In most large houses, and in many small ones, heating is effected by means of steam or hot water, on account of the ease with which heat is in this way transmitted to places which could not be reached with hot air from a central furnace. Of both steam and hot-water heating there are two varieties, the direct system, by which a radiator, through which the hot water or steam circulates, is placed in the room to be warmed, and the indirect system, by which the radiators are placed in the basement, and serve to heat a current of fresh air, which is brought in from the outside, and, after warming, is conducted to the room to be heated. The obvious advantage of the latter method is that the rooms are not only warmed, but supplied with fresh air, while, under the direct system, no fresh air is introduced, the radiators serving simply to heat the air already in the room. In practice, the two systems are frequently used together, the main rooms, for example, in a house being warmed by hot air from indirect radiators in the basement, while the halls, and often the chambers, are heated by direct radiation from radiators standing in them. Occasionally, also, a dwelling-house is heated by steam or hot water on the so-called "direct-indirect" system, where the radiators standing in the rooms are supplied with fresh air through small openings in the wall behind them, or from pipes brought to them through the floors; so that the occupants of the room can have the direct heat of the radiator, with a certain amount of fresh, warm air in addition.

Direct system.

Indirect system.

Each of the two principal systems has its advantages and disadvantages, which are shared proportionally by the mixed systems. A complete indirect system is more expensive in first cost, and involves about twice as great an expense for fuel, as a direct system which will do the same amount of heating; but the constant supply of fresh air which it affords is a great advantage, and an indirect system is more compact, and less likely to be injured by freezing, or to do injury by leaking valves, than a direct system. On the other hand, a direct system, besides the economy which it offers in cost of installation and maintenance, is more easily managed, as the boiler is smaller, and there are no air conduits to look after.

Most boilers for house heating, both for steam and hot water, are made of cast iron, in sections, so that, by varying the number of sections, the power of the boiler can be adapted to the circumstances of the case. Some of the larger Direct-indirect system boilers are enclosed in a brick setting, but the majority of those used for house heating are either covered with asbestos or magnesia, to prevent loss of heat, or are left bare. In the early days of cast-iron sectional boilers, when used for steam, a section, owing to imperfect circulation of the water, would occasionally burst; but this accident, owing to the small proportion which a single section bore to the whole boiler, rarely had any worse consequence than the upsetting of the household economy while repairs were being made. Now, however, the designing of such boilers is better understood, and an accident of the sort is of rare occurrence.

Relative advantages.

Boilers.

Hot-water heating, for which the same boilers are used, dispensing with the safety-valve, and water and pressure gauges, and some other accessories, has certain advantages, for dwelling-houses, over steam heating. As water boils, under normal atmospheric pressure, only at a temperature of 212°, no steam can be formed, or forced into the radiators, at any lower temperature than this, except by means of air-pumps, condensers, or other devices, which are too complicated and costly for use in ordinary houses; and, with the light pressures used in house apparatus, the steam can be raised very little above this temperature, as a slight increase in the temperature increases the pressure to the point at which the safety-valve opens. The ternperature of a steam radiator can, therefore, under ordinary circumstances, be varied only within narrow limits, and if it is powerful enough to heat a given room satisfactorily on a cold day in winter, it will be far too powerful for the same room in mild weather; yet nothing can be done to mitigate its heat except to shut off steam altogether, which will leave the room cold, or to open the windows, wasting the heat, and endangering sensitive people in the room. This inconvenience is particularly felt in houses heated by direct steam at night, when, as the temperature of the radiators cannot be materially varied, it is necessary either to keep up steam all night, making the rooms as warm at night as in the daytime, or let the steam go down to a point below 212°, when the radiators at once become perfectly cold. With an indirect, or a direct-indirect system, something can be done to modify the temperature by opening or closing the registers, and thus varying the air-supply; and direct radiators are sometimes made double, so that one-half only may be used at night; but these devices increase the cost, and, if steam is to circulate at all, a large fire must be kept up through the night.

Hot-water heating.

With hot water, on the contrary, the heat of the radiators can be varied to almost any extent, as the water simply flows through them, bringing with it whatever temperature may have been imparted to it, from the freezing-point to the boiling-point. It is, therefore, not necessary to keep up a bright fire through the night, a low fire being sufficient to keep the radiators at whatever moderate temperature may be desirable. For this reason, hot-water heating by direct radiation is very popular among the owners of small houses, as the heat is capable of perfect regulation, while the boiler is small, durable, and easily managed, and the cost of fuel is less than for any other system of heating equally efficient.