All fireplaces should be built in accordance with the few simple essentials of correct design given herein if satisfactory performance is to be realized. They should be of a size best suited to the room in which they are used from the standpoint of appearance and operation. If too small, they may function properly but do not throw out sufficient heat. If they are too large, a fire that would fill the combustion chamber would be entirely too hot for the room and would waste fuel. The location of the chimney determines the location of the fireplace and too often is governed by structural considerations only. A fireplace suggests a fireside group and a reasonable degree of seclusion, and therefore, especially in the living room, it should not be near doors to passageways of the house. CHARACTERISTICS The principal warming effect of a fireplace is produced by the radiant heat from the fire and from the hot back, sides, and hearth. In the ordinary fireplace practically no heating effect is produced by convection, that is, by air current. Air passes through the fire and up the chimney, carrying with it the heat absorbed from the fire; at the same time outside air of a lower temperature is drawn into the room. The effect of the cold air thus brought into the room is particularly noticeable farthest from the fire. Heat radiation, like light, travels in straight lines, and unless one is within range of such radiation, little heat is felt. Tests made by the Bureau of Agricultural Chemistry and Engineering showed that about five times the amount of air required for even liberal ventilation may be drawn into a living room by the operation of a fireplace. Such excessive ventilation may cause chilling drafts. Persons located at advantageous points in the room will be comfortable under such conditions, but those out of the radiation zone will not. [Illustration: Figure 19.--In 1744 Franklin promoted a metal fireplace of this type to be set out into a room. These are known as Franklin stoves and sometimes are equipped with andirons for burning wood or a grate for burning coal. The metal blower, shown in front of the opening was used with grates and set in place when starting the draft and then removed so that the cheery heat of glowing coals could be enjoyed. At one time this type of stove was highly thought of because it threw out more heat than the built-in fireplace. A few manufacturers specialize in Franklin stoves because of the present-day demand.] Tests conducted by this Bureau indicate that, as ordinarily constructed, a fireplace is only about one-third as efficient as a good stove or circulator heater. Nevertheless, they have a place as an auxiliary to the heating plant and for their cheerfulness and charm. In milder climates, fireplaces may suffice as the sole source of heat; also certain materials often wasted may be utilized for fuel. The disadvantages of the ordinary fireplace are lessened by "modified" fireplaces. MODIFIED FIREPLACES