THE HOT-AIR FURNACE
Of the methods of heating dwellings other than by stoves, that of the
hot-air furnace is the most common. Of the various modes of furnace
heating it is the least expensive in first cost and most rapid in
effect. In the use of steam heat, the water in the boiler must be
vaporized before its heat is available. With hot-water heating, the
whole mass of water in the entire system must be raised considerably in
temperature before its heat can affect the temperature of the rooms,
and consequently in first effect it is very slow. In the use of the
hot-air furnace the heat from the register begins to warm the rooms
when the fire is started.
Hot-air furnaces are made by manufacturing companies in a great variety
of styles and forms to suit purposes of every kind. In practice the
furnace is built in sizes, to heat a definite amount of cubical space.
The maker designs a furnace to heat a certain number of cubic feet of
space contained in a building. It must be sufficiently large to keep
the temperature at 70°F. on the coldest nights of winter when the
wind is blowing a gale. It is evident that with the variable factors
entering the problem, the designer must be a person of experience in
order that the furnace meet the requirements.
The following table taken from a manufacturer’s catalogue shows the
method of adapting the product of the maker to any size of dwelling.
The volume of the house is calculated in cubic feet and from this
result the size of furnace most nearly suited is selected from the
table.
------------------------+--------+--------+--------+--------+--------
Furnace number | 1 | 2 | 3 | 4 | 5
------------------------+--------+--------+--------+--------+--------
Weight without casing, | 984 | 1,111 | 1,340 | 1,531 | 1,934
lb. | | | |
------------------------+--------+--------+--------+--------+--------
Estimated capacities in | 8,000 | 12,000 | 20,000 | 35,000 | 60,000
cubic feet | to | to | to | to | to
| 12,000 | 20,000 | 35,000 | 60,000 |100,000
------------------------+--------+--------+--------+--------+--------
Capacity in number of | | | | |
rooms of ordinary size | 3 to 5 | 5 to 7 | 7 to 9 |9 to 12 |12 to 15
in residence heating | | | | |
------------------------+--------+--------+--------+--------+--------
CONSTRUCTION
[Illustration: FIG. 45.--Interior view of a hot-air furnace.]
The furnace, in general construction, consists of a cast-iron fire-box
with its heating surfaces, through which the flames and heated gases
from the fire pass, on the way to the chimney; these with the passages
and heating surfaces for heating the air compose the essential
features. Fig. 45 shows such a furnace with the sides broken away to
show the internal construction. The flames and gases from the fire-box
_F_ circulate through the cast-iron drum _D_ and are discharged at _C_
to the chimney. The drum _D_ is made in such form that it presents to
the heat from the fire a large amount of heating surface and at the
same time offers as little opposition as possible to the furnace draft.
The air to be heated enters the furnace through the cold air duct at
the bottom, and after circulating through the drum, passes out at
the openings _R_ to the conducting pipes. The cast-iron box _W_ is a
water tank that should be attached to every hot-air furnace. The water
contained in the tank is for humidifying the air as it passes through
the furnace. In this furnace the outside casing is of sheet iron,
reinforced with wrought-iron flanges. The front, which contains the
doors of the fire-box, ash-pit, etc., are of cast iron of ornamented
design.
As the air to be heated passes through the furnace it receives part of
its warmth by radiation but most of it is absorbed by coming directly
into contact with the heating surfaces. Since air is a poor conductor
of heat its temperature is raised very slowly; it should, therefore,
be kept in contact with the heating surfaces as long as possible to
insure an economical furnace. In common practice the ratio of heating
surface to grate surface average 35 to 1; that is, for each square foot
of grate surface there is 35 square feet of heating surface to warm the
passing air. Should this ratio be increased to 50 to 1 the efficiency
of the furnace would be much improved.
If the ratio of heating surface to the grate surface is too small for
its requirements, the temperature of the air-heating surfaces must be
very high to provide the desired amount of heat. Under such a condition
the efficiency of the furnace would be low, since in all cases where
rapid combustion is required the available amount of heat per pound of
coal consumed is low. With a large amount of heating surface, the air
remains in contact with the hot surface a relatively longer period and
the desired temperature is reached with the expenditure of a smaller
amount of fuel. A momentary exposure of the air to a red-hot surface
is far less effective than a prolonged contact with a surface having
only a moderate temperature. Time is an element of great importance
in heating air. In considering the relative merits of two furnaces
with the same amount of grate surface, that with the larger amount of
heating surface will evidently be the most efficient.
The supply of heat comes primarily from the burning coal on the furnace
grate. The grate surface should be large enough in area to permit the
required quantity of heat to be generated by the burning fuel with
a moderate fire. If the grate surface is too small for the required
purpose, a hot fire will be necessary, when the normal amount of heat
is demanded by the house. During extremely cold weather, particularly
when accompanied by high wind, the extra heat demanded to keep the
house at the desired temperature makes necessary the use of an amount
of fuel that cannot be burned on the grate unless the fire is forced.
Hot fires can be kept up only at the expense of a large amount of heat,
and the resultant efficiency of the furnace is reduced.
High furnace temperatures are always attended by a large loss of heat.
The vastly greater quantity of air necessary to create the combustion,
the high temperature of the chimney gases and the increased velocity of
the heated gases through the furnace, all tend to increase the amount
of heat that is sent up the chimney, and to decrease the percentage
of heat that is delivered by the furnace. In order to heat the house
economically the furnace must be large enough to easily generate the
required amount of heat demanded in the most severe weather.
=Furnace-gas Leaks.=--The presence of furnace gas in the atmosphere of
a house is not only annoying but may be a source of danger. Gas leaks
are commonly due to the imperfect union of the various parts of which
the furnace is composed.
Cast-iron furnaces are constructed in sections that are assembled to
form a complete plant. In assembling, the various parts of contact must
be carefully joined to prevent the gases in the fire-box from escaping
into the air-heating space. In the manufacture of cast-iron furnaces it
is practically impossible to form gas-tight joints by the contact of
the metal alone. In the erection of the furnace all doubtful joints are
filled with stove putty. Furnaces of good design require the use of the
least amount of this material.
Stove putty is composed of finely divided graphitic carbon that is
made into a paste suitable for filling all imperfect joints. When
the putty hardens it withstands the heat to which it is subjected,
without shrinking. In the course of time, however, the putty may be
displaced and leave openings through which the furnace gases may leak
into heating space and thus enter the house. Leaks of the kind may be
stopped by renewing the putty which may be obtained from any dealer in
stoves.
=Location of the Furnace.=--The location of the furnace will generally
be governed by the exposure of the house and the location of the
chimney. In all exposed rooms on the windward side of the house the
temperature will be lower and the air pressure higher than in other
parts of the house. The increase in atmospheric pressure makes it
necessary to supply to such rooms the hottest air practicable. The
conducting pipes, therefore, should be most directly connected with the
furnace and with the least run of horizontal pipe. The proper place for
the furnace is as near as possible the coldest place of the house.
It is a common practice to place registers near the inner corner of
the room, in order to economize in conducting pipe, in horizontal
runs. A small amount of economy in first cost is thus secured but the
efficiency of the apparatus is sacrificed.
The greatest objection to placing the registers and conducting pipes in
the outer walls of buildings is that of loss of heat, due to exposure
to the outside cold and the resulting loss in circulation. Losses of
this kind may be prevented by covering the ducts with the necessary
non-conducting material. The registers should occupy a place in the
room nearest the entering cold air.
[Illustration: FIG. 46.--Method of conducting warm air from the furnace
to the registers.]
=Flues.=--It is customary to place the conducting pipes for the first
floor in such a way as to use only the shortest connections. The flues
used for the second floor produce, as in a chimney, a greater velocity
of flow to the air and as a consequence larger _horizontal_ pipes are
used at the furnace. All horizontal pipes should have upward slant, as
much as the basement will permit.
The velocity of the air in the conducting flues will depend on two
factors: the height of the flue, and the temperature of the air. To
prevent the loss of the temperature of the air, the flue should be
covered with at least two layers of asbestus paper bound with wire.
Wall flues are commonly flattened and occupy a place in the wall
between the studding. Each flue should have a damper at the furnace,
that will permit the heat to be shut off from any part of the house.
Rules for proportioning of registers and conducting flues to suit
rooms of various sizes are entirely empirical. The sizes of registers
and flues found satisfactory in practice is generally a guide for the
designer. The following table is taken from a manufacturer’s catalogue
and gives a list of sizes that have proven satisfactory under a great
variety of conditions and may be taken as good practice:
FIRST FLOOR
-----------+-------------+------------+-----------
Sizes of | Diameter of | Size of | Height of
registers | pipes | rooms | ceilings
in inches | in inches | in feet | in feet
-----------+-------------+------------+-----------
12 by 15 | 12 | 18 by 20 | 11
10 by 14 | 10 | 15 by 15 | 10
9 by 12 | 9 | 14 by 15 | 9
8 by 12 | 9 | 13 by 13 | 9
-----------+-------------+------------+-----------
SECOND FLOOR
-----------+-------------+------------+-----------
10 by 14 | 10 | 18 by 20 | 10
9 by 12 | 9 | 16 by 16 | 9
8 by 12 | 8 | 13 by 13 | 8
8 by 10 | 7 | 12 by 12 | 8
-----------+-------------+------------+-----------
The furnace is not only a means of heating the house but may be a means
of ventilation as well; to this end it is desirable to arrange the air
supply of the furnace to connect with the outside air. This arrangement
assures a supply of oxygen even though no special means is arranged for
discharging the vitiated air from the rooms.
[Illustration: FIG. 47.--Interior construction of a combination
hot-water and hot-air furnace.]
=Combination Hot-air and Hot-water Heater.=--In the case of large
houses heated by hot air it is sometimes better to use two or more
furnaces than to attempt to carry the heat long distances in the
customary pipes. Where heat is required in rooms located at a distance
more than 30 feet, it is advisable to use a combination hot-air and
hot-water heater, the distant rooms being heated by hot-water radiators.
A furnace arranged for such a combination is shown in Fig. 47. This
furnace contains, first, the essential features of a hot-air furnace;
next, it includes a hot-water plant. The fire-box and air-heating
surfaces are easily recognized. The arrows show the course of the air
entering at the bottom of the furnace, which after being heated by
passing over the heating surfaces, escapes at the openings marked _warm
air_, to the distributing pipes.
[Illustration: FIG. 48.--The hot-air furnace as it appears in the
house.]
Inside the air-heating surfaces are three hollow cast-iron pieces _W_,
that form a part of the walls of the fire-box. These pieces, with their
connecting pipes, form the water-heating part of the furnace, which
supplies the hot water for the radiators. The pieces _W_, with the
connecting pipes and radiators, form an independent heating plant, with
a fire-box in common with the hot-air furnace.
The returning water from the radiators enters the heating surfaces _W_,
through the pipe marked _return pipe_. The heated water is discharged
from the heaters into that marked _flow pipe_ which conducts it to the
radiators. Such a furnace is, therefore, two independent systems, one
for hot air and the other for hot water, but with a single fire-box.
This furnace, like the simple hot-air furnace, is rated, first in the
amount of space it will heat with hot air and in addition, by the
number of square feet of hot-water radiating surface that will be kept
hot by the hot-water heater.
In Fig. 48 is shown the location of the furnace in a cottage with the
conducting pipes to the various rooms. The registers in the first floor
are generally set in the floor but if desired they may be placed in the
walls. Those on the second floor are placed in the walls because of
convenience. The conducting pipes pass through the partitions between
the studding.
[Illustration: FIG. 49.--Details of air ducts and damper regulator used
with the hot-air furnace.]
In all well-arranged hot-air heating plants provision is made so that
the air for heating may be taken from the outside. It does not follow
that the supply of fresh air should always come from outdoors; there
are times during extremely cold weather, accompanied by high winds,
when ventilation is ample without the outside source of supply. Since
it is never desirable to take the air supply from the basement, such
an arrangement as is shown in Fig. 49, or a modification of the same
plan is commonly employed. The duct _A_ from the outside and _B_ from
the rooms above connect with the air supply for the furnaces. A damper
_C_ arranged to move on a hinge, is so placed as to admit the air from
either source as desired. The damper may be placed so as to take part
or all of the air from the outside by adjusting the handle at the
proper place.