103. The methods of house heating now in vogue maybe divided into two classes, called direct and indirect. The distinction between them consists solely in the mode of supplying the heat. In the direct method, the heat is emitted from stoves or radiators contained within the room to be warmed; in the indirect method, the heat is supplied by a current of hot air which is brought in from some outside source.

Several modifications and combinations of these methods are also used. The so called direct-indirect method is one in which the room is warmed by the direct action of a radiator or heater, and, in addition, a current of fresh air is allowed to enter the room from the outer atmosphere. This fresh air, however, is compelled to pass through the heater and become warmed before it mingles with the air in the room. The direct-indirect system is a combination of a system of direct heating with one of direct ventilation, the ventilation usually being limited to the room containing the heater. If there is no vent by which air may flow out of the room at the same time that the fresh air flows in, the current cannot be maintained, and the heater then operates like any direct heater.

Indirect-heating systems are usually combined with a system of ventilation, but are sometimes operated without it. Indirect heating is sometimes practised by means of a heater, or stack, which takes cold air from a room, and, after warming it, returns it to the same room through the ordinary hot-air flues and registers. This method is highly objectionable from a sanitary point of view, but it is sometimes used for warming hallways or large rooms which contain only a very few people. The air within a room may be heated and kept warm, without introducing any fresh air, by either the direct or indirect method. Thus, it will be seen that ventilation is not inseparably connected with either system of heating.

The direct-heating system is usually operated without any provision for ventilation, and it is not suitable, therefore, for warming dwellings or rooms which are occupied by people.

104. General Requirements of a Heating System.

In order to heat and ventilate a room in a satisfactory manner, it is necessary to secure the following desiderata:

1. A uniform distribution of heat throughout all parts of the room, as far as possible.

2. A thorough diffusion of fresh air throughout the level or zone in which persons breathe.

3. A prompt and complete removal of all foul air from the room.

4. A means of preventing all waste of heat, caused by the premature escape of the heated air.

5. A means of avoiding perceptible currents or drafts of either warm or cold air.