In residence-heating it is frequently the cus tom to heat the first floor by the indirect method and the upper stories by the direct. When an owner will pay for it, the indirect method is used throughout the building. Such a system is much to be preferred to the direct.
The simplest method of connecting steam radiators is by the gravity system, and it is usu ally employed unless steam exhausted by en gines is available for heating. This system comprises distributing-mains connecting with the top of the boiler, and with vertical riser pipes from which horizontal branches lead to the radiators. Usually a return pipe is con nected to the opposite end of the radiators from that at which steam is admitted, this return connecting, through return risers and mains, with the boiler at a point below the water-line. As the steam in the radiators condenses, the resulting condensation flows back by gravity through the return pipes to the boiler. The flow and return pipes are made sufficiently large to ensure a practically uniform pressure through out the system. The system is simplicity itself, as the fire only needs attention. When the boiler is once filled, no more water is required.
It is only recently that the steam exhausted by engines and pumps has been used for heat ing. Before this time steam direct from the boilers was used in direct radiators for heating mills and factories. The radiators consisted of coils of pipe suspended from the walls or ceilings. Sometimes the condensation was re turned to the boilers by a pump or other device; sometimes it was allowed to go to waste. As the steam exhausted by engines, pumps, etc., contains a very large percentage of the heat that it contained upon entering the engine, some one conceived the idea of utilizing this steam for heating buildings, thereby saving the steam direct from the boilers that would otherwise have to be used. This practice is now almost universal where exhaust-steam is available, and the saving that it has occasioned is very great. By placing what is known as a back-pressure valve in the exhaust-pipe, sufficient pressure is maintained to cause the exhaust-steam to cir culate through the pipes and radiators of the heating-system, the latter being connected to the exhaust-pipe between the engine and the back pressure valve. The condensation that occurs
in the heating system can be collected and re turned to the boilers by various methods. Usu ally a pump or similarly acting device is em ployed.
A hot-water system arranged on the gravity principle has flow and return pipes similar to the gravity-system of steam-heating described. The entire system is filled with water. As the water is warmed in the boilers it becomes lighter in weight per cubic foot, making a difference in pressure between the flow and return pipes and causing a circulation to begin. The water rises in the flow pipes to the radiators and is there cooled. On its return to the boiler the water is again heated, and so the circulation is main tained. As the difference in weight between the water in the flow and return pipes is very slight, the motive power producing the circula tion is very slight also. Hence the pipes have to be relatively larger than for steam-heating and very carefully connected to, avoid excessive friction, which would stop or retard the cir culation. As large pipes are costly, in some large plants heated by hot water, a circulation is brought about by pumps.
Direct steam-radiators emit about 250 Brit ish thermal units per square foot of radiating surface per hour, and hot-water radiators about 180 heat-units per square foot. Consequently about one-third more radiating surface is neces sary with hot water than with steam. The pipes also must be larger, hence the hot-water sys tem is the most expensive in first cost. Hot water, however, is cheaper to operate, for water will circulate with a very low fire and supply the small amount of heat required to warm a building in mild weather.
With direct steam-heat, operating on the gravity system, it is impossible to vary to any appreciable extent the temperature of steam in a radiator; hence with this system the alterna tive is all the heat the radiator will supply or none at all. This is the principal objection to heating by means of direct steam. Air warmed by the relatively cooler hot-water radiators is thought by some to be more agreeable than air heated by steam-radiators.