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.