(C.) The third method is known as the direct-indirect, which, as its name implies, is a combination of the two above described.

In this system the radiator is placed in the room to be heated, and the air is reheated as often as it comes in contact with the radiator. To furnish a certain amount of fresh air, a duct is so located that air from the outside passes into the room through the radiator, replacing impure air which is removed through vents or foul-air ducts by natural or forced draft, the latter of which should be used if the most reliable results are wanted. This method is used a great deal in heating large halls, theaters, churches, etc., and gives quite satisfactory results, though the indirect method with a system of forced draft ventilation is generally considered the best device for heating large buildings; its expense, however, prevents its universal adoption.

The term direct, indirect, or direct-indirect, as applied to a radiator, refers to the relation of the radiator to the air supply and the room to be heated, and not to any peculiarity of construction or circulation of steam or hot water in the radiator itself.

4. Steam heating. - A steam-heating system may be installed after the house is built; this is often a great advantage, but as it is expensive to maintain for a small house, its principal use is to heat large buildings; in many locations, steam is conducted by the central-heating-plant system to all of the buildings within a radius of several hundred feet.

In places where steam power is generated, the radiators may be heated by exhaust steam, and the expense of heating, while the exhaust is being used, will be practically nothing. This is the method followed wherever possible. If direct steam is used, it is at a very low pressure, from three to six pounds being sufficient to send the steam through the pipes and insure the return of the condensation to the boiler.

Steam radiators are connected by two systems; the one-pipe system, in which there is only one pipe to supply the steam, and to return the condensation to the boiler to be reheated, and the two-pipe system, in which each radiator or coil has a supply pipe, and a return to the main return pipe. Either system, if properly installed, will give satisfactory results. In neither system should there be pockets or sagging pipes in which the returning condensation may be trapped, as this will prevent the system from doing its work and, if the pipe should freeze, a new piece of pipe would have to be put in. In neither system should there be any part which cannot be drained.

5. Hot-water heating. - The initial cost of a system of hot-water heating is greater than that of a steam system, as more radiating surface is required, but it is less expensive in operation. As in steam heating, the pipes and radiators may be installed after the house is built. It is a very popular system for use in dwellings, as it requires but little care besides keeping the fires, which need less fuel than any other equivalent system.

In the pipes of this, as of the steam system, there should be no pockets or drops, and both should be so built that they can be thoroughly drained, to prevent freezing if the house is to be left vacant.

6. Ventilation. - Ventilation forms a part of most modern heating systems. Ventilation consists of more than furnishing a sufficient supply of fresh air; it should provide also for removing the air which has been breathed, or which has been polluted by coming in contact with unclean bodies or clothing. An opening in the ceiling of a room is not satisfactory, if it is the only means of ventilation, as it allows a great deal of heat to escape. A ventilator of this sort should be used judiciously, especially in cold weather; if a building or auditorium intended to accommodate large gatherings is constructed properly, the ceiling will be high enough to allow foul air to be well above the heads of the people, and the ventilator need not ordinarily be opened at times when the loss of heat would be a serious inconvenience.

The heat of an indirect system usually enters a room near the ceiling, and in its downward passage carries with it impurities and the most poisonous gases. These descend to the floor, and unless removed, will accumulate until the air is absolutely poisonous. To provide an easy exit for these gases, registers should be placed in the floor, or in the wall near the floor, through which the gases may be conducted out of doors by one of two methods, the natural draft, or the forced draft. The former allows gases to follow their own inclination, and while ventilators are made which, by various devices, accelerate the movement, the efficiency of the system depends, to a great extent, upon the condition of the outside atmosphere, which, if dead and heavy, checks the air current.

The forced draft is independent of the atmospheric conditions, and a current of air may always be maintained, as the draft is caused by a fan driven at a high rate of speed, which draws the impure air from the inside of the building, and forces it out of doors. This air, of course, is replaced by pure heated air, or in warm weather by artificially cooled air. Thus the air is kept continually in motion.

The contractor in building a house generally will sublet the heating and ventilating, as this work, in order to insure satisfactory results, should be planned and installed by men who have made it a study and who are familiar with all the appliances and methods which will assist in making it efficient and satisfactory.

7. Plumbing. - It is the usual custom that the carpenter should do all of the cutting necessary to allow the plumber to lay pipes and to set his work properly. The soil pipes and all others which are to be laid in the walls or floors should be in place before the house is lathed or the floors laid.

The carpenter should not attempt to say whether the work is properly done or not unless he has had considerable experience with plumbers' work; if he is responsible for the work done upon a house for which there is no architect, he should engage some competent person to inspect the work thoroughly before it is covered. A guarantee from a responsible plumber is often accepted. In most cities where there are sewer and water systems, there is an official inspector of plumbing, who should be given every opportunity to look over work, for if a faulty place is discovered after the house is finished, considerable expense and annoyance may be caused in making it right.