more pipe than a one-pipe system, increasing the cost.

In the final system, a two-pipe direct return system, a return pipe from each radiator or convector takes the shortest route back to the boiler. This diversion of most of the water before it reaches the end results in hotter spaces at the beginning and cooler spaces at the end of the run. This is a generally unsatisfactory system.

Hot water circulating through pipes causes pipe expansion, so expansion joints are inserted into long pipe runs. Clearance around all pipes passing through floors and walls is also provided to accommodate pipe expansion. Air vents keep air from accumulating at high points in the piping or at convector branches. Access is required for air vents in piping, which have to be bled— have the excess air let out—by hand. Water vents prevent water in the low points of a drained, inoperative system from freezing and bursting the tubing and fittings. A water vent is also located at the bottom of a boiler to allow it to be drained.

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