105. The circulation of air, and consequent distribution of heat throughout a room, depends greatly upon the locations of the air inlet and outlet, and upon the location of the radiator or stove relatively to the cold exterior walls.

The circulation which is caused by the introduction of warm air and the expulsion of foul air, at various points, is shown in Figs. 37 and 38. Fig. 37 shows the interior circulation in a room which is heated in the ordinary old-fashioned way. The hot air enters at a and escapes through a ventilating register b, which is located near the top of the wall at the opposite end of the room. The direction of the currents is shown by the arrows. The air composing the main current c is much warmer than that in the space d. There will be some circulation in this space, as shown by the dotted arrows, but it is usually very sluggish. There will usually be a layer of cold air, more or less foul, near the floor at e. The current of fresh hot air passes through the room so directly and quickly that it will not diffuse into the main body of air at d, to a sufficient extent to freshen it properly. There will be a considerable difference in temperature of the air at the floor and at the breathing level, and the inmates of the room are likely to suffer more or less discomfort in consequence. The hot air escapes from the room at a high temperature, and a large percentage of the available heat is thereby wasted.

Fig. 37.

Fig. 38.

If the outlet in Fig. 37 is brought down near the floor, the inlet remaining in the same position, the premature escape of the hot air is prevented and the distribution of heat is somewhat improved, but it is still quite imperfect.

Fig. 39.

In Fig. 38 the location of the outlet is changed by placing it at or near that end of the room at which the fresh air enters. The fresh-air current passes forward towards the opposite wall, spreading out and moving more slowly as it proceeds, and it gradually settles downwards and returns along the floor to the outlet b.

The fresh air is thus induced to travel the length of the room twice, instead of once, as in Fig. 37, and, if the arrangements are not at fault otherwise, the distribution of heat and the freshening of the apartment will be very satisfactory.

106. The general circulation within a room will be greatly influenced by the nature and extent of the cooling surfaces, such as windows and exterior walls. If there is a large window in the room the air will tend to circulate as shown in Fig. 39. Being rapidly cooled by contact with the glass, it will flow downwards with considerable velocity, and the current will spread out upon the floor, thus forming a cold stratum at a. The upward currents of air in other parts of the room, to compensate for the downward current, will be so diffused and slow as to be imperceptible.

A radiator placed at a would prevent cold air from falling to the floor, and would tend to equalize the temperature of the room.

107. In general, the supply of hot air should be brought into the room at a level above the heads of the inmates, and not up through the floor. The object is not only to secure proper circulation, but to prevent the incoming current of hot air from impinging upon people in the room and thus becoming a source of discomfort. The minimum height for hot-air inlets is found in practice to be about 8 feet above the floor, in rooms of moderate dimensions and with currents of low velocity.

In large rooms having high ceilings, the inlets should be placed at a greater elevation, generally not less than two-thirds the height of the ceiling. In large audience rooms it is advisable to introduce the hot air at a low velocity through openings in the ceiling, and to take out the spent air through openings in or near the floor.

The number of hot-air inlets which will be required will depend upon the dimensions of the room. The air must be introduced at a sufficient number of points to secure a thorough distribution of the heat throughout the room. One inlet is usually sufficient for the rooms commonly found in dwellings and offices, but, in large parlors, music rooms, etc., two or even more may be employed to good advantage. Hot air should not be introduced through floor inlets, except in hallways and anterooms, for the purpose of warming persons who have been exposed to the cold.

108. Floor outlets for foul air should be arranged to avoid a draft in the vicinity of the outlet. If only a single outlet is provided, having about the same area as the inlet, the current near its orifice will have nearly the same velocity as the hot air coming from the inlet, and it will cause a draft at that point. It is, therefore, necessary that the outgoing currents be divided, by increasing the number or area of the outlets sufficiently to avoid drafts of objectionable velocity.

In the case of audience rooms, the outlets should be made in the form of long, narrow slits extending horizontally at the base of the walls, or around the edges of platforms. If the floor is stepped, the outlets may be made in the risers.

The plan of taking the foul air out through a large grating in the floor in the center of a hall, is not a good one.