HEATING AND VENTILATION. 693 HEATING AND VENTILATION. Fig. 3. hot-water heating— fsfal method of piping. now than before electric power be- came so common. The great disad- vantage of either steam or hot water, when employed in direct radiation sys- tems, is that they merely warm the air already in the room, instead of bringing in a constant snpply of fresh warm air. Tliis matters little where an independ- ent system of ventilation has been pro- vided and is intelligently used; bnt in small buildings, particularly houses, there rarely is a system of ventilation. It is to meet this objection that either the indirect system or the direct-indirect is often employed in place of direct radiation. Both steam and hot-water systems require a boiler or heater, a pipe system, and radiators. As the hot water is circulated at 140° F., against 212° F. for low-pressure steam, the radiating surface for hot water must be larger than for steam. Hot-water fitting requires greater care in design and construction than steam, which, coupled with the fact that steam- er fitting has been practiced extensively for many years, has rendered the a(h)p- tion of hot water less common than steam. Of late, however, hot water has been coming to the front. It has this great advantage over steam, name- ly, that it begins to yield heat very soon after the fire is starU'd and con- tinues to do so until the water cools, while there can be no heat from st<'ani plants until a water temperature of 212° F. has been attained, and none after it falls below that point. On the other hand, steam heat may be shut olV more quickly than water, .since the lat- ter continues to give ofl' its heat for a much longer period. Steam boilers and hot-water heaters alike require a furnace, with a grate surface for burning the fuel and a heat- ing surface for imparting the heat thus derived to the water in the boiler or heater. Where steam for both power and heat is to be generated, the boiler . Fig. 4. STEAM-EU^TING — TWO-PIPE SYSTEM, Fig. 5. STEAM-HEATING — P^VKTIAL-CIItCUIT SYSTEM. will be chosen principally for its suita- bility for power; or wholly so if the exhaust steam from the engine is to be used for heating. Where the boiler is employed for heating only, low-pressure steam is likely to be "used, and the boiler need not differ very much from a water heater, except that the former has a resei'voir for steam mounted above the heating surface, while the latter has not. Low-pressure steam systems, it may be explained, show some 5 to 10 pounds pressure at the boiler, and lower through the pipes and radiators. The choice of a boiler, from the many ty]jes and styles availalde, will depend upon a variety of local and even individual conditions. Thus a ver- tical boiler may be eho.sen where hori- zontal space is scarce and vertical space is ample ; or a water-tube boiler may be selected where a high pressure is to be carried for power purposes. Where fuel is cheaj) or the service is likely to
